Early History of Particle Accelerators*

ADVANCES I N ELECTRONICS A N D ELECTRON PHYSICS, VOL.

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Early History of Particle Accelerators* M. STANLEY LIVINGSTON? Los Alamos ScientiJic Laboratory Los Alamos, New Mexico

I. Introduction

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11. Origins of Accelerators . . . .

A. B. C. D.

........... Electrical Discharge in G Electrostatic Machines . ...................... Surge Generators . . . . . . . . . . . . . . . . . . . Tesla Coil. . . . . . . . . . .................

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Iv. Resonance

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C. Ernest Lawrence and the Cyclotron . . . . . . . . . . . . . . . . . . . V . The Betatron . . . . . . . . . . . . . . . . . . ...................... VI. Synchronous Accelerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ A. Story of the Development of Phase Stab B. Electron Synchrotron . . . . . . . . . . . ............. C. Synchrocyclotron .................. ............. ........................... D. Proton Synchrotron . . . . VII. Linear Accelerators . . . . . . . . . . . . . . . . . . . . ............ A. Early Linear Accelerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Origins of Modem Linear Accelerators . . . . . . . . . . . . . . . . . . . . . . . . . .............. C. Resonance Linear Accelerator- Alvarez Linac ........ ....... D. Electron Linacs . . E. SLAC Two-Mile A .. ... ......... F. Linacs for Special Purposes . . . . . . . . ..................... .............. VIII. Alternating Gradient Accelerators . . . . . . . . . . . . A. Origins of Strong Focusing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Alternating Gradient Proton Synchrotrons . . . . . . . . . . . . . . . . . . . . . . . ................ C. Alternating Gradient Electron Synchrotrons . .

34 38 38 41 43 45 48 50 52 55 60 61 63 65 65 71 75

* Work done under the auspices of the U.S. Department of Energy. t Massachusetts Institute of Technology, Retired. 1 Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-014650-9

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M. STANLEY LIVINGSTON D. Sector-Focused Cyclotrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Separated Function Proton Synchrotrons . . . . . . . . . . . . . . . . . . . . . . . IX. Storage Rings and Colliding Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

78 80 82 85

I. INTRODUCTION Particle accelerators are devices for giving kinetic energy to ions or electrons. They are used to study the properties of atomic nuclei by producing nuclear disintegrations and other interactions. Most of our present knowledge of nuclear physics has been obtained from experiments using the particle beams from accelerators. These beams of electrons, protons, heavier ions, and a growing number of secondary particles and radiations are the probes with which scientists sample nature. The rapid growth of the research field of nuclear physics has been due in large part to the experiments performed with such beams, coming from a sequence of electronuclear machines of ever-increasing energy. As particle energies have increased with each new generation of accelerators, the number and complexity of particle interactions have also increased, increasing our knowledge of nuclear science. This rapid increase in the energy of particle accelerators has also opened a new field of research-that of “particle physics.” With the greatly increased energies, evidence has been found for new fundamental particles having much higher mass values than were previously known. This evidence has disclosed the existence of scores of new particles (or excitation states of particles) and proof (or disproof) of the many theoretical interpretations that have been advanced. Very high energy accelerators are almost the only instruments with which progress can be made at this frontier of knowledge. So the demand from scientists for accelerators of still higher energy continues. The accelerator field has been characterized by a sequence of new concepts or inventions, each leading to a new type of machine capable of still higher energies and stimulating the development of a new generation of machines. At times these developments have come so fast that it was difficult to determine which laboratory or machine held the current voltage record. There are several examples of the simultaneous and independent development of the same new concept by scientists in different laboratories. And at times the competition has taken on the aspects of a race for high voltage, with several laboratories seeking to be the first to achieve a new voltage record. The speed of development of accelerators and the rapid increase in energy has been due in large part to the rapid development of other engineering technologies. As the accelerator field has developed it has paral-

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FIG.1 . Diagram of apparatus used by Rutherford to observe the disintegration of nitrogen nuclei by alpha particles. (From M. S. Livingston and J. P. Blewett, "Particle Accelerators," 1962. Reprinted by permission of McGraw-Hill Book Company, New York.)

leled the progress in the electronics industry and in other branches of engineering, and has quickly utilized new materials and new technologies as they have become available. By now the controls of all major accelerators are computerized, and remote-handling has displaced mechanical techniques for most maintenance. On the other hand, accelerator designers have themselves contributed much to the state of the art in other fields of science and engineering, with a variety of new concepts and innovative techniques. When Ernest Rutherford ( 1 ) in 1919, first disintegrated the nucleus of the nitrogen atom using alpha particles from radioactive sources, a new era was opened in science (Fig. 1). The dream of the alchemist had been achieved; matter could be transmuted from one form to another. However, scientists realized immediately that better tools would be needed than the particles from natural radioactivity if this new field of study of atomic nuclei was to be effectively explored. During the following ten years, studies of the type initiated by Rutherford using natural alpha particles were continued, and the magnitudes of the nuclear binding energies were estimated. These were of the order of several millions of electron volts. None of the existing techniques for producing high voltages could approach this energy range. Nevertheless, scientists were confident that protons or other light ions could eventually be accelerated to energies sufficient to produce disintegrations of light nuclei. By 1926-1927 work had started in the Cavendish and in several other laboratories to develop the high-voltage electrical devices needed. The initial goal was not voltage itself but disintegration of atomic nuclei using artificially accelerated par-

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ticles. In his President’s Address (2) before the Royal Society in 1927, Sir Ernest Rutherford reemphasized the need for higher energy voltage sources and described the status of voltage generating devices throughout the scientific world. The first to succeed were Cockcroft and Walton in the Cavendish Laboratory. Urged on by Rutherford, they had chosen to develop a device of modest size and energy based on the principle of the voltage multiplier. Early in 1932 they observed the disintegration of lithium nuclei by protons of 500 keV energy. Their work is reported in a series of papers (3) in the Proceedings of the Royal Society which give full details of the technical developments, voltage calibrations, and experimental observations. Other laboratories followed in rapid succession. Within a few months the cyclotron of Lawrence and Livingston (4) of the University of California at Berkeley, which had been brought into operation at 1.2 MeV proton energy in early 1932, was adapted to duplicate and extend the disintegration results of Cockcroft and Walton. Next, a group at the Bureau of Terrestrial Magnetism of the Carnegie Institution at Washington, D.C., composed of Tuve, Hafstad, and Dahl (5), produced a beam of 0.6-MeV protons with an electrostatic generator of the type devised by Van d e Graaff (6),and also started nuclear studies. By 1934, Lauritsen and Crane (7) at the California Institute of Technology had progressed far enough in their utilization of the cascade transformer system located in the Caltech Engineering Department to start nuclear studies at 700 keV energy. So, within two years at least four laboratories were using artificially akcelerated particles (protons) to start programs of studies on the disintegration of atomic nuclei. The race was indeed a close one! Five waves of development have swept the accelerator field, characterized by different concepts in the acceleration, focusing, and use of the particles. The first wave was the application of direct voltage techniques in which particles are accelerated through a single large potential drop. Several types of voltage sources were put to use, including transformer-rectifier circuits, the voltage multiplier circuit, electrostatic generators, and others. The magnitude of the potential developed was increased to its practical limit by using electrode terminals of large radius of curvature and by improving insulation. Voltage breakdown of accelerating tubes was minimized by subdividing the potential between several gaps along the length of the discharge tube. However, the voltages obtained in direct voltage accelerators were limited (until quite recently) to about one million volts (1 MV) at atmospheric pressure, by the breakdown of insulation and of the surrounding medium. The second wave was based on the concept of resonance acceleration, in which particles are accelerated by an rf electric field, and arranged to pass many times through this field to obtain a final energy many times the

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applied voltage. The chief examples are the cyclotron and the early linear accelerators. The compact arrangement and simplicity of construction made the cyclotron the dominant instrument for nuclear studies for many years, in the energy range of 5 to 20 MeV. The energy limitation was due to the relativistic increase of mass of the accelerated particles with increasing energy, so the ions would fall out of resonance. This method of producing high-velocity particles without the use of high voltages was a major step forward, and the cyclotron was used around the world. Resonance linear accelerators were also built, but due to limitations in focusing techniques and in the rf systems available in those early days, they did not have sufficient energy to compete with the cyclotron in nuclear studies. The third wave of development came from application of the principle of phase-stable or synchronous acceleration to resonance accelerators. Under suitable conditions, the resonant particles can be made to oscillate in the phase with which they traverse the accelerating gaps, around an “equilibrium” phase at which they are maintained in resonance. The action is similar to the “hunting” in phase of the rotor of a synchronous motor, hence the name. In principle, it is possible to keep particles in resonance for an indefinitely large number of accelerations. Several families of synchronous accelerators utilize this property; the most important are the electron synchrotron, the synchrocyclotron, and the proton synchrotron. Electron synchrotrons are the simplest, but the first round was limited in energy to about 1 GeV (billion electron volts) because of the drain in energy through “synchrotron radiation” as the electrons were deflected in the containing magnetic field. In the synchrocyclotron synchronous acceleration avoids the relativistic limit of the standard cyclotron, and energies are much higher. The energy limit is set by the physical size and cost of the large solid-core magnet; in practice this limit has been 700 MeV for protons. The most important application of the principle is the proton synchrotron, which uses a ring magnet containing a doughnutshaped vacuum chamber between poles within which ions are accelerated. Again, the maximum practical energy is determined by the size or cost of the machine; in practice this limit has been less than 10 GeV. The fourth wave of development came through a new principle of focusing for particle beams which involves the use of alternating gradient magnetic fields. With such “strong focusing” the size and cost of magnets for circular accelerators such as proton and electron synchrotrons have been greatly reduced, making circular machines of very much higher energy economically feasible. And the use of .“quadrupole” magnetic focusing has provided high quality focusing for linear accelerators for the first time. The result has been a new generation of machines which are much larger, produce much higher energies, and cost much more to construct. Of these, the largest is the giant accelerator at the Fermi National

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Laboratory at Batavia, Illinois, which has produced protons of 500 GeV. A close second is the Super Proton Synchrotron (SPS) at CERN near Geneva, of 400 GeV. And the highest energy electron linac which depends on quadrupole focusing is the 20-GeV machine at the Stanford Linear Accelerator Center (SLAC), in Palo Alto. The fifth wave of development is the use of intersecting or colliding beams. This technique is essentially a method of improving the efficiency of the targeting system, and it greatly increases the energy available for excitation in particle interactions. The technique starts with the development of storage rings, or circles of constant-field deflecting and focusing magnets into which beams of accelerated particles are injected and made to circulate continuously, with lifetimes of many hours. If a beam of protons of, say, 100 GeV strikes a fixed target, most of their energy will be used in the transfer of momentum to the target atoms, so only a fraction (about 20 GeV) will be available for excitation of the products of the interaction. However, if two beams of 100-GeV protons are made to collide head-on, the entire energy of 200 GeV is available for excitation. Dramatic scientific results have already been obtained with colliding beams of electrons and positrons, and of protons on protons. The ultimate will be achieved when antiproton beams become available to use in colliding beam systems with equal-energy protons. The purpose of this article is to present the historical background of the field of particle accelerators and to describe the conceptual and technical growth of each of the major types. We will first describe the status of high-voltage technology before accelerators were conceived, which will show the technical knowledge available to the earliest generation of accelerator builders. This was in the period between World War I and World War 11, when electrical engineering was in its infancy and only the crudest of rf systems were in use. Original contributions will be identified where possible, and references cited to the early publications. In the following sections, we will describe the origins and early developments of each of the major accelerator types, in turn. We will show how new ideas and concepts swept through the accelerator field and brought new types of machines with higher and higher energies. We will show how invention followed invention so rapidly at times that it was difficult to identify the origin, and how these concepts spread from one laboratory to another and were continuously improved. In the early days the accelerator field was in many respects an art, known and practiced by a relatively small number of scientists and engineers, and many of the developments spread by word of mouth from one laboratory to another. This was a new and exciting field, and most of the early designers were physicists devoting their time and talents to the engineering development of

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accelerators in order to use the output beams for research in the newly developing field of nuclear physics. In later years a new breed of accelerator designers and engineers entered the field, who applied modern engineering technology to this growing field of particle accelerators and made reliable machines out of the early experimental prototypes. 11. ORIGINS OF ACCELERATORS A. Elerirical Discharge in Gases

In a general sense, the first particle accelerators were the gas discharge tubes of the late 1800s, built during the era in which the properties of gas discharges were being studied and the components of the discharges identified. In most of these devices the discharge was contained within a glass envelope, so only light and X rays emerged. To cite a few examples: Hittorf (1869) studied the conduction of electricity in ratified gases and observed the “cathode glow;” Crookes (1878) showed that “cathode rays” cast shadows and must be corpuscular in nature: Goldstein (1886) observed the “canal rays” which emerged through holes in the cathode and showed that they were positively charged; Philip Lenard (1892) mounted thin foil metal windows on discharge tubes through which the cathode rays emerged into air, and by magnetic deflection showed them to be negatively charged; Roentgen (1895) observed and studied the “X rays” produced when canal rays were stopped on a target, and which penetrated the glass envelope. Others in this early group of experimenters studied the “electrodeless” discharge, an early form of plasma. Then, in 1895, J. J. Thomson showed that cathode rays were negatively charged particles with e l m values 2000 times greater than other gaseous or aqueous ions, demonstrating conclusively the existence of the electron. During the early years of the century the developing field of electrical engineering produced a number of devices to exploit the field. The thrill of experimenting with high voltages was a challenge to scientists and engineers alike. A determined effort was made to develop generators of high voltage, primarily for the testing of electrical equipment, even before the present uses of such machines were envisioned. In the pages to follow the major types of voltage generators developed during this period will be described. and their eventual uses indicated. B . Electrostatic Machines The voltages applied to gaseous discharge tubes were produced by a variety of generators. One type was an electrostatic generator which pro-

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duced charge by dielectric induction, then separated the charge and stored it in capacitors. (An early form of capacitor was the “Leyden jar.”) Machines using rotating dielectric disks which produced potentials up to 100 kV were built by Toepler, by Hoeltz, and by Wimshurst in the 1890s. Such machines were widely used for classroom demonstrations of static electricity in the early 1900s; many examples are now in science museums. In the early days of accelerator development an attempt was made by Dahl(8) (then at the Carnegie Institution) to build a Hoeltz-type machine for nuclear studies. He constructed a 24-in. disk machine which developed 200 kV with a current capability of 10 mA. This was too low a voltage for the anticipated needs, but he suggested mounting several such machines on insulated platforms and connecting them in series for higher potentials. However, this type of machine was not as steady as the belt-type electrostatic generator which was also being developed, and was abandoned. With the improvement of industrial techniques, the induction-type electrostatic generator has been developed to be a practical voltage source by the SAMES Company of Grenoble. Their generators use rotating dielectric cylinders in a hydrogen atmosphere. SAMES has built and marketed a series of compact, reliable machines for voltages between 50 kV and 1 MV which have found application in many European laboratories. Other electrostatic generators depend on separation of charge. Lord Kelvin is reputed to be the originator of the “charged water drop” generator; water drops electrified by friction on issuing from a nozzle fell into an insulated metal container which became charged. Righi (1890) used a belt formed of alternate links of insulating and conducting material to carry charge produced by friction to a hollow metal spherical terminal. The modem development of the belt-charged generator was started by Van de Graaff in 1930. The Van de Graaff type of electrostatic generator has become such an important instrument that it deserves detailed treatment, and is described in a following section. C . Surge Generators

The “Marx” circuit was widely used by electrical engineers to produce surges of high voltage for testing electrical equipment. In this device a stack of capacitors is charged in parallel from a relatively low-voltage dc supply, and then discharged through cross-connected spark gaps. The potential surge obtained momentarily across the stack of capacitors can be extremely high and can cause sparks many feet long in air, but the duration of the pulse is only a few microseconds and it is oscillatory in character.

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In Berlin, Brash and Lange (9) started work on high-voltage sources and discharge tubes in 1927. They built a surge generator using the Marx circuit and developed sectionalized evacuated tubes to which they applied the pulses. Their discharge tube seems to have been the first to use rubber gaskets between metal rings. The vacuum was rough and the discharge practically exploded the tube in each pulse. They claimed accelerating electrons to 2.4 MeV and bringing them out into air through a thin window, and protons up to 900 keV. However, they did not report any nuclear studies with their high-energy particles. The highest voltage surge generator was built at the Pittsfield, Massachusetts plant of the General Electric Company in about 1932; it was capable of producing voltage surges of over 6 MV and served as an engineering test facility for many years. It may be of interest to report an attempt to use the high potentials developed in the atmosphere during electrical storms. In 1932, Dr. C. Urban and associates stretched an insulated cable across a valley between two peaks in the Alps, and from this suspended a conducting cable supporting a terminal. During thunderstorms sparks several hundred feet long were obtained. Plans were made to install a discharge tube for acceleration of particles, but were abandoned when Dr. Urban was accidentally electrocuted. D . Tesla Coil

Another phenomenon exploited during the early years of the century to obtain high voltages was electromagnetic induction. Circuits were developed with coils linked by magnetic flux, with or without iron cores, to produce high alternating voltages. One early form was the Tesla coil, in which a resonant primary coil of a few turns induced high alternating voltages in a multiturn secondary coil. The technique has been used for both pulsed excitation and for steady alternating current excitation. For pulse operation a capacitor in the primary circuit is charged sufficiently to discharge a spark gap, producing an oscillatory surge of current through the primary circuit. This induces bursts of high potential across the terminals of the secondary coil. Voltage amplitude is at a maximum when the primary and secondary circuits are tuned to resonance. This device was investigated as a possible source for particle acceleration by a team under Breit (10)at the Department of Terrestrial Magnetism of the Carnegie Institution. They used a secondary coil wound on a glass tube which was equipped with spherical metal terminals, and immersed it in an insulating oil bath; they reported peak potentials of 3 MV at atmospheric pressure. Breit, Tuve, and Dahl (11) also developed multielectrode discharge tubes to distribute the potential drop between several gaps; this was one of the early uses of the multielectrode

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discharge tube. However, the oscillatory character of the potential obtained from the Tesla coil made it unsuitable for particle acceleration. The Carnegie group abandoned it around 1932 in favor of the belt-charged electrostatic generator developed by Van de Graaff. However, they were able to utilize the multielectrode accelerating tubes in their work with the electrostatic generator.

E . Resonance Transformer Modifications of the Tesla coil have taken several forms, including ac and rf systems. In 1933, Sloan (IZ),a student of Lawrence’s at the University of California, developed an rf version called a “resonance transformer.” Two resonant coils were mounted within a larger copper-lined tank which was evacuated. The secondary was made of heavy (1-in.), internally cooled copper tubing of 10 to 12 turns, supported by one end from the top of the tank, with which it formed a highly efficient resonant circuit at about 6 MHz frequency. The primary was a single turn of water-cooled tubing at the top of the tank, with ends emerging through insulators. The circuit was excited by a vacuum tube oscillator circuit using a laboratory-built water-cooled power tube, which was tuned to the resonance frequency. Such a resonance transformer was installed at the University of California Hospital in San Francisco in 1933 and used as an X-ray generator at voltages up to 1.25 MV. This installation gave many years of service for deep X-ray therapy. A similar machine was later installed at the Presbyterian Hospital in New York City. Also, a I-MV resonance transformer of this type was installed in Lawrence’s laboratory in Berkeley and was equipped with a tubular electrode hung from the lower, high-potential end of the resonant coil, through which electrons could be accelerated. A cathode was mounted on one side of the enclosing tank so electrons were accelerated once on entering the tubular electrode and again after emerging. The electron beam was brought out into air through a thin window where it formed a dense blue ionization glow extending several feet into the air. This unpublished work was done by Sloan, Livingood, and Kinsey. Another modification of the induction coil was the low-frequency (180 Hz) resonance transformer developed by Charlton, Westendorp, Dempster, and Hotaling (13) before 1934 at the General Electric Company. In this version the secondary coil was made of a stack of compact coils wound with many turns of fine wire, with an internal laminated iron core, which resonated at 180 Hz; the coil and core were mounted inside a pressure housing for insulation. A primary coil at the low-voltage end, made to resonate at the same frequency, was supplied from a 60-Hz power

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supply through a frequency-tripling circuit. An evacuated accelerating tube installed along the axis of the coil was used for acceleration of electrons, which struck a target at the base of the housing for the production of X rays. This system was produced and sold as an X-ray generator operating at 1 MV. F. Cascade Transformer

In the early 1920s the Southern California Edison Company established a laboratory at the California Institute of Technology in Pasadena to build a test system for high-voltage power equipment needed for long-distance transmission at 50 Hz. In this laboratory, Sorensen built a system using three 250-kV transformers in cascade which would produce 1 MV peak from line to ground. The transformers were mounted on insuIkted platforms. Each transformer had a low-voltage primary and a high-voltage secondary, and also a third “exciter” winding at the highpotential end of the secondary which was used to supply the next transformer. This system was used for several years for voltage breakdown tests on electrical components. The cascade transformer system was taken over in about 1930 by C. C. Lauritsen of Cal Tech to use as a voltage source for the acceleration of particles. First, X-ray tubes were developed operating at potentials up to 750 kV. Then a series of tubes were built for positive ion acceleration. By 1934, Crane, Lauritsen, and Soltan (7) reported the first results of a program of nuclear research, using protons of up to 1-MeV energy. Eventually, the superior qualities of the belt-charged electrostatic generator were recognized, and the transformer system was replaced by an electrostatic generator. This Cal Tech nuclear research laboratory has trained a notable succession of research students, and for many years was a major source of nuclear physics. The generators described above have yielded either pulsed or alternating potentials. This was a basic fault for application to ion accelerators. The techniques that have been successful are those which develop steady direct voltages and which are controllable to good precision. The earliest systems that did produce steady direct voltages are described in the following section. 111. DIRECTVOLTAGE ACCELERATORS A . Transformer -Rectifier Systems The typical X-ray system uses a high-voltage transformer to produce alternating voltage for an evacuated X-ray tube, which also acts as a rectifier in the circuit. A typical hospital installation uses an X-ray tube with

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Rectifier

FIG. 2. Transformer-rectifier circuit for the production of high dc potential. (a) X-ray tube acts as a self-rectifier. (b) A rectifier is used for positive ion acceleration. (From M. S . Livingston and J. P. Blewett, “Particle Accelerators,” 1962. Reprinted by permission of McGraw-Hill Book Company, New York.)

thermionic cathode and radiation-cooled target, and operates at about 250

kV. To accelerate positive ions it is necessary to have a rectifier in the cir-

cuit, otherwise the tube could support a large electron current during the negative half-cycle. It is also necessary to maintain the terminal at a negative potential. A filter circuit is added to provide a steady dc potential, and a positive ion source is located within the high-voltage terminal (Fig. 2). Descriptions of several low-voltage ion accelerators using this simple transformer-rectifier potential supply have been published (14, 15). With sufficientlyhigh intensity ion beams, nuclear disintegrations can be observed at quite low potentials; for example, the disintegration of Li nuclei by deuterium ions has been reported at potentials as low as 60 kV. B . Voltage Multiplier The voltage multiplier is a circuit for charging capacitors in parallel and discharging them in series. The circuit was invented by Greinacher (16) in 1921, and its early applications were to electrical engineering

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problems such as circuits for the testing of high-voltage electrical equipment. It differs from the surge generator in that it operates on alternating current, using rectifiers to charge capacitors during one half-cycle and other rectifiers to transfer the charge during the other half-cycle, so a steady direct voltage results. The circuit can be adapted to add stages of multiplication as desired. With N capacitors and N rectifiers, one can obtain a voltage multiplication by a factor of N. Cockcroft and Walton in the Cavendish Laboratory were searching for a device of modest size and energy which might still be able to disintegrate nuclei. Their first accelerator circuit (17) used a simple half-wave rectifier and a single-section tube to accelerate protons to 300 kV. They found this potential was about the limiting value for a single-section accelerating tube. Next, they chose to develop the voltage multiplier circuit (18) invented by Greinacher and to extend its capabilities to produce a planned maximum of 700 kV. They also spent several years in the development of single-section and later, multisection accelerating tubes. Their arrangement used vertical stacks of four capacitors and four rectifiers in the multiplier to obtain a fourfold voltage multiplication, and a twosection discharge tube also mounted vertically for acceleration of ions from a hydrogen gas ion source at the top. Each stack was surmounted by a rounded corona shield to limit sparking. Cockcroft and Walton were in the midst of their development when they became aware of the theoretical predictions of Gamow (19) [and also Condon and Gurney (20)] using the new tools of the wave mechanics, which showed that protons of quite low energy would have a significant probability of penetrating the nuclear potential barriers and so of disintegrating light nuclei. This more modest goal stimulated them to try an experiment even though their equipment would only produce 500-kVprotons at the stage of development. They used the lightest practical target (metallic lithium) and a scintillation counting technique for observation similar to that used by Rutherford in his early alpha-particle experiments. The scintillations they observed were from the alpha-particle fragments of the reaction: Li' + H1+ He4 + He4. Their results were published in early 1932; this was the first disintegration of a nucleus by particles accelerated in the laboratory. A series of papers (3)give full details of the technical development, voltage calibrations, and experimental observations. These papers are classics in nuclear physics and have brought enduring fame to their authors, who were awarded the Nobel Prize in Physics for 1951 (see Figs. 3 and 4). In the following year (1933) Oliphant, Harteck, and Rutherford (21), set up a single-stage transformer-rectifier system of lower voltage but considerably higher beam intensity, and observed disintegrations from several light elements at potentials between 100 and 250 kV. Also in 1934,

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this group reported on their first use of heavy hydrogen ions (deuterons) in a series of nuclear disintegration experiments. A few years later a 1.25-MV voltage multiplier was installed in the Cavendish Laboratory, which was engineered and constructed by the Philips Company of Eindhoven. The voltage multiplier system has been widely copied and modified in other laboratories. The engineering theory of the circuit has been published by Bouwers and Kuntke (22) in Germany, and has been extended and experimentally checked by Arnold (23, Peck (249, and Lorrain (25). The theory shows that high frequency is an advantage, to minimize ripple voltage and reduce the size of capacitors. For example, Lorrain describes a 500-kV generator using 24 stages and operating at 32 kHz. The increasing availability of solid state rectifiers and other components in more recent years has allowed the development of highly efficient systems at very high frequencies which operate with a minimum of maintenance problems. However, the energies available from the voltage multiplier have been limited to 1-1.5 MV, due to breakdown of insulation or of the surrounding medium. In recent years most of the commercial production has been by two firms: Philips, of Eindhoven, and Haefely, Inc., of

FIG. 3. Stacks of rectifiers, capacitors, and accelerating tube sections of the Cockcroft- Walton voltage multiplier. (Courtesy of the Cavendish Laboratory.) [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

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FIG.4. Cockcroft at the base of the accelerating tube for protons, used for the disintegration of lithium nuclei. (Courtesy of the Cavendish Laboratory.) (From M. S. Livingston and J. P. Blewett, “Particle Accelerators,” 1962. Reprinted by permission of McGrawHill Book Company.)

Basel. Their well-engineered and reliable systems are used primarily as preaccelerators for higher energy accelerators. For example, the 800-MeV Linac at the Los Alamos Meson Physics Facility (LAMPF) uses three 1-MV units built by Haefely, one 5 r protons (H+),one for negative ions (H-), and one for polarized ions (H-).

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C . The Belt-Charged Electrostatic Generator

As a Rhodes scholar in Oxford in 1928 R. J. Van de Graaff became interested in the need for high-voltage machines in the developing new field of nuclear physics. Following his return to Princeton he conceived and built the first electrostatic generator of the type now associated with his name. He described this table model of an electrostatic generator before the American Physical Society (6) in 1931. It was simple and inexpensive. Two 2-ft diameter spherical aluminum electrodes were supported on 7-ft glass rods, each with a motor-driven silk belt to transport charge to the terminal. Charge was sprayed onto the belt by corona points at the grounded end of the belt and removed within the terminal by another set of points. One unit was charged positively and the other negatively; when they were moved toward each other long sparks were drawn between them. The potential difference was estimated to be 1.5 MV, and was limited by corona from the terminals. The simple construction and the steady direct voltage made the device attractive as a voltage source for particle acceleration. Groups in several other laboratories in the United States became interested and joining the development. One of the first to recognize the advantages of the belt-charged generator was M. A. Tuve at the Department of Terrestrial Magnetism (D.T.M.) of the Carnegie Institution. Tuve, Hafstad, and Dahl(5, 26) built the first machines specifically intended for particle acceleration, with the continuing advice and support of Van de Graaff. First a spherical shell of 2-m diameter mounted on insulating Textolite legs was equipped with a charging belt and used to test the technique and measure a voltage to ground of 2 MV. Next, a I-m diameter spherical shell was built and equipped with a sectionalized accelerating tube and an ion source. This model operated at 600 kV, and with it the D.T.M. group was able to accelerate protons and observe their first nuclear disintegrations in 1933. Starting in late 1933 the 2-m machine was reconstructed with a 1-m terminal mounted inside as a potential divider, and equipped with a multisection accelerating tube (Fig. 5 ) . This machine was highly successful and contained many features still used in electrostatic generator systems. An important technical development was the study of voltage calibration for air-insulated generators. Sphere gap calibrations were unreliable in the megavolt range; generating voltmeters which measure field intensity at the terminal surface were also found to be unreliable due to corona losses. A high-energy proton beam brought out through a thin window could have its range measured, but range -energy relations were quite uncertain in the early 1930s. The first satisfactory calibration was obtained by magnetic deflection of the proton beam, using accurately placed slits. The

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FIG.5 . View of the 2-m electrostatic generator at the Carnegie Institution. Dahl is on the ladder, Hafstad kneeling, and Tuve standing on right. [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

most accurate calibrations were based on use of a column of precision resistors, totaling 10,OOO megohms (MR), which paralleled the discharge tube. Current measurements through the resistor column were used to calibrate the terminal voltage at which certain nuclear resonances were observed such as C ( p , y ) at 400 and 480 KV, three F(p,y) levels, and others (27). Such nuclear resonances have since been determined with

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such precision that they are used as standards for calibration of instruments and particle energies in different laboratories. Meanwhile, parallel developments started in other laboratories, based on the use of high gas pressure to insulate the terminal and increase the potential. The first to experiment with this technique were Barton, Mueller, and Van Atta (28) at Princeton. But the major development was by Herb and his associates at the University of Wisconsin, in the design of a series of pressure-insulated generators. The first model (29) reached 0.75 MV; the next (30) operated at 2.4 MV and was the first to be equipped with an electronic voltage stabilizer system; the third model, reported by Herb, Turner, Hudson, and Warren (31) in 1940, used three concentric electrodes for potential division and reached a potential of 4.0 MV (Fig. 6). The Herb design, using a horizontal arrangement with pressure tank for insulation and several concentric terminal shields, found many supporters. However, the practical limit was reached at about 4 MV, primarily owing to difficulties of supporting the terminal and the horizontal discharge tube as size increased. For a time, other laboratories exploited the vertical mounting with its apparent advantage in mechanical stability. Large pressurized generators were built at the Westinghouse Research Laboratory (32), at the Carnegie Institution, and at the University of Minnesota (33). These operated at relatively low gas pressure (60 to 120 psi). All were restricted to less than their theoretical voltage limits, operating at about 3 MV. Van de Graaff went to the Massachusetts Institute of Technology in Cambridge in 1932. There, with the support of President Karl T. Compton, he started on the design of a really large generator. It was located in an airship hangar at the Round Hill estate of Col. E. H. R. Greene near South Dartmouth, Massachusetts. The machine had two 15-ft diameter spherical aluminum terminals, each supported on a 6-ft diameter Textolite cylinder 24-ft long, and each mounted on a movable platform rolling on rails. Within each cylinder was a 4-ft wide belt for charging. The plan was to mount a discharge tube horizontally between the two terminals, through which a beam of ions could be accelerated, with an ion source in one terminal and a target and nuclear laboratory for observation in the other. The concept was exciting and the scale tremendous for that time. It was essentially completed by 1936 and described by L. C. Van Atta, C. M. Van Atta, Northrup, and Van de Graaff (34). The machine developed 2.4 MV on the positive terminal and 2.7 MV on the negative one, a possible total of 5.1 MV. The difficulties of mounting an evacuated discharge tube between terminals were extreme, and the machine never performed satisfactorily as an accelerator. Furthermore, the high humidity near the ocean

FIG.6. Sectional drawing of the 4-MV pressurized electrostatic generator at the University of Wisconsin. (Courtesy of Dr. R. G . Herb.)

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and unclean conditions in the hangar (bird droppings on the terminals) made it clear that this arrangement and location were unsuitable. The Round Hills generator was removed to M.I.T. in Cambridge in 1937, where it was installed within a tight, metal-domed building in which dust and humidity could be controlled. The two columns were mounted adjacent to each other with the terminals in contact; one held the charging belt and the other a vertical discharge tube with an ion source in the terminal. The beam was brought down through the floor to a basement laboratory where experiments could be performed. It was completed as an accelerator in 1940, and was used to accelerate electrons and positive ions to 2.75 MeV energy (Fig. 7) (35).After years of service for research this original “Van de Graaff” became obsolete as a scientific instrument and was moved to the Boston Museum of Science, where it is now located as a permanent exhibit and operated on occasion to produce sparks. The sparks from the terminal to the housing or down the column in this giant voltage source are awe-inspiring. To many people this installation typifies the atom amasher of the nuclear physicist. At M.I.T. a new series of developments was started in about 1938 in which J. G. Trump of the Electrical Engineering Department was prominent. Trump and a growing staff made intensive studies of the problems of high-voltage insulation and other limitations. In 1939 Trump and Van de Graaff reported on a series of electron accelerators intended as sources of X rays for medical and industrial purposes. Studies were made of many problems, including flashover potentials of dielectrics inlvacuum , andlin compressed gases, the influence of electrode material on breakdown potential, the relative dielectric strengths of various gases such as Freon, CCl,, and SFs, and ionization as a function of depth in tissue-like material for X rays and electrons of different voltages. Years of intensive study made this group the leader in the design of electrostatic generator systems. A large part of our technical knowledge of high-voltage engineering has come from this comprehensive program. This knowledge was put to use in the design of several large, vertical, pressurized proton accelerators, at M.I.T. and elsewhere. A culmination at M.I.T. was the Rockefeller generator completed in 1950, operating at voltages up to 12 MV. In 1947 Trump, Van de Graaff, Denis Robinson, and others formed the High Voltage Engineering Corporation (HVEC) in Cambridge, Massachusetts, for the commercial production of Van de Graaff generators. This was the first commercial firm engaged solely in the business of building electrostatic accelerators, and it holds a unique position in the field. “Hi-volts,” as it is called, produces several models of pressure-insulated generators with sealed-off electron accelerating tubes, for the production of X rays. It also makes several classes of vertical-mount pressurized ma-

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FIG.7. Original 15-ft spheres of Van de Graaffgenerator assembled at Massachusetts lnstitute of Technology as an accelerator. Between columns are C. M. Van Atta, D. L. Northrup, and L. C. Van Atta. (M.I.T. photo.) (From M. S. Livingston and J. P. Blewett, “Particle Accelerators,” 1962. Reprinted by permission of McGraw-Hill Book Company.)

chines with pumped discharge tubes for ion acceleration, to be used in research laboratories.

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Another commercial firm that produces electrostatic generators is the National Electrostatics Corporation, of Middleton, Wisconsin. This firm utilizes the experience of Herb and his associates at the University of Wisconsin. They make and market generators using the “pelletron” principle for charging, in which the charging belt consists of a chain of metal pellets connected by insulated links. The most recent development is a horizontal, double-ended generator called a “tandem,”* in which negative hydrogen ions produced in a source at ground potential are accelerated “up” to the positively charged terminal in the center (or top) of the machine. Here, they traverse a foil or a gas jet in which they are stripped of their electrons to become protons. They are then repelled by the positively charged terminal and accelerated back to ground potential through another discharge tube, to emerge in the laboratory with an energy corresponding to twice the potential of the high-voltage terminal. The first of these units was built by HVEC and installed at the Chalk River Laboratory in Canada (36). It has produced beams of H+ or D+ ions with energies up to 10 MeV, and units have been delivered to laboratories around the world for use in nuclear physics research. In more recent years the “tandem” generator has been extended to over 20 MeV.

IV. RESONANCE ACCELERATION A . Origin of the Resonance Principle The first proposal for accomplishing the resonance acceleration of particles through a linear array of accelerating electrodes was made by G. Ising (37) of Sweden in 1925 (Fig. 8). He suggested the use of highfrequency electric fields generated by a spark-gap oscillator, and transmission lines to supply the rf fields to the accelerating electrodes. There is no record of any experimental test of this proposal. The first experimental test of the principle of resonance acceleration was made by Rolf Wideroe (38) in 1928 (Fig. 9). His apparatus was the direct ancestor of all resonance accelerators, both linear and circular. It consisted of a set of three coaxial cylinders in which the central one was driven to oscillate in potential by an rf source. Particles which traversed the first gap when the phase of the electrode was accelerating gained an increment in energy; those which were in resonance and which also crossed the second gap at an accelerating phase gained another increment

* Credit for the first suggestion of this use of an electrostatic generator goes to L. W. AIvarez.

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in energy and emerged with the energy equivalent of twice the applied rf voltage. It was an elementary two-stage linear accelerator. The central electrode acted as a “drift tube” to shield the ions while the rfvoltage was of the opposite polarity. Wideroe did not have an rf source of high enough frequency to resonate with the lightest ions, but used Na+ and K+ ions from a “Kuntzman” source. He chose the length of the central electrode, the applied frequency and the applied voltage to resonate with the chosen

I

I

Erdc

FIG.8. Diagram of linear accelerator from G . Ising’s pioneer publication (1924) (37) of the principle of multiple acceleration of ions.

FIG.9. Diagram of prototype resonance accelerator of Wideroe in 1928. [From Livingston (39).Reprinted by permission of Harvard Univ. Press.]

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ions, and measured their doubled energy by deflection of ihe accelerated particles. From this start came a sequence of accelerator types using magnetic fields to produce circular orbits, all of which used the basic concept of resonance acceleration of the circulating ions by an applied rf electric field. B. Row Wideriie and the Resonance Principle

Rolf Wideroe might be called the first designer of accelerators, and he was the originator of one of the most important concepts in the field, that of resonance acceleration. In order to do justice to this innovative engineer, his contributions deserve more attention than mere references can provide. To this end, a summary of his career is given below. Rolf Wideroe was born in Norway in 1902 and took his early education in Oslo. As a student 16 years of age he first read in the newspapers about the disintegration of nitrogen atoms with alpha particles by Ernest Rutherford in England. This story made a great impression on him. He realized that the dream of the alchemists had come true, that atomic nuclei could be disrupted, and that mankind was on the first step in the study of the nature of atomic nuclei. He also realized that electrical machinery would be needed to attain the higher voltages and higher energies required for successful study of atomic nuclei, and decided to study electrical engineering. In the next years he finished his studies in Oslo and went to Karlsruhe in Germany to study engineering. There, as early as 1924, he conceived of the use of magnetic induction to accelerate electrons-the principle of the betatron. His idea was to use the electric “vortex” field surrounding a region of changing magnetic field to accelerate electrons inside a doughnut-shaped evacuated tube. In 1925 Wideroe went to the Technical University at Aachen to study for a doctorate in engineering. As a thesis (1927) he made an experimental trial of the betatron principle, but was not successful. (Many others had recognized this principle of acceleration by magnetic induction and had also made contributions to the development. Ultimately, it took a detailed theoretical analysis of orbit theory and focusing by Serber to provide Kerst with the information with which to build the first working betatron, in 1940.) During his years at Aachen Wideroe conceived of still another way of accelerating particles without the use of direct high voltage. This was a method of resonating the particles with a radiofrequency electric field in order to add additional energy on each traversal of the field. In this case his experiments on a prototype of a linear resonance accelerator were successful. He published this result, along with his work on magnetic induction, in a now-famous article in the Arch. fur Ekkfrotechnik (B), in 1928. The use made of this article by E. 0. Lawrence in conceiving the resonance principle of the cyclotron is cited by all historians in the field. The resonance principle is basic to all modern linear accelerators and to most of the several types of circular magnetic accelerators. Wideroe has been continuously active in electrical engineering, first on electrical power systems and later on physical instrumentation. He has also studied and published many papers in the fields of radiotherapy. From 1946 to the present time he worked at the Brown-Boveri plant in Baden, Switzerland, primarily on physical research in radiotherapy. He has published 180-odd papers in scientific and engineering journals and has

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made application for more than 200 patents. Wideroe has also continued his major interest in accelerators. In 1943 he conceived of the use of colliding beams of particles meeting head-on as a method of increasing the interaction energy, and filed for a German patent on the concept. In 1946 he took out a Norwegian patent on an accelerator having the principle of synchronous acceleration -an independent suggestion of the synchrotron. He has continued his active interest in the accelerator field. He travels widely and attends most of the conferences on accelerators, and has met most of the major accelerator designers. He is now retired (1979) but still attends conferences and visits the latest installations.*

C . Ernest Lawrence and the Cyclotron

This section must necessarily be more personal than other portions of this history of accelerators, since the writer was a student and close associate of Professor Lawrence during the first four years of the development of the cyclotron and, in fact, constructed the first three sizes of the cyclotron. A more detailed description is given in a monograph by the writer (39) published by the Harvard University Press. Ernest 0. Lawrence had been a young associate professor at the University of California for several years when he conceived the idea of magnetic resonance acceleration which became the cyclotron. In discussions with Lawrence in later years I learned that he conceived the idea in the early summer of 1929 while browsing through the current journals in the library at the University. In the Archivfiir Elektrotechnik for 1928, he saw the illustrations in a paper by Rolf Wideroe, and recognized the resonance principle involved, although he could not read German readily. Wideroe’s paper described an experiment in which positive ions of Na and K were accelerated to twice the applied voltage while traversing two gaps at the ends of a tubular electrode to which an rf voltage was applied. This was an elementary, two-stage, resonance linear accelerator-the first of its type. In the paper the author describes his method of confirming the doubled voltage by electrostatic deflection measurements on the ions. Lawrence was aware of the importance of finding a method of accelerating particles to study “nuclear excitations” and realized the limitations of the techniques involving direct voltages. He recognized that extension of Wideroe’s resonance principle to really high energies would require a very long array of electrodes. So he speculated on variations of the resonance principle, including the use of a magnetic field to deflect particles in circular paths so they would return to the electrode and reuse the electric field in the gap. He derived the equations of motion of particles in such a combination of magnetic and electric fields, and found that the particle would have a constant frequency of rotation independent of its energy or

* Personal communication from R. Wideroe to M. S. Livingston, August, 1978.

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the size of the orbit. By applying the correct frequency to a pair of suitably shaped electrodes mounted perpendicular to a uniform magnetic field, ions could be made to resonate with the rf field, and cross and recross the gap between electrodes many times, each time gaining more energy and traveling in a larger orbit. On trips to the East Coast that summer, Lawrence discussed his idea with several friends and became convinced that it was sound. In the spring of 1930 he asked a graduate student, N. E. Edlefsen, who had completed his thesis and was awaiting the June degree date, to make a quick experimental study of the resonance principle. Although Edlefsen did not observe true resonance, Lawrence considered the results promising. He described the concept at a meeting of the American Association for the Advancement of Science in Berkeley that spring and submitted a brief article (40) (with Edlefsen) to the journal Science. This was the first published description of the resonance principle. I was a graduate student at Berkeley at that time, and in the early summer of 1930 I asked Professor Lawrence to propose a topic for an experimental thesis. He suggested that I demonstrate the validity of this resonance principle (now known as cyclotron resonance). I started experimental work that summer. First, I reassembled and recalibrated the 4-in. magnet used by Edlefsen, built a similar glass vacuum chamber with internal electrodes, and observed similar effects. The current to an unshielded electrode at the periphery exhibited a broad resonance as magnetic field was increased. However, I soon found that this was not due to hydrogen ions but to residual air ions. Then I built a vacuum chamber out of a short section of brass ring having brass end plates sealed with wax (Fig. 10). For an accelerating electrode I used a hollow, half-pillbox of copper mounted on an insulated stem, with the opening facing a slotted bar placed across the diameter of the chamber. Due to its shape this electrode was called a “D,” a term still used by cyclotronists. An rfpotential was applied to this electrode from a Hartley oscillator circuit using a 10-W vacuum tube as an rf power source. Hydrogen gas was admitted to the chamber and was ionized by electrons from a thermionic cathode mounted near the center. The important difference was that the collector at the edge of the chamber was mounted inside a shielded box so only those particles could enter which traversed a set of slits and a transverse deflecting electric field. I first observed sharp resonance peaks in the collector current when the magnet was tuned through a narrow range, in November of 1930. The deflecting field and slit system in front of the collector gave a rough check of ion energy. But the basic proof was that the magnetic field at resonance was just that calculated from the resonance equation using the measured value of the applied radio frequency and the e l m value of hydrogen

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FIG. 10. Brass vacuum chamber used by M . S. Livingston to demonstrate cyclotron resonance. (Reported in a Doctorate Thesis at the University of California, April 14, 1931.) [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

molecular ions. The small laboratory magnet used for the first studies was limited in field to 5200 G. A stronger magnet was borrowed for a time which would produce 13,000 G, and the oscillator circuit was tuned to the expected resonance value for molecular ions H$.Resonances were ob-

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served for a wide range of frequencies and magnetic fields, to a maximum value of 30 m wavelength or 10 MHz frequency, at 13,000 G. The calculated energy of the H2+ions was 80 keV (thousand electron volts) for the radius of orbit built into the chamber. This was obtained with an applied rf potential of about 1 kV, so the ions traversed a minimum of 40 turns (80 accelerations). I used these results in my doctorate thesis, completing my Ph.D. in May 1931. And the results were reported by Lawrence and Livingston at the American Physical Society Meetings and were published in the Physical Review (41). In early 1931, Lawrence applied for and was awarded a grant from the National Research Council for $1000, for construction of a machine that could produce 1-MeV protons. I was given an instructorship in the Physics Department for the following year, so I could continue with the development of a larger “magnetic resonance accelerator,” as we called it at that time. During the late spring and summer of 1931 Professor Lawrence and I designed a magnet of 10-in. pole diameter and a vacuum chamber which would fit between poles, and placed orders for the components. During the summer and fall I assembled the magnet in Room 239 of LeConte Hall, the Berkeley Physics Building, and built the other elements such as the rf oscillator. As before, the vacuum chamber was a flat brass box (square this time) and the cover plate was sealed with wax. A single D-shaped electrode was mounted on a Pyrex insulator, facing a slotted “dummy-D,” for the rf electrode. The oscillator used a Federal Telegraph water-cooled power tube in a circuit which produced peak potentials of up to 50 kV across the accelerating gap, and at frequencies up to 20 MHz. I was greatly aided in the development of this first high-power rf oscillator by David Sloan, another graduate student who had been a ham radio operator and was an ingenious student of high-frequency radio techniques. This first practical cyclotron (42) produced HZ+ions of 0.5 MeV energy : ions (protons) of 1.22 MeV, with beam curby December 1931 and H A in January 1932 (Fig. 11). This was the first time in rents of about scientific history that artificially accelerated ions of over 1 million volts had been produced. The original vacuum chamber of this 1.2-MeV cyclotron is now on permanent exhibit in the Kensington Museum of Science in London. We had barely confirmed our results, and I was working on revisions to increase beam intensity when we received the issue of the Proceedings ofthe Royal Society describing the results of Cockcroft and Walton at the Cavendish on the disintegration of lithium with protons of only 500 keV energy. We did not have any instruments for making such observations at that time. So Lawrence sent a call to his friend and former colleague at Yale, Donald Cooksey, who came to Berkeley for the summer with a stu-

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FIG. 1 1 . First practical cyclotron built by Lawrence and Livingston in 1931-1932, which produced 1.2-MeV protons and was used for first disintegration experiments in the United States. [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

dent, Franz Kurie. They helped us develop the necessary instruments and counters for observing disintegrations. With the help of Milton White, a Berkeley graduate student, we installed a target mount in the chamber, mounted a thin mica window on the outer wall opposite the target, and placed our counters outside the window. Within three months after hearing the news from Cambridge we were able to observe and measure disintegrations from Li and other light elements. These results (43) were published that fall. Lawrence was planning his next step even before I had completed the 10-in. machine as a working accelerator. This was in the midst of the “great depression” and funds were hard to obtain. He was forced to use many economies and substitutes to reach his goal. In late 1931 he located a magnet core from an obsolete Poulsen arc magnet with a 45-in. core at the Federal Telegraph Company plant in Palo Alto, which we used for the next size machine. The two cores were machined down to 273411. diameter pole faces. Magnet windings were formed of copper strip wound in flat layers and immersed in oil tanks for cooling. (The oil tanks leaked. We all

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wore paper hats when working between coils to keep the oil out of our hair.) This magnet was installed in December 1931 in a frame warehouse near the Physics building later known as the “Old Radiation Laboratory,” which was the center of cyclotron activities for many years. Early in 1932 I turned the 10-in. machine over to White to use for his thesis problem, and applied most of my time to construction of the larger machiqe. The vacuum chamber was a 28-in. brass ring fitted with iron plates for top and bottom lids, with the top lid sealed with soft wax as before (Fig. 12). Initidly a single D-shaped rfelectrode was used, facing a slotted bar as a “dummy D.” This arrangement allowed us to locate the deflection electrode and collector at any chosen radius. The accelerated, resonant beam was first observed at small radius, and shimming and other adjustments were made to maximize intensity. Then the collector was moved to a larger radius, and the tuning and shimming repeated. So we learned by trial and error of the necessity for a radially decreasing field to maintain focusing. Eventually we reached a practical maximum radius of 10 in. and

FIG. 12. Livingston and Lawrence beside 27t-in. cyclotron at University of California, 1933. [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

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installed two symmetrical D’s with which higher energies and intensities could be obtained. Progress during this period of development, from 1-MeV protons to 5-MeV deuterons, was reported in several publications (44, 45) from 1932 to 1934. Other young scientists joined Lawrence’s “Radiation Laboratory,” and more graduate students came from the Physics Department. We joined in teams for taking data and publishing results. David Sloan built a sequence of linear accelerators following Wideroe’s resonance concept. Wesley Coates and Bernard Kinsey worked on a resonance transformer which produced electrons and X rays of 1 MeV energy. And Livingood made quartz-fiber electroscopes with which to observe the new induced radioactivities. Malcolm Henderson came in 1933 and developed counting equipment and magnet control circuits. Incidentally, Henderson invented the name “cyclotron,” first used as laboratory slang, then picked up by news reporters and publicized. Edwin McMillan joined the group in 1934 and made many contributions to the planning and interpretation of research experiments. R. L. Thornton also came in 1934, but left for a time to design and build cyclotrons elsewhere. Our self-appointed laboratory assistant was Commander Telesio Lucci, retired from the Italian navy, who was a friend to all. We were the first laboratory to use deuterons in an accelerator. Professor G. N. Lewis of the Chemistry Department had succeeded in concentrating heavy water with about 20% deuterium from battery acid residues; we electrolyzed it to obtain gas for our ion source. Soon after we had tuned in the first beam of deuterium ions we observed alpha particles from a Li target with much longer ranges and higher energies than any observed in natural radioactivities. We installed a wheel of targets on a greased joint, with targets of many light elements which could be turned into the beam opposite our detector system. This 274-in. cyclotron (46) was able to produce deuterons of 5.0 MeV in December 1933. Chadwick reported the discovery of the neutron in 1932, produced from Ra-Be natural radioactive sources. As soon as we had developed linear amplifiers and thin ionization chambers with which to observe single particles, we used a paraffin layer in front of the ionization chamber and were able to observe the recoil protons from neutrons. When deuterons became available for bombardment, we observed neutrons in large intensities from essentially every target used. The first observation of neutrons was in September 1933. We had other exiting moments: Early in 1934 (February 24), Lawrence brought a copy of the Comptes Rendus into the Laboratory which described the discovery of induced radioactivity by Curie and Joliot in Paris, using natural alpha particles on boron and other light elements. They predicted that the same activities could be produced

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by deuterons on other targets, such as carbon. We had a deuteron beam in use, a carbon target in the chamber, and a Geiger counter and counting circuits in service at that time. We quickly arranged the cyclotron to bombard for 5 min and then turned off the beam and studied the delayed emissions from the target. Within a half-hour after hearing of the Curie-Joliot results we were observing induced radioactivity. These results were reported by Henderson, Livingston, and Lawrence (47) in March 1934. I left the Laboratory in July 1934 to go to Cornell, and later M.I.T., as the first missionary from the Lawrence cyclotron group. Don Cooksey returned to stay permanently at Berkeley and joined in Lawrence’s next stage of development, which was to expand the pole faces to 37-in. diameter and to build a larger chamber (48) which soon produced 8-MeV deuterons. Other young scientists joined the group and the first professionally trained engineers arrived, notably W. M. Brobeck and W. W. Salisbury. Dr. John Lawrence, Ernest’s brother, arrived in 1935 to start the first biological experiments. Lawrence obtained support for the 60-in. “Crocker” cyclotron (49), to be used primarily for cancer therapy using neutrons. This machine was a beautifully engineered and reliable instrument, and became the prototype of scores of cyclotrons around the world (Fig. 13). The 60-in. machine was completed in 1939 and soon attained its design goal of 20-MeV deuterons or 40-MeV He2+ions. The year 1939 is also notable as the year in which Ernest Lawrence received the Nobel Prize. Meanwhile, cyclotrons were constructed in many other laboratories, at first largely designed by graduates of the Berkeley school. Soon these laboratories were able to make important contributions to the development. Among those contributing to progress in the early years were: Michigan, Cornell, Columbia, Princeton, Rochester, Washington University at St. Louis, Yale, Purdue, Carnegie Institution of Washington, Harvard, and Massachusetts Institute of Technology. The modern cyclotron is a composite product of many laboratories and scores of individual contributions. A large number of the technical developments have not been published, but have passed from one laboratory to another by visits and design sketches. For a time, several commercial firms joined in the development of the cyclotron. The Collins Radio Company in Cedar Rapids, Iowa, designed and constructed two 60-in. machines, one for the Brookhaven National Laboratory and one for the Argonne National Laboratory, under the engineering direction of W. W. Salisbury. The General Electric Company built a machine of the same size for the National Committee of Aeronautics Laboratory in Cleveland, Ohio. The Philips Laboratory at Eindhoven, and Brown-Boveri in Zurich have also built several cyclotrons for science laboratories in Europe. The cyclotron was an immediate success and was widely copied.

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FIG. 13. The 60-in. Berkeley cyclotron built for medical applications in 1938. Photograph shows L. W. Alvarez on magnet-coil tank, E. E. McMillan on the "D" stem casing, and standing (left to right) D. Cooksey, D. R. Corson, Lawrence, R. L. Thornton, J. Backus, and W . W. Salisbury. (Lawrence Radiation Laboratory photo; reprinted by permission of Lawrence Berkeley Laboratory, Univ. of California.)

By 1945 there were at least 15 installations in the United States, mostly in universities, and 10 installations abroad. Engineering techniques have improved steadily throughout the development. Several excellent descriptive papers have been published (50), and a few review papers (51) describing the variety of developments. A detailed study of cyclotron orbit theory was prepared by Cohen (52). At the peak of its use, there were over 100 installations in service around the world. But the useful scientific life of the standard cyclotron was limited. In 1937 Bethe and Rose (53)published the first analysis of the energy limits due to the relativistic increase of mass of the ions with increasing energy, which causes the ions to go out of resonance with the fixed-frequency rf electric fields used for acceleration. This energy limit has proved to be about 25 MeV for protons and deuterons. Up to this energy cyclotrons have been very productive in the study of nuclear physics. They have also been used widely as sources of neutrons, both for scientific research and for medical therapy uses against cancer. The cyclotron has become a

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symbol of nuclear science. With the availability of higher energy machines, cyclotrons were no longer pushed to attain the maximum energies, but one-by-one were converted to special purposes. One development of special interest is the acceleration of multiply charged carbon, nitrogen, and other heavy ions to energies of well over 100 MeV, at Oak Ridge and at Berkeley. High intensity sources of heavy ions have made it possible to produce a wide range of radioactive products. Such heavy ions have been particularly important in the study of the transuranic elements. V. THE BETATRON In the betatron, electrons are held in a circular orbit by a transverse magnetic field and are accelerated by the electric field induced by the changing magnetic flux linking the orbit. The principle is identical with that of the transformer, with circulating electrons replacing the secondary coil in the transformer. The usual arrangement is a circular magnet with ring-shaped pole faces between which a doughnut-shaped vacuum chamber is located, and a central core which provides the flux for acceleration. Since the magnet is normally excited by alternating current, to provide the changing flux, the magnet is laminated as in a transformer. Acceleration occurs during the quarter-cycle while the flux is rising, and while the magnetic field at the orbit has the correct value to produce motion in a circle. As the electrons approach maximum energy, the deterioration of the field can be used to divert them against a target, producing X rays. The X rays occur in a sequence of short pulses at the frequency of the magnet power supply. The betatron is not a resonance accelerator and does not depend on an rf field for acceleration. The accelerating field is given by the time rate of change of the flux linking the orbit. One formulation for the induced voltage per turn is $ E ds = -d&'dt, where the integral of the electric field around one turn is given by the time rate of change of the total flux linking the orbit. A simple analysis shows that the flux change must be twice the value obtaining if flux density were uniform and equal to the field at the orbit. This is the famous 2: 1 rule. There must be a strong central field linking the orbit and a weaker field at the orbit. This is obtained by careful design in which the gap between pole faces in the central core is made shorter than the gap which provides the guide field for the orbit. There will be one orbit radius within the chamber at which the 2: 1 rule holds, which is the location of the equilibrium orbit. Many scientists have recognized the possibilities in this principle of acceleration by magnetic induction. It has the advantage of avoiding the

EARLY HISTORY OF PARTICLE ACCELERATORS

35

problems of insulation breakdown which plague the direct voltage accelerator, and the magnetic field retains the electrons in circular orbits so the accelerator is compact in size. Since electrons are light and reach high velocities even at low energies, they can make many revolutions and acquire a high energy in a short time. The first patent application for an accelerator using this principle was by J. Slepian in 1922; it was issued in 1927. However, there is no evidence that this patent was reduced to practice, and the experimental trials by others were unsuccessful for many years. In 1927, Breit and Tuve built an apparatus using a spark discharge of a large capacitor through a coil to produce the magnetic field; however, they did not have a properly shaped field to focus the electrons at the orbit location, and the experiment failed. Wideroe (38),in 1928, made an experimental study in which he did recognize the need for focusing, but he used an external electron gun for which the capture efficiency was very small; the electrons were observed to make only 1+ turns. Walton (54),in 1929, developed the theory of orbit stability, and Jassinsky (55), in 1936, worked out the injection theory. Still others applied for patents: Steenbeck in 1937 and Penney in 1941, with no practical progress. Others worked on the concept without publication or patents; reports suggest that studies were made by Abbott at the University of Washington, James Tuck at the Clarendon Laboratory, and F. Dunnington at the University of California. An excellent review of the historical development is given by Kerst (56) in a publication in 1946. The first operating magnetic induction accelerator was built by D. W. Kerst (57) at the University of Illinois, and he gave it the name “betatron.” He obtained the idea by studying the patent issued to Steenbeck. And he was assisted in the design of the magnetic field by a study of the orbit stability and focusing of the circulating electrons made by R. Serber (58L which was published jointly with Kerst shortly after. The first model was small and compact. It produced 2.3-MeV electrons and X rays with an intensity equivalent to the y rays from a gram of radium. Extension to higher energies was obvious, and it was clear that the betatron could become an important source of X rays for medical and industrial purposes. Kerst went next to the General Electric Company, where with the cooperation of this experienced staff a 20-MeV betatron (59) was completed in 1942 (Fig. 14). Commercial production of betatrons for research laboratories, hospitals, and industrial plants started promptly at General Electric, and also at Westinghouse and at Allis-Chalmers. Other developments came from the Philips Laboratory at Eindhoven, and at BrownBoveri in Switzerland, supervised by Wideroe. In 1945 Westendorp and Charlton (60) of the General Electric Company built a 100-MeV betatron.

36

M. STANLEY LIVINGSTON

FIG. 14 Diagram of betatron magnet pole tips and vacuum chamber, showing orbit location and the central core supplying flux linkage for acceleration. (From M. S . Livingston and J. P. Blewett, “Particle Accelerators,” 1962. Reprinted by permission of McGraw-Hill Book Company.)

Meanwhile, Kerst returned to the University of Illinois to build first an 80-MeV “model” (61) and ultimately a 300-MeV betatron (62) which was the largest and probably the last of this line. It seems clear that the complete stability theory of Kerst and Serber and the careful and thorough magnet design calculations of Kerst were the reason for Kerst’s success in the rapid development of the betatron. Orbital stability requires spatial focusing for particles which deviate in direction from the equilibrium orbit. Such stability will occur in a radially decreasing magnetic field, such as would be specified by a radial variation: B, = B0(ro/r)”,in which the exponent n lies Detween zero and one. The radial restoring force will be proportional to n. The frequencies of oscillation around the equilibrium orbit (radial and vertical) will both be lower than the orbital frequency, fo, and will be given by f, = (1 - n)l’zJ, and f, = nl’zfo.These are the well known “free” or “betatron” oscillation frequencies. To achieve this type of stability, the pole faces defining the fields around the vacuum chamber (and including the equilibrium orbit) are sloped to provide a radially increasing gap length and, hence, a radially decreasing field. It can be shown that the amplitude of these oscillations are damped with increasing energy, so the orbits tend to collapse onto the equilibrium orbit at maximum energy. Electrons are injected into a betatron by a hot-cathode gun, placed near the equilibrium orbit and aimed in a direction parallel to the orbit. The electrons oscillate about an instantaneous orbit whose location is de-

EARLY HISTORY OF PARTICLE ACCELERATORS

37

termined by the injection energy. The problem of injection is to avoid having the electrons strike the back of the injector gun on succeeding turns. Two factors affect this result: the damping of the free oscillations, and the motion of the instantaneous orbit toward the equilibrium orbit, both of which are affected by the increase of the magnetic field with time. These factors operate to best effect if the field index n is close to 0.75, and the earliest betatrons were designed with this value of n. Later results show that the value of n is not critical and can be as low as 0.50. Best results are usually obtained if the injector gun is placed radially outside the orbit; if the field shape is such that the field index increases with radius outside the orbit, an injector location near n 1 seems to help the process of injection. Most betatrons are intended to produce an X-ray beam, but for some purposes it is desirable to have an emergent electron beam. For production of X rays it is only necessary to have a suitable target located radially outside the orbit, and to allow the beam to move in its natural direction as the magnetic field deteriorates near maximum energy. For emergent electrons, a device called a “peeler” has been used; this consists of a short section of laminated iron shielding which provides a channel where the field is weaker than normal. This is placed outside the orbit and electrons are expanded into the channel rather than against a target. They can emerge through a thin metallic window set in the side of the vacuum chamber. The first betatrons were designed to use 60-cycle ac powering. It was soon clear that use of a frequency tripling circuit to produce 180-cycle excitation would triple the induced voltage per turn and triple the output pulse rate. Another important concept was that of field biasing. It was noted that separation of the magnet structure into two components, one producing the field at the orbit and another producing the central flux, would allow the components to be excited independently. An increase in energy by almost a factor of 2 can be achieved by having the flux in the central core reverse direction while orbit field rises from zero to maximum. This can be accomplished with either of the two circuits by applying a direct current bias to the winding. With this technique both central flux and orbit field can approach the limits set by saturation of the iron, and output energy can be nearly doubled for the same magnet weight. With such separation of functions the power required to produce a desired central flux can also be greatly reduced by eliminating the air gap in the central core. With no air gap the inductance of the exciting winding is increased and the size and power rating of the resonant capacitor bank is reduced. In practice, two methods have been used to provide the advantages described above. One is called “field biasing,” in which the dc current component is applied to

38

M. STANLEY LIVINGSTON

the field windings; the other is “flux biasing” in which the dc component forces the flux to change from a negative maximum to a positive maximum during a quarter-cycle. Both systems have been used. A charged particle moving in a circular orbit experiences a central accelerating force and so radiates energy. This radiation emerges in a forward cone around the instantaneous direction of the electron. Such radiation disturbs the betatron relation and ultimately sets an upper limit on the energy which could be obtained. The energy loss sets in at about 100 MeV and increases rapidly above this value, rising with the fourth power of the energy. Compensating corrections can be applied, which allowed the 300-MeV machine to operate to its full design energy. But designs for much higher energies have proved to be impractical. For several years the betatron was the favored instrument for production of X rays for medical and industrial purposes, and it still has a wide area of use. But the synchrotron can produce electrons of much higher energy and has displaced the betatron for most scientific uses. And eventually compact linacs which could be mounted in gimbals were found to be more flexible for medical therapy and for X-ray photography. So the betatron has now become obsolete in its primary applications, and other instruments are being substituted. It has had a short life but a good one!

VI. SYNCHRONOUS ACCELERATORS

A . Story of the Development of Phase Stability

Synchronous acceleration uses the basic principle of resonance acceleration of the particles with an rf field, as in the standard cyclotron, and the particles travel in circular paths under the influence of a transverse magnetic field. However, it differs from the cyclotron in that resonance is maintained through an indefinitely large number of turns, through selection of a band of orbits within certain phase angles of the radio frequency, which have a type of stability. For such orbits, any change from the so-called “equilibrium” phase, radial position, or energy of the orbit is automatically compensated and shifted back toward the properties of the equilibrium orbit. This is similar to the motion of the rotor of a synchronous motor-hence the name. For example, since the angular velocity of particles in the magnetic field is in general a function of energy, any deviation in energy from the equilibrium value results in a phase change, which means a variation in the energy per turn (or per traversal of an accelerating gap in the linear accelerator). Within the stable phase band, such a change in energy shifts

EARLY HISTORY OF PARTICLE ACCELERATORS

39

FIG. 15. Edwin E. McMillan and Valdimir I . Veksler, independent discoverers of the principle of phase-stable or “synchronous” acceleration. From M. S. Livingston and J. P. Blewett, “Particle Accelerators,” 1962. Reprinted by permission of McGraw-Hill Book Company.

the phase so that the energy moves toward equilibrium. Individual particles oscillate in phase about the equilibrium value, and so also oscillate in their rate of increase in energy and in their radial position. This results in a “breathing” type of oscillation for each particle about the equilibrium position, which is gradually damped as the particle energy increases. This concept was developed independently at the end of World War I1 by E. M. McMillan (63) of the University of California and by Vladimir Veksler in the U.S.S.R. (Fig. 15). In both countries the end of the war

40

M. STANLEY LIVINGSTON

brought a desire to restart the research programs in nuclear physics, which had been postponed for the duration, and both scientists had been aware of the relativistic limitations in energy of the standard cyclotron at high energies. Veksler (64) first conceived of an “electron cyclotron” (or “microtron”) in which both frequency and magnetic field are held constant and the electrons describe orbits that are tangent at one point but require successively 1, 2, 3, . . . periods of the driving frequency. An rfaccelerating cavity is located at the point where the orbits coincide, but elsewhere the electrons traverse orbits of different discrete radii. Such machines have subsequently been built, but the practical usage was minimal and they have not been an important accelerator type. Next, Veksler recognized that the skipping of cycles was equivalent to a stepwise variation in frequency and made the natural extension to use a smooth frequency variation. He also realized that with a constant frequency, the magnetic field could be vaned, the principle of another type of synchronous accelerator (65). Meanwhile, while McMillan (63) was still at Los Alamos on his wartime assignment, he conceived of this same technique of increasing particle energy, and promptly published his concept. On his return to Berkeley he started building an electron synchrotron for 300 MeV, an energy sufficient to produce pairs of the newly discovered particles called mesons, and later muons. The initial publications in the U.S.S.R. and in the United States referenced above alerted others to this simple concept for extending acceleration to higher energies than could be achieved with cyclotrons. Goward and Barnes (66) in England were the first to report an experimental test. They modified the circuits of an existing 4-MeV betatron to make a synchrotron which operated at 8-MeV electrons. Also, at the General Electric Company, Pollock (67) built a 70-MeV machine, using parts originally intended for a biased betatron, and observed acceleration of electrons and X rays in early 1946. Incidentally, he was the first to observe the “synchrotron light” projected in the forward direction by the accelerated electrons. Soon, Ivan Getting at M.I.T., and Robert R. Wilson at Cornell, began work on synchrotrons for 300 MeV energy. Even though the rapid utilization of McMillan’s and Veksler’s proposals was an immediate result in many accelerator installations, it is still of interest to search out other independent origins of the idea of phase stability. For example, a device similar to the proton synchrotron was proposed by M. L. Oliphant in 1943, at the University of Birmingham in England, even though it was unpublished due to wartime restrictions. In its original form the guide field was to be provided by air-core coils because

EARLY HISTORY OF PARTICLE ACCELERATORS

41

of the high rate of change of magnetic field required, and acceleration was to be a accomplished with great speed by a somewhat naive polyphase accelerating system. Nevertheless, when modified later to use a slower rate of acceleration and to take advantage of phase stability, it operated much as planned when installed at the University of Birmingham (68). Others claim early awareness of the principle. For example, the possibility of longitudinal stability in a linear accelerator was noted by V. K. Zworykin (69) in 1945; Leo Szilard included a description of phase stability in a patent issued in 1934; N. Christofilos made a patent disclosure in 1946; and R. Wideroe claims early knowledge of the principle. Four important types of accelerators (plus the microtron) use the phase stability principle: (1) the electron synchrotron, (2) the nonrelativistic linear accelerator, (3) the synchrocyclotron or frequency modulated cyclotron, and (4) the proton synchrotron. These will be discussed in more detail in the sections to follow.

B . Electron Synchrotron The synchrotron is the simplest of all phase-stable accelerators. And unlike other accelerators which required slow and tedious development from small sizes at low energies, the electron synchrotron was conceived in its full stature as a high-energy accelerator. The first installation designed by McMillan and built at Berkeley starting in 1945, was for 300 MeV, in order to produce mesons and start research on subnuclear particles. Even the basic limitation, that imposed by radiation loss by the orbiting electrons, was recognized and designs included rf power sufficient to compensate for the energy loss. In the synchrotron, particles are accelerated to high energies within a doughnut-shaped vacuum chamber placed between the poles of a ringshaped magnet. No central core is required for induction acceleration as in the betatron. Rather, a simple rf circuit in the form of a resonant cavity is built into the chamber, designed to provide an accelerating gap for the circulting electrons. The magnet is operated cyclically from low to high magnetic fields to match electron momentum as the electrons are accelerated to high energy. The magnet cores are laminated as in a transformer to limit magnetic losses during the cyclic excitation. A constant frequency rf voltage is applied across the accelerating gap. The electrons are trapped within a stable phase band and accelerated synchronously, automatically increasing in energy to follow the rate of rise of the magnetic field. At maximum energy, the electrons can be diverted against a target to form X rays, which emerge from the machine in a sequence of pulses at the repetition rate of the magnet cycle.

42

M. STANLEY LIVMGSTON

The first 300-MeV electron synchrotron (70) built by McMillan at Berkeley can be used to typify the early synchrotrons. The magnet was made of laminated silicon steel, bonded into bundles for assembly by through-bolts and external clamps. The poles and pole tips were also made of laminated steel, formed of cemented blocks; in the poles the laminations were radial. A number of “flux bars” were arranged around the inside of the orbit, to provide the “betatron start.” Excitation windings were also made of stranded wire, as in a transformer, to reduce eddy currents. The magnet was excited by discharge of a capacitor bank through an electronic switch consisting of ignitrons; the full cycle required 1/32 sec and was repeated six times per second. Acceleration of electrons occurred during the first quarter-cycle, inside a fused silica “doughnut,” made in sections and sealed together by rubber bands at the joints. The rf accelerating system was a quarter-wave resonator formed by copper plating one of the fused silica sectors of the vacuum chamber, with a gap in the plating at one end for acceleration and with the plating cut into strips to minimize eddy currents. This resonator operated at about 50 MHz frequency (wavelength equal to the circumference of the orbit), and the peak potential for acceleration was about 3 kV. The synchrotron was brought into operation at full design energy early in 1949, and the X rays were used for many experiments. The first round of electron synchrotrons, those with energies up to 500 MeV, were built with circular magnets and circular orbits. An innovation in design called a “racetrack” was developed by Crane (71) at the California Institute of Technology for his 1200-MeV electron synchrotron. This broke up the magnet structure into duants or semicircular sectors spaced by straight sections, in order to provide space for installing injection systems, rfaccelerating cavities, and ejection devices to bring out an emergent beam of electrons. In later higher energy synchrotrons the use of duants or quadrants became the preferred arrangement, and it initiated a trend toward the present pattern used in very large circular accelerators. The synchrotron quickly replaced the betatron as a source of highenergy electrons. The much lighter ring magnet which provides the guide field is simpler and less costly than the laminated core magnet of a betatron. In the synchrotron the radiative losses by electrons are corrected automatically by phase shifts if the rf cavity resonator provides sufficient voltage; the complicated compensating devices devised for high-energy betatrons are not needed. Even as sources of X rays in the 20-50 MeV region, with their many medical and industrial applications, the smaller and cheaper synchrotrons have competed successfully with betatrons. A review of electron synchrotrons was published by Thomas, Kraushaar,

EARLY HISTORY OF PARTICLE ACCELERATORS

43

and Halpern (72) in 1952. Only with the advent of strong focusing has this first generation of electron synchrotrons been displaced, and only because alternating gradient synchrotrons could produce much higher energies. This development will be described in a later section.

C . Synchrocyclotron

Both Veksler and McMillan realized that the conditions for phase stability could be met in the cyclotron at energies above the relativistic limit of the standard cyclotron, by modulating the driving frequency to match the decreasing frequency of the ions in the uniform magnetic field. At the University of California Radiation Laboratory, the 184-in. magnet originally intended to be used as a giant standard cyclotron, which had been put to other uses during World War 11, was ready to be converted to peacetime research uses. As a test, the 37-in. magnet was temporarily equipped with pole faces machined to give an exaggerated radial decrease in field, thereby simulating the type of orbits to be expected at much higher energies. Then the applied frequency was modulated (decreased) cyclically and the resonant particles were observed to follow the frequency change and remain in resonance until they reached the periphery. The frequency was modulated by a rotating capacitor in the resonant D circuit. The experimenters found that the beam was phase-focused so strongly that some particles could be swept out to the edge even when the variable capacitor was turned manually. With this justification, the 184-in. was immediately redesigned to use frequency modulation, and was rapidly built by most of the available laboratory staff. The name “synchrocyclotron” was suggested by Professor Lawrence. This project, as well as the electron synchrotron, was supported by the Manhattan Engineer District which had supported development of the atomic bomb, and later by the Atomic Energy Commission (AEC). The test on the 37-in. magnet was reported by Richardson, Wright, Lofgren, and Peters (73) in 1948, by which time the reconstruction of the 184-in. had long been completed and it was in operation. The effort to rebuild the 184411. cyclotron into a synchrocyclotron was carried on as a “crash” program, using the manpower and techniques so successful during World War I1 (Fig. 16). The machine was operated at almost its first trial late in 1946. A brief description was given by the team: Brobeck, Lawrence, McKenzie, McMillan, Serber, Sewell, Simpson, and Thornton (74)in 1947. A single very large D was used, mounted on the end of a quarter-wave resonant line grounded through a mechanically rotated variable capacitor. The resonant frequency vaned between 12.6 and 9.0 MHz, covering the range needed for accelerating either deuterons or

44

M. STANLEY LIVINGSTON

FIG. 16. Berkeley 184-in. synchrocyclotron before shielding was added. (Lawrence Radiation Laboratory photo.) [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

He2+ions in the magnetic field which fell from a central value of 15,000 to 95% of this value at the outer radius of 81 in. The machine produced deuterons of 195 MeV and He2+ions of 390 MeV on an internal probe target. Early experimental work was done with the beam of emergent neutrons produced by the deuterons striking the target. Later a deflector system was installed which brought the particle beams out through channels in a shielding wall to an experimental area. An improved rf modulation system was installed in late 1948 which allowed higher frequencies so protons could also be accelerated. Eventually, in 1957, the ultimate record was achieved with an emergent proton beam of 740 MeV, deuterons of 460 MeV and He2+ ions of 920 MeV (75). Other synchrocyclotrons were built at Rochester, Columbia, Harvard, Chicago, and Carnegie Tech, of somewhat smaller size and energy, and also abroad, at Harwell (England), Amsterdam, Uppsala, Liverpool, at CERN near Geneva, and in the U.S.S.R. at Dubna. The highest energy machines next to Berkeley are 680 MeV at Dubna, and 600 MeV at CERN. These synchrocylotrons have been most productive in research, adding greatly to our knowledge of nuclear physics and particle physics.

EARLY HISTORY OF PARTICLE ACCELERATORS

45

D. Proton Synchrotron

The proton synchrotron (PS) operates on the same basic principle as the electron synchrotron, and the magnetic field increases with time as the protons gain energy to maintain constant orbit radius. But unlike electrons which approach the velocity of light at relatively low energies (v = 0 . 9 8 ~at 2 MeV) and so have an essentially constant frequency of revolution during acceleration to higher energies, protons do not reach the equivalent limit until they have acquired about 4 GeV energy. So in a proton synchrotron the velocity and the frequency of revolution increase during the entire acceleration interval. The applied radio frequency must synchronize with the changing orbital frequency of the particle, requiring frequency modulation over a wide range, determined by the ion frequencies of revolution at injection and at maximum energy. This feature introduces new and complicated technical problems in the design of the accelerating electrodes and of the high-frequency oscillator. The same type of phase focusing exists to bunch the particles about an equilibrium phase of the accelerating field as for the electron synchrotron, and if the applied frequency is correct, the protons maintain a constant average orbit radius. An error in frequency could cause the protons to gain too much or too little energy and to spiral inward or outward. The required schedule of frequency modulation does not follow any simple law, but depends on the rate of increase of magnetic field, which is itself a function of the properties of the magnet iron and the constants of the power supply. So new problems of frequency control enter, which are unique to the proton synchrotron. However, much experience had been acquired in earlier synchronous accelerators; the magnitude of fields could be measured with good accuracy; and adequate theoretical analysis was available to compute the frequencies. So the physical problems of design were not so formidable as might have been expected. For economic reasons the proton synchrotron is the obvious choice for acceleration to very high energies. The ring-shaped magnet is much lighter and cheaper, for a given orbit radius, than the solid-core magnets of synchrocyclotrons. At the present state of technology the costs of linear systems greatly exceed those for a ring magnet system, both for construction and for power. The controlling parameter for the cost of ring-shaped magnets is the size of the aperture, so there is a high premium on providing a minimum factor of safety for the computed particle oscillation amplitudes. The first proposal of a proton accelerator using a ring magnet, in which both magnetic field and frequency of the applied rf are varied, was made in 1943 by Professor M. L. Oliphant of the University of Birmingham, to the British Directorate of Atomic Energy. Due to wartime security

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M. STANLEY LIVINGSTON

restrictions the proposal was not published, nor was construction started at that time. It is reported in a detailed study by Oliphant, Gooden, and Hyde (68) published in 1947 and accompanied by a theoretical analysis of orbit stability by Gooden, Jensen, and Symonds (76). The original proposal anticipated the discovery of phase stability by several years, and there is no evidence that stability had been adequately considered. The publications in 1947 did include the concepts and proofs of McMillan (63) and Veksler (65). An accelerator following these designs was built at the University of Birmingham at the end of World War 11, and it was operated at 1.0 GeV energy for several years following 1953. Professor Oliphant’s return to his native Australia and the untimely death of Dr. Gooden, the chief scientist on the project, slowed completion of this machine until after others were operating. For a few years following the announcement of phase stability, several laboratories were involved in developing synchrocyclotrons and electron synchrotrons. Design studies for proton synchrotrons started early in 1947 in two laboratories supported by the AEC: The newly formed Brookhaven National Laboratory on Long Island, and the University of California Radiation Laboratory. At Berkeley, Dr. W. M. Brobeck (77) made a preliminary design for 10-GeV protons in 1948 which was primarily a study of a pulsed magnet power supply for the large ring magnet envisioned. At the same time, preliminary designs for a multi-GeV accelerator started at Brookhaven, under the direction of the author, on leave from M.I.T., as chairman of the Accelerator Project. These early plans were reported by Livingston (78) and others before the American Physical Society in 1948. When preliminary designs and cost estimates became available in 1948, a decision was made by AEC and representatives of the two laboratories for the construction of two machines: a 2.5-3.0 GeV “cosmotron” at Brookhaven and a 5.0-6.0 GeV “bevatron” at the University of California. In both laboratories teams of scientists and engineers were assembled to complete designs, and the results must be recognized as the joint efforts of many individuals. The first proton synchrotron to be completed (May 1952), and the first multi-GeV accelerator, was the Brookhaven cosmotron, at 2.3 GeV protons (Fig. 17). It was soon brought to its design energy of 3.0 GeV. A description was published as the machine approached completion, in 1950, by Livingston, Blewett, Green, and Haworth (79). Twiss and Frank (80)made a theoretical study of orbital stability in 1949, and others have described special features. Following completion the entire staff collaborated in adetailed description of all components of the operating machine,

EARLY HISTORY OF PARTICLE ACCELERATORS

47

FIG. 17. The first proton synchrotron and the first multi-GeV accelerator, the Brookhaven cosmotron, photographed in 1952 before shielding was added. (Brookhaven National Laboratory photo.) [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

which occupies a full issue of the Review ofScientiJc Instruments, edited by M. H. Blewett (81). At the University of California the designers of the bevatron chose to build first a quarter-scale model to determine requirements and to demonstrate resonance. They also planned alternative pole-tip arrangements, a large aperture design which would produce 3.5 GeV, and a small aperture which would be capable of reaching 6.4 GeV. Following the initial performance of the Brookhaven machine which showed the smaller aperture was adequate, they chose the high-energy alternative. Initial operation at about 5 GeV was reported in early 1954, followed the next year by fullenergy operation. Publications of the Berkeley group have been largely through laboratory memoranda and AEC progress reports. Lofgren (82) published a brief description in 1950. The most inclusive survey is an internal report in 1957 by Brobeck (83). During the 1950s several other proton synchrotrons were built, patterned on either the cosmotron or the bevatron. The largest was the

48

M. STANLEY LIVINGSTON

10-GeV synchrophasotron of the U.S.S.R. Joint Institute of Nuclear Research at Dubna, based on the bevatron, which was completed in 1957. Technical reports in the Russian literature can be traced through the paper by Veksler (84) in the U.S.S.R. Journal of Nuclear Energy. Three machines are based on the cosmotron design, a 3-GeV “Saturne” completed in 1958 at Saclay, a 3-GeV rapid-cycling machine at Princeton completed in 1961, and a scaled-up model at 7 GeV called Nimrod at Harwell. At the Argonne Laboratory near Chicago a “zero-gradient synchrotron” (ZGS) completed in 1962 uses a magnet of the bevatron type, but with a uniform field and with end shaping of the eight octants to provide focusing. The basic parameters of these proton synchrotrons mentioned above are given in Table I (85). VII. LINEARACCELERATORS In linear accelerators particles are accelerated in straight lines through a linear array of electrodes to which a n rf electric field is applied. Many electrode structures have been devised, with the purpose of improving the energy given to the particles per unit of input power. These structures have one feature in common-their field patterns include a traveling wave component whose phase velocity is the same as that of the accelerated particles. One way of visualizing the acceleration is that the particles coast down the front of the traveling wave as a surfboard rider coasts down the advancing front of a water wave. With heavy particles, velocity increases as the particles gain energy, so the system must provide a phase velocity that increases with distance along the accelerator. This is usually accomplished by using a sequence of hollow tubular electrodes in which the lengths increase with the (calculated) particle velocity. Particles are accelerated in resonance with the rf voltage, and the electrodes act as drift tubes to shield the particles from the fields during the decelerating portions of the cycles. At relativistic energies when the particles approach the velocity of light, the structure becomes constant in spacing and simpler in design. With electrons this occurs at quite low energies-0.98 of the velocity of light at 2 MeV energy. So relativistic speeds can be produced in the preaccelerator or injector, and the multi-MeV accelerator itself can have constant and uniform spacings. The chief advantage of the linear accelerator lies in the natural collimation of the beam, compared with the spreading emergent beams from circular accelators. This provides ease of extraction of the beam and reduces complexity and cost of radiation shielding. The disadvantage is the

TABLE I PARAMETERS OF C ONVE NT IONAL PR OT ON SYNCHROTRONS

Location Name Maximum energy (GeV) Orbit radius (m) Number of magnet sections Peak magnetic field (kG) Magnet weight (tons) Aperture Width (cm) Height (cm) Pulses per minute Injection energy (MeV) Number of accererator cavities Harmonic order Date of completion

(85)

Birmingham, U.K.

Brookhaven, NY

Saclay, France

Berkeley, CA

Dubna, U.S.S.R.

Princeton, NJ

Harwell, U.K.

Argonne, IL

1.O 4.5

Cosmotron 3.0 10.7 4 13.8 1660

Saturne 2.5

Bevatron 6.4 18.2 4 16.

Synchrophasotron

P.P. A. 3.0 12.2 16 13,8 350

Nimrod 7.0 23.6 8

ZGS 12.5 27.4 4 21.5 4000

91 22 12 3.7

60

21 6 0.46

91 24 28

9.0

18 7 1140 3.0

15.

81 15 15 50.

1 1

1 1

1953

1952

1 4 1962

2 8 1962

1

12.6 810 50

11.

4 15. 1080

10,OOO

10 30.5 4

13. 35,000

10 19 3.6

122 30 10 9.8

1

1

2

4

2 1958

1 1954

1

8 1962

150

40 5

1957

14. 7000

50

M. STANLEY LIVINGSTON

requirement of large amounts of rf power, which cannot be reused as in circular resonance accelerators. However, due to this high power, high beam intensities can usually be produced from linear accelerators with comparative ease. For a time, designers hoped that the apparent economic advantage of construction costs being linearly proportional to energy, while circular machines require magnets whose costs increase with the third power of energy, might provide an economic advantage at high energies. However, the sequential developments of circular accelerators, from the cyclotron with its solid-core magnet, to the synchrotron with its lighter ring magnet, to the alternating gradient synchrotron which still further reduces the magnet dimensions and cost, have retained the economic advantage of magnetic machines. The linear accelerator must rely on other advantages, associated with its sharply collimated emergent beam and high beam density, to justify its competitive position. The most important use of linear accelerators is as preaccelerators or injectors into higher energy machines, where the specific advantage is the compact, well-focused beams. Another important use is in the production of very intense beams of heavy ions, which have become very important in nuclear chemistry. A recent application is the “meson factory,” where very high intensity, well-focused beams are finding important uses at intermediate energies. Accelerator builders have shortened the title of linear accelerator to “linac,” which is used both for proton and electron machines. Some specialists argue for more distinguishing titles for the basic types. But so far the only one which has been accepted is the name “hilac” for heavy ion linear accelerators.

A . Early Linear Accelerators

The earliest proposal for a linear accelerator in the literature was by Ising (86) in Sweden in 1925. He suggested the use of an array of tubular electrodes of increasing length, with voltages applied from a spark-gap 0scillator through transmission lines of increasing length. However, Ising did not reduce his ideas to practice. Rolf Wideroe (38)was inspired by Ising’s proposal to conceive of another system for providing voltage to the electrodes-a resonant rf power supply. In his experimental test of the concept he used three tubular electrodes, with the outer two grounded and the central one excited by the rf source. Particles that traversed both accelerating gaps at the correct

EARLY HISTORY OF PARTICLE ACCELERATORS

1

1

1

f

--

---

I

51

rf

osc.

FIG.18. Schematic diagram of Sloan-type linear accelerator, using tubular electrodes of increasing length connecting alternately to the terminals of an rf voltage source. (From M. S. Livingston and J. P. Blewett, ”Particle Accelerators,” 1962. Reprinted by permission of McGraw-Hill Book Company.)

accelerating phase emerged with an energy equivalent to twice the applied rf voltage. This was an elementary resonance linear accelerator, although it consisted of only two stages of acceleration. However, this demonstration by Wideroe of resonance acceleration inspired Lawrence at the University of California to invent the magnetic resonance accelerator known as the cyclotron, and started a chain of developments which has led step-by-step to the present-day giant magnetic accelerators. Lawrence also noted the possibility of using a linear array of electrodes for acceleration of particles and suggested it to students as another line of experimental development. David H. Sloan, then a student at Berkeley, extended Wideroe’s idea. He designed and built linacs with 10 and later 30 hollow tubular electrodes of increasing length supported alternately from two bus-bars (Fig. 18). In this case the particles are in resonance when they traverse one gap separation length in one half-cycle of the radio frequency. Heavy ions (Hg+) were used because of limitations to relatively low frequencies. Since particle velocity increases with the square root of the energy at low energies, the electrode lengths increase with the square roots of a series of integers starting with one. The overall length of the 30-electrode linac was 1.14 m and the resonant frequency was 10 MHz. With a peak voltage of 43 kV across the accelerating gaps, singly charged mercury ions could be accelerated to 1.26 MeV energy. When these ions were incident on a target, they produced soft X rays characteristic of mercury and the target element, but no nuclear effects were observed. The results were first reported by Sloan and Lawrence (87) in 1931. Later, Sloan and Coates (88) reported production of 2.8-MeV Hg+ ions, with a longer array, in 1934. Others in the Berkeley laboratory worked with linear accelerators. Kinsey (89) built a linac for Li+ ions, which were accelerated to 1.0 MeV, in 1936. Again, no evidence was found of nuclear disintegrations. These heavy ions proved ineffective for nuclear research in competition with the cyclotron. So after a few years of unrewarding work this program of linear accelerator development at Berkeley was abandoned.

52

M. STANLEY LIVINGSTON

3. Origins of Modern Lineur Accelerators

Present linac developments are based on new concepts of rf systems which arose during World War I1 in the radar and communications fields. For example, several high-power rf power sources were developed for powering high-frequency communication systems, such as water-cooled triodes capable of continuous operation at frequencies up to 200 MHz. And magnetron power tubes were available to provide pulsed power for radar systems at still higher frequencies, of 3000 MHz. Another development was the use of hollow cavity resonant circuits, including hollow waveguides for the transmission of rf power. An early concept was the “rumbatron,” intended as a device for accelerating electrons when it was first developed by W. W. Hansen (90) of Stanford University in 1934. This was an empty copper container with a natural resonant frequency which could act as a highly efficient rf circuit, and was capable of developing very high voltages across the extremities with the application of a moderate amount of rf power. Several subharmonics at higher frequencies would also be excited within such a cavity resonator, depending on its shape. The shaping of resonant cavities to increase the efficiency of a chosen harmonic led directly to present-day linear accelerators, which are loaded along the central axis with disk-shaped or tubular shaped electrodes. Two rather different types of linear accelerators came from these wartime developments, one for protons and one for electrons. In both cases the particles are accelerated by rf fields within a linear array of electrodes. The two types differ markedly in appearance for a simple reason-the choice of the most efficient operating frequency. For protons, velocities at the low-energy end of the accelerator are low and the protons are strongly affected by the transverse electric field components. For any reasonable choice of beam size (which itself depends on the physical dimensions of the beam from the positive ion source), the beam aperture must be quite large, of the order of several centimeters. This requirement leads to the use of quite large apertures within which the field must be essentially constant, and so to relatively large electrodes and large cavity dimensions. The choice made by Alvarez for the first proton accelerator built after World War 11, was a cavity 39 in. in diameter and 40 ft long, loaded with an axial array of drift-tube electrodes varying in outer diameter from 5 to 3 in. (Fig. 19). This array was resonant at 200 MHz. Most linacs built since this first one, for proton energies around 200 MeV, are in the same frequency range. Electrons, on the other hand, have much higher velocities even at low energies, so the effect of transverse field components is much smaller and

EARLY HISTORY OF PARTICLE ACCELERATORS

53

FIG.19. Schematic diagram of Alvarez-type linear accelerator in which frequency is the same in each of the successive ‘‘cells.’’ (From M. S. Livingston and J. P. Blewett, “Particle Accelerators,” 1%2. Reprinted by permission of McCraw-Hill Book Company.)

apertures can be smaller. Also, beam sizes from electron sources are usually small (less than 1 mm). So, the size of uniform field required within the electrode apertures need only be 3 to 4 mm, and external cavity dimensions can be much smaller. With external cavity dimensions of only 3 to 4 in., the operating frequencies are in the range of 3000 MHz. One of the earliest electron linacs built by Hansen at Stanford in 1947, had a disk-loaded waveguide cavity of 4 in. outer diameter and 12 ft long, and operated at 3000 MHz. The accelerating system for an electron linear accelerator consists of a tubular waveguide separated into a sequence of identical small cells or cavities by a set of irises (Fig. 20). Each cell resonates in the TM,,, mode in which the major electric field component is parallel to the axis. These cavities are coupled through the axial holes in the irises, and the entire system resonates at the basic frequency of a single cell. Another description of this resonant mode is that it is a traveling wave in which the phase velocity (well above the velocity of light for the tubular waveguide) is reduced to be equal to the velocity of light by the loading provided by the irises. The parameters of iris-loaded waveguides have been studied ,by several groups. Among these are the Stanford (69) group, that of Slater (91) at M.I.T., and a British group at Harwell (92, 93). Choice of iris

54

M. STANLEY LIVINGSTON

FIG.20. Schematic diagram of iris-loaded waveguides used for electron linear accelerators. Spacings for two resonant configurations are shown. (From M. S. Livingston and J. P. Blewett, "Particle Accelerators," 1%2. Reprinted by permission of McGraw-Hill Book Company .)

parameters determines the phase velocity, the shunt impedance, the wavelength, and the power losses (94). An early electron linac built by Fry (92) in England, had an iris spacing of 1 cm (10 irises/wavelength) at 300(-MHz, with which he accelerated electrons from 45 to 538 keV in a length of 40 cm. At Stanford the iris spacing of Mark I linac was 24 cm (quarter-wavelength); it was 14 ft long and produced electrons of 7 MeV energy. In the early days it was not clear which would require less power, the traveling wave or a standing wave. It is now known that for efficiency, traveling waves must travel far enough so they are attenuated to about l/e of their input amplitude. For this condition the power requirement for the traveling wave is the same as for a standing wave system. So the choice must be based on considerations other than power.

EARLY HISTORY OF PARTICLE ACCELERATORS

55

An important result of linac design is the power requirement. Analysis of field patterns in different electrode structures, and of the resistive losses in the copper walls of the resonant cavities, leads to a relationship between particle energy T, accelerator length L , operating frequency (expressed as free-space wavelength A), and the required rf input power P: P

=

c T* (h)1’2/L

where C is a constant depending only on the geometric structure of the accelerating system. We note that power is inversely proportional to accelerator length; so power can be decreased by increasing length, and the designer can choose an optimum length for which total cost of power plus accelerator construction is a minimum. Also, power is directly proportional to wavelength, or inversely to frequency, which suggests the highest practical frequency be used. Since frequency or wavelength is determined by the factors described earlier, the important choice is the maximum frequency compatible with good beam quality and good beam geometry. However, in a practical sense, the choice of frequency often involves the availability of suitable power sources (oscillator tubes) in the desired frequency range, an economic consideration. The cost of the rf power supply for a linac is usually one of the largest cost items in the construction budget. C . Resonance Lineur Accelerator -Alvurez

Linac

The first proton linear accelerator to be used for research was built by L. W. Alvarez at Berkeley at the end of World War 11. The essential concept was the use of a cavity oscillator, as discussed in the previous section. The resonant cylinder (39 in. in diameter and 40 ft long) was formed of shaped copper sheet mounted within a steel vacuum chamber and was water cooled; both the copper cylinder and the enclosing vacuum chamber were formed in two halves which could be opened to install and service the drift tubes. The resonant cavity was loaded with an array of 45 drift tubes of increasing length (to match particle velocity) mounted along its axis. The loaded cavity operated in the 21r mode, a modification of the TMolomode, and resonated with a standing wave pattern enclosing each drift tube. Electrodes had constant inner diameters through which the beam traveled, but lengths increased and outer diameters were varied to tune each “cell” to the basic frequency of 202.5 MHz. Electrodes were supported on slender stems through which water was circulated for cooling. Radio-frequency power was fed from coupled oscillators outside the chamber through loop couplings into the cavity. Protons were preaccelerated to 4 MeV in a horizontal electrostatic generator before injection into the linac.

56

M. STANLEY LIVINGSTON

Surplus radar and electronics equipment was made available from the armed services, including power oscillator tubes to excite the cavity at 200 MHz and their power supplies. The machine was brought into operation at 32-MeV protons in 1948, and was promptly put to use for research studies. Several years later Alvarez and nine collaborators (95) published an article describing the machine. This machine became the prototype of proton linear accelerators, and many others were built following this basic pattern. Another factor in the development of this first resonance linac was recognition of the applicability of phase stability to linear accelerators and its impact on focusing of the particle beam. The design of the linac by Alvarez was proceeding at the same time that McMillan was designing his first electron synchrotron, and when the 184411. was being converted into a synchrocyclotron. It was recognized at an early stage that phase stability also applied to the linear accelerator and caused the particles to condense into a stable phase band on the rf wave. For example, consider particles moving along the axis of the electrodes in phase with the traveling wave component of the rf field. There are two phase positions on the rf wave at which the particle would gain the correct amount of energy to stay in resonance, one on the rising side of the wave and one on the falling side. First consider the particle that crosses the accelerating gap on a rising phase, but with a phase error such that it is delayed compared to the equilibrium particle. This particle will gain too little energy, will acquire a velocity below the average, and will take slightly longer to traverse the fixed spacing to the next gap. So it will arrive with its phase shifted closer to equilibrium phase, and will continue this shift until it reaches and exceeds the equilibrium phase. Now this particle will experience the reverse effect: It will gain too much energy, increase velocity above average, and shift phase back toward equilibrium. The result is that each particle will oscillate in phase about the equilibrium point. More detailed analysis shows this oscillation to be stable and to be damped to smaller amplitude as acceleration continues. Next consider a particle that crosses the gap when the rffield is falling. In this case an analysis similar to that just given shows that the phase shifts will be such as to increase the deviations and the particle will eventually be lost from resonance. So, we conclude that the stable phase position is on a rising phase, when the voltage across the gap is increasing while the particle is crossing the gap. This requirement of a rising phase for stability affects another feature of the linac-the focusing or defocusing by the electric fields at the accelerating gap. Now, the general shape of the electric field pattern between cylindrical electrodes provides some slight focusing for static fields, when the field is accelerating. To understand this, it can be noted that the field

EARLY HISTORY OF PARTICLE ACCELERATORS

57

lines in the entry region where particles enter the gap have a shape which is convergent, and so the entry half of the gap provides focusing; in the exit half of the gap the field lines are divergent, producing a defocusing effect on the particles. However, particle velocity increases during traversal of the gap so the time spent in traversing the exit half is shorter than the time in the entry half. So the net result is a slight (weak) focusing, for static fields. In a resonance accelerator the particles in the stable phase band are in a phase where the electric field strength is increasing while the particles cross the gap. This means that the divergent forces in the exit half of the gap become larger than the convergent forces of the entry half, and the net result is defocusing. This defocusing effect due to the increasing electric field exceeds the weak focusing (due to the velocity increase) described above for static fields. So the overall result is defocusing at each gap in a resonance accelerator. This defocusing by the rf fields was a serious problem for early linac designers. Alvarez first used thin metallic foils on the entry faces of the gaps to remove the defocusing component; however, these foils were fragile and soon burned out. Next, he installed grids on the entry faces of the gaps designed to shape the field lines so as to reduce the defocusing component (Fig. 21). Such grids resulted in a significant loss of intensity due to particles impinging on them. It was not until quadrupole magnetic lenses (strong focusing) were developed in the early 1950s that the focusing problem for linacs was satisfactorily solved. In the 20 years since the first linac was built there have been many changes and improvements in the engineering techniques used for construction, and the output energy has been increased to 200 MeV. But the basic principle of the drift-tube linac and the basic arrangement of structures for acceleration have been retained without significant modification. The major changes have been (96) 1. Improvement of mechanical tolerances in construction 2. Improvement of rfproperties of materials and joints 3. Use of improved pumps, seals, and vacuum-conditioning techniques 4. Use of automatic temperature controls to stabilize frequency 5 . Use of quadrupole lenses in drift tubes for focusing 6. Use of copper-clad steel in tank construction 7. Use of post couplers to change operation from 27r to 7r/2 mode 8. Radiation “hardening” with ceramic insulation.

A listing of the major Alvarez-type linacs built by 1971 is given in Table 11.

58

M. STANLEY LIVINGSTON

FIG.21. Alvarez-type linear accelerator at the Argonne National Laboratory which produces SO-MeV protons. Outer vacuum casing is opened to show structure. (Photo from Argonne National Laboratory.) [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

TABLE I1 LISTING Machine Alvarez, Berkeley Kharkov I , USSR Bevatron, Inject I Univ. Minnesota

ELA, Harwell

OF

ALVAREZ-TYPE LINEAR PROTON ACCELERATORS~

Energy (MeV)

Frequency WHz)

Focusing type

Number of drift tubes

32 20.5 10 10 40 68

202.5 139.4 202.5 202.55

Grids Grids Grids Grids

45

10

202.56

Grids Quads Quads Quads

30 50

CERN-PS, Inject AGS, Inject I

Nimrod, Inject , Bevatron, Inject I1 ZGS, Inject ITEP-PS, Inject Serpukov, Inject AGS, Inject I1 200 GeV, Inject LAMPF, Inject

10 30 50

202.56

50

201.06

I5 19.3 50 6 24 38 73 100 200 200 100

115 199.3 200 148.5

Quads Quads Quads Quads Quads

148.5

Quads

201.25 201.25 201.25

Quads Quads Quads

50

42 41 37 24 41 40 26 41 40 26 124 48 73 124 18 33 93 41 26 295 295 165

Taken in part from "Linear Accelerators," (97), North-Holland Publishing Co., Amsterdam, 1970.

Number and type of tanks

Year completed

1, liner 3, liner

1948 1950 1953 1955

3, liner

1959

3, liner

1959

1, copper clad 1, liner 1, copper clad 1, copper clad 2, liner

1960 1961 1%2 1%3 1966

3, liner

1967

9, copper clad 9, copper clad 4, copper clad

1971 1971 1971

1, liner 1, liner

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M. STANLEY LIVINGSTON

D . Electron Linacs As indicated earlier, the beginnings of the modern electron linacs can be traced directly to Hansen (90) at Stanford University. In 1934 he started a program of studies of devices and systems to be used for the study of nuclear physics. Since his past experience was in X-ray research, he was influenced to accelerate electrons rather than heavy particles. He also had much experience in rf systems, and this led to his invention of the “rumbatron,” a resonant rf cavity intended for the acceleration of electrons. His first goal was 1 MV. His work was interrupted by the onset of World War 11. During this period Hansen joined with Russell Varian and Sigurd Varian in their work on microwave systems, which resulted in their invention of the klystron, a more efficient generator of microwaves. At the end of World War 11, Hansen returned to his interest in accelerators for nuclear physics. He found that his cavity oscillators would be suitable to form resonant circuits for linear accelerators and started specific development. With the assistance of J. R. Woodyard and E. L. Ginzton and support initially from the U.S. Office of Research and Inventions, he started a program of development of electron linacs. The first machine, the Stanford Mark I, was an iris-loaded cavity 12 ft long which resonated at 3000 MHz; it produced 1-MeV protons in 1947. In his status report to the supporting agency at that time Hansen submitted the single sentence “We have accelerated electrons.” This was the beginning of a sequential development of electron linear accelerators (and klystron power tubes) at Stanford which continued for many years and has achieved the highest energy of any linear accelerator. Support came from the Office of Naval Research and later the AEC. The most complete survey is given in the “Linear Accelerator Issue” of the Review of Scienrijk Instruments (97). Development at Stanford continued with the Mark 11, built in 1948-1949. It was intended to be an intermediate step on the way toward an ultimate one billion volts (1 GeV), and to act as a prototype of one section of the larger machine. This unit was 14 ft long and was powered by klystrons developed in a parallel program to provide a peak output power 20 MW. The Mark I1 was first operated in 1949 and ultimately achieved electron energies of nearly 40 MeV. The next step was the Mark 111, which was itself a sequence of steps in the development toward 1 GeV. In 1949, while it was in development, Professor Hansen died and his responsibilities were assumed by E. L. Ginzton. The first three sections of Mark 111 were 30 ft long and powered by three klystrons, each providing 8 MW peak power; an electron beam of 75 MeV was produced in late 1950. The accelerator grew in 10-ft steps to reach 210 ft length in late 1953, and eventually operated at 600 MeV.

EARLY HISTORY OF PARTICLE ACCELERATORS

61

For a period of several years the Mark 111 was applied to research, primarily to a program of electron scattering carried on by R. Hofstadter. Hofstadter was awarded t h e Nobel Prize for these studies of atomic nuclei in 1961. This was a high point for the laboratory. Meanwhile, in late 1957 a 90-ft extension was added to Mark 111, which was completed in July 1960 with research operations up to 900 MeV energy. Finally, by March 1964 the Mark 111 was producing beams of electrons with energies up to 1.2 GeV, and a continuing program of research was underway. E . SLAC Two-Mile Accelerator

The success of Mark 111 during the 1950s stimulated plans at Stanford for a much higher energy electron linear accelerator. Beginning in 1955 members of the Physics Department started discussions on the scientific justification for such a big venture. It was dubbed “the Monster,” and officially called Project M; opinion is divided as to whether M stood for Monster” or “multi-GeV.” A formal proposal for construction of a “two-mile” accelerator was prepared in April 1957 and submitted to the AEC, the National Science Foundation, and the Department of Defense. Congress took the first step in funding the Stanford project in 1970 by authorizing funds for design and engineering studies. The AEC authorized the project on September 15, 1961, and a contract was signed in April 1962, four years after the initial formal proposal. Ground was broken in July 1962. The schedule for construction was five years and the budget was $1 14 million. The parameters of the SLAC linac are given in Table I11 (98). “

TABLE I11 PARAMETERS OF THE SLAC LINAC (98) Beam energy Length of accelerator pipe Average beam current Average beam power Beam pulse length Pulses per second Number of klystrons Peak power per klystron Total project power, 1967 Operating staff and scientists Size of site, on Stanford campus Construction cost Operating cost, initial

20 GeV“ 10,Ooo ft 30 CLA 600 kW 1.7 psec 360 240 24 MW 80 MW I 100 480 acres $ 1 14 million $20 million/year

Beam energy can be increased to 40 GeV by using 960 klystrons.

62

M. STANLEY LIVINGSTON

The principal credit for completing this huge job on time and within the allotted budget goes to the director of the Stanford Linear Accelerator Project W. K. H. Panofsky (“Pief” to all his friends), and to the associate director, R. B. Neal. It was completed with the first beam traveling the full two-mile length of the accelerator in May, 1966, and was brought to the design energy of 20 GeV by January, 1967. SLAC is located on Stanford property about two miles from the main campus area (Fig. 22). The accelerator housing is an underground tunnel

FIG.22. Aerial view of the SLAC site. The 20-GeV linac is in an underground tunnel beneath the long klystron gallery. (Stanford Linear Accelerator photo; reprinted by permission of Stanford Linear Accelerator Center.)

EARLY HISTORY OF PARTICLE ACCELERATORS

63

formed by the cut-and-fill method of trenching. Over the tunnel is the klystron gallery which contains all the power supplies and maintenance systems. The end station includes areas for a beam switchyard and many target stations including large magnetic spectrometers. The Stanford Linear Accelerator Center (SLAC) is a national €acility, a large scientific laboratory built and operated by Stanford University, originally for the AEC and now for the Office of Energy Management. Its purpose is the study of the properties of matter in its most fundamental forms, usually known as “elementary particle physics.” Users include qualified physicists from all parts of the United States and abroad, as well as the physicists from SLAC and from the Stanford Physics Department. F. Linacs f . r Speciul Purposes

The linear accelerator has proven to be ideal for the acceleration of multicharged heavy ions, such as N3+, U6+, and so on.* The heavy ions are produced, usually in the singly charged state, in special ion sources, and are preaccelerated to 0.5 or 1.0 MeV in a direct voltage accelerator such as a Cockcroft-Walton. Before injection into the hilac, the ions pass through a “stripper,” which is a gas jet in which the ions lose electrons to become the desired multicharged ions. After stripping, the ions traverse the hilac where they acquire a final energy equal to the hilac energy times the number of charges on the ions. Two of the early hilacs were built in a joint project between groups at Yale and at the Radiation Laboratory of the University of California. The first machine was built around a cylindrical cavity 10 ft in diameter and 15 ft long; the frequency of operation was 70 MHz. It provided ion energies of up to 10 MeV/nucleon. Several electron linacs have been built to supply the growing needs for high-intensity electron beams at about 500 MeV energy, for both scientific and industrial purposes: 1. The Bates (99) linear accelerator is sponsored by M.I.T. and supported b y the AEC and the Department of Energy. It is a traveling wave accelerator 180 m long, utilizing 40 MW of rf power at 2856 MHz frequency. With pulsed operation at 1.8% duty cycle it produces electrons of 430 MeV, or at 5.8% duty cycle, of 220 MeV energy * 2. The Saclay electron linac is similar to the Bates linac with a wave guide of 185 m length and a peak design power of 60 MW at 2856 MHz frequency. It was designed to produce 640 MeV electrons at 1% duty cycle or 250 MeV at 2%, and has generally achieved the design figures. * Accelerators for this purpose are called “hilacs.”

64

M. STANLEY LIVINGSTON

3. The Amsterdam electron linac is also similar to those above, designed for 500-MeV electrons with 2.5% duty cycle and 250 MeV with 10% duty cycle. The waveguide is 200 m long and the peak design power is 72 MW at the same frequency.

A wide variety of smaller accelerators of electrons have been built for use in applied programs such as cancer therapy or for industrial radiographic inspections. A recent application of the proton resonance linear accelerator is for the production of mesons to be used in the study of nuclear physics, as contrasted with the study of elementary particle physics. A growing awareness of the importance of this field of research developed in the late 1950s. The need was for proton accelerators in the intermediate (< 1 GeV) energy range and with high beam intensities, sufficient to produce meson beams for nuclear physics studies and for cancer therapy experiments. This category of high-intensity accelerators acquired the name of “meson factories.” A competition developed between proton linacs and fixedfield cyclotrons, resulting in 1962 in approval by the AEC for the con-

FIG.23. Side-coupled-cavity linac in tunnel of 800-MeV LAMPF high-intensity linac. (Photo from Los Alamos Scientific Laboratory; reprinted by permission of Los Alamos Scientific Laboratory.)

EARLY HISTORY OF PARTICLE ACCELERATORS

65

struction of a meson-producing proton linac of 800 MeV energy and of up to 1.0 mA proton beam at the Los Alamos Scientific Laboratory. The Los Alamos Meson Physics Facility (100) (LAMPF) was completed in 1972 and brought into full-scale operation for research in 1974. It has also started studies on the irradiation of certain types of human cancer. This linac has a unique design of coupled resonant cavities operating at 800 MHz, effective after the beam is accelerated to 100 MeV in an Alvarez-type linac operating at 200 MHz (Fig. 23). The spacings between accelerating gaps decrease steadily along the half-mile length of the linac, to match the proton velocity. Power is supplied by 44 specially designed klystrons of 14 MW each, operating on a pulsed duty cycle of 6%. Even at present intensity levels (350 PA) the LAMPF linac produces a beam of protons of higher beam power (beam current x energy) than any other accelerator in the world. The linac is arranged with three separate injectors, so it can accelerate simultaneously a high-intensity proton beam or, if (H+) beam and a lower intenjty negative hydrogen ion (H-) desired, a beam of polarized H- ions. This greatly increases the flexibility for research. The chief parameters of the LAMPF linac are given in Table IV. PARAMETERS OF

TABLE IV LAMPF PROTONLINAC(100)

THE

~

Beam energy Energy variability Energy resolution Duty factor Average beam current, maximum Beam power, maximum Radio-frequency power input Length of accelerator Cost of accelerator Cost of project

___

~~~

800 MeV 200-800 MeV 0.4% 6%

ClA

800 kW 6.1 MW 2600 ft $22 million $60 million

VIII. ALTERNATING GRADIENT ACCELERATORS

A . Origins of Strong Focusing At the time when the strong focusing principle was conceived, plans were being made in Europe for the construction of a proton synchrotron of the highest practical energy, to be located at the soon-to-be-established laboratory near Geneva now called CERN (European Organization for

66

M. STANLEY LIVINGSTON

Nuclear Research). Preliminary estimates suggested an energy of 10 GeV with the funds available. A delegation of scientists and engineers representing the CERN planning staff was scheduled to visit Brookhaven and Berkeley that summer (1952) to assess the cosmotron and the bevatron as potential models for their 10-GeV machine. The writer had served as chairman of the Accelerator Department at the Brookhaven National Laboratory from 1946 to 1948, while the cosmotron was being designed, and then had returned to M.I.T. The Brookhaven staff anticipated that the cosmotron would be completed in early summer of 1952, and I made plans to spend the summer at Brookhaven, with a graduate student and some instrumentation, to start research experiments with the mesons expected when the cosmotron started operations. However, on arrival I found that the cosmotron was not yet ready for operation but needed some engineering consolidations. In anticipation of the visit of the CERN delegation I felt it would be useful to review the design features of the cosmotron, to see if it could be extended to 10 GeV. As a start, I considered how to improve the efficiency of the magnet. I had been largely responsible for choosing the C-shaped cross section for the yoke of the magnet, with the return circuit on the inside of the orbit, which had certain advantages but was known to suffer from a reduced useful radial aperture at high fields. The pole faces of the cosmotron which formed the magnetic aperture were nearly flat and parallel, and the vertical focusing was provided by a slight radial decrease in field of about 6% across the 30411. width of the radial aperture. This can be expressed in terms of the magnetic field index, n, the exponent defining the radial change in field: B, = Bo(ro/r)". For the cosmotron the n value had been chosen to be 0.6. Note that for vertical focusing this index must lie between 0 and 1. The problem was that with the return circuit of the C-shaped iron circuit on the inside of the orbit, the asymmetric saturation of the return leg at high fields led to a considerable reduction in width of the region for which n = 0.6. It seemed to me that the asymmetric saturation of the C-shaped cores could be compensated, and yet the advantages of the C-shape retained, by alternating the locations of the return yoke from inside to outside the orbit. In designing pole faces for this arrangement, for which both types of cores would be shaped to give n = 0.6 at low fields, I found that at high fields saturation would result in positive magnetic gradients for one type and negative gradients for the other. My first concern was whether this alternation in gradients would destroy orbit stability. I discussed this question of orbit stability with my theorist colleague Ernest Courant, and he took the problem home with him that evening. The next morning he reported, with some surprise, that preliminary calcu-

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lations showed the orbits to be stable and to have even smaller transverse amplitudes than in the constant gradient of a standard synchrotron. As I recall, the set of gradients used by Courant in this first calculation was: n, = + 1 .O and n2 = - 0.2, chosen to give an average value of n = + 0.6 as used in the cosmotron. The significance of this result was discussed with others in the laboratory, notably Hartland Snyder, and no fault could be found with Courant’s analysis. If a little alternating gradient was good, more should be better! Courant’s next calculations were for n values of about &lo, which showed even stronger focusing and smaller amplitudes of oscillations. It became clear that the average value of ii = +0.6 was unimportant, but that a new type of stability was associated with the alternation in gradients. This was the start of an exciting period at Brookhaven. Larger and larger gradients were assumed in further stability calculations, with n values of 100, 1000, and even more. As they increased I developed sketches of magnetic circuits to provide the high-gradient fields. The magnet poles became sharply tilted and narrower. Analysis showed that the pole faces should be shaped to a rectangular hyperbola to provide a uniform gradient across the aperture. As the experimentalist on the team, I kept busy designing the strangely shaped magnetic circuits with hyperbolic poles and small cross sections. As gradients grew larger and both vertical and horizontal particle oscillation amplitudes became smaller, much smaller apertures were needed to contain the beams. Soon our speculations led to such large gradients and small apertures that construction was obviously impractical. When the largest vacuum chamber that could be installed between poles became less than 1 in. in diameter, I objected that we had passed the bounds of practicality -at least for a high-energy accelerator. We ended up with designs for n = f 300 as the most practical size (Fig. 24). Courant also studied the synchronous oscillations in gradient fields and found them to be stable as in the normal synchrotron. Furthermore, the orbits of particles having a spread in momentum were found to be compacted into a narrow radial band whose width varied inversely as the n value. So, as n values increased and pole faces became smaller, the acceptable momentum spread remained large. Suitable configurations of focusing (F)and defocusing (D) magnets and straight sections (0) were devised for the arrangement of magnet units in circular orbits, such as: FODO, FOFDOD, and FOFODODO. Hartland Snyder recognized and developed the generality of the stability principle. He noted that the alternating magnetic forces on charged particles resulted in a type of dynamic stability that has many analogs in mechanical, optical, and electrical systems. For example, an inverted

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FIG. 24. Originators of the alternating gradient focusing principle at Brookhaven National Laboratory, 1952. Left to right,,E. D. Courant, M. S. Livingston, H. S. Snyder, and J. P. Blewett. (Brookhaven National Laboratory photo.) [From Livingston (39).Reprinted by permission of Harvard Univ. Press.]

pendulum is unstable under static forces and will fall to one side with any small displacement from the vertical. However, if the base is oscillated rapidly up and down through a short stroke, the pendulum is stable in the inverted position over a wide range of oscillation frequencies. The use of gradient fields as lenses for charged particles in linear beams was also studied, including the use of constant magnetic fields. A magnet was proposed for such applications having four poles of alternating polarity, with pole faces shaped to the four arms of a rectangular hyperbola (Fig. 25). Field direction alternated around the four poles. A doublet formed of two such “quadrupoles” in which the gradients in the second unit are oriented at 90”to the first, forms a lens doublet that focuses divergent charged particles in both transverse directions (Fig. 26). This was the origin of the quadrupole lens systems now commonly used in accelerator laboratories for control of linear particle beams. The strength of focusing possible with such quadrupole lenses greatly exceeds that of a solenoidal magnetic field, and power requirements for focusing highenergy beams are much less. We noted that similar lenses could be

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formed of permanent magnets, and speculated on the use of smalldiameter beam pipes surrounded by permanent magnet quadrupoles in cables many miles long.

FIG.25. Cross section of quadrupole magnetic lens. [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

FIG.26. Schematic diagram illustrating two-dimensional focusing resulting from a particle beam traversing two quadrupole lenses which are rotated by 90" from each other. [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

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Meanwhile, John Blewett showed that alternating electric field gradients had the same focusing properties as magnetic field gradients. Transverse electric field gradients, alternating in sign, between sets of electrodes of hyperbolic cross section in a quadrupole array, will also focus particle beams along the axis. This feature was first utilized in the “electron analog” of an alternating gradient accelerator built at Brookhaven, to test the alternating gradient (AG) principle and other design features. When the CERN delegation, consisting of Odd Dahl, Frank Goward, and Rolf Wideroe, arrived in Brookhaven, the AG concept had been developed sufficiently to be presented to them as a significant improvement of the cosmotron design. They were sufficiently impressed with the potentialities to plan for studies in British and European laboratories on their return. However, they were planning to go on to Berkeley to complete their tour, and we realized that we had not yet informed the Berkeley group of the AG developments. So Leland Haworth, assistant director of Brookhaven, made a long-distance telephone call to Edwin McMillan at Berkeley to inform them of the Brookhaven developments before the CERN delegation arrived. We later learned that the Berkeley staff were in the embarrassing position of being unable, owing to security classification, to describe their own developments of focusing in the cyclotron by azimuthally varying fields. The use of “sector focusing” in cyclotrons has since led to the development of a category of high-intensity “isochronous” cyclotrons in the energy range up to 500 MeV. By the time these two lines of development merged a few years later, it was evident that sector focusing was a special case of the general theory of AG focusing, but applied to constant magnetic fields. So it seems that the Berkeley group had been working on a type of AG focusing at that time, but were unable to describe it in the open literature until it was declassified in 1956. Most of the developments at Brookhaven occurred within a few weeks time and involved primarily four staff members. The first report was sent to the Physical Review on August 21 and was published (101) in the December 1, 1952 issue, presented by E. D. Courant, M. S. Livingston, and H. S. Snyder. A companion paper by J. P. Blewett (102) described the parallel case of electric field AG focusing. The next major step was reported by Adams, Hine, and Lawson (103) of the Harwell Atomic Energy Research Establishment in England early the next year. They identified and studied the problem of orbital resonances which might threaten orbit stability. For example, if the frequency of the transverse (betatron) oscillations were an integral multiple of the orbital frequency, the effect of even a very small orbit perturbation could be cumulative and could build

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up to disastrous amplitudes. To a lesser extent the same is true of halfintegral and other subintegral resonances. This required the avoidance of such resonances in the design of the pole faces, and resulted in much care being required in design and in the control systems. It became a difficult but acceptable problem in the design of high-energy AG accelerators. As frequently happens, this concept was developed independently elsewhere. N. C. Christophilos, an electrical engineer of American birth, educated and working in Athens, had been studying accelerators as a hobby for some years. An unpublished report of his dated 1950 presents the concept of AG focusing, and he also applied for U.S. and European patents. A copy of his report was privately transmitted to the Berkeley Radiation Laboratory, but was not given serious consideration at that time. After the Brookhaven publication in 1952, Christophilos came to the United States and demonstrated his priority. This was recognized by a brief note published by the Brookhaven group (104) in 1953. Christophilos joined the staff of the Brookhaven and later the Livermore laboratories, where he continued his speculative designing of accelerators and other devices. B. Alternuting Gradient Proton Synchrotrons

Design studies of high-energy proton synchrotrons using AG magnets started in 1953 at Brookhaven and in the CERN design group, in the energy range of 25 to 3 0 GeV. Since magnets were so much smaller for AG systems than for standard synchrotrons, they could be built for larger orbits and higher energies. At CERN this came at just the right time to provide a really high energy machine for their new joint laboratory. At Brookhaven design started promptly but construction plans were not formally supported by the AEC until late 1953. In the interim, a “quickie” design study was started at M.I.T. under my direction, with a staff of only three to four people. This resulted in an M.I.T. publication (105) in June, 1953, entitled “Design Study for a 15 GeV Accelerator,” which was the earliest relatively complete design study. When Brookhaven received support for their study and authorization for construction of a 30-GeV machine, this effort at M.I.T. was transferred to the design of an electron AG synchrotron, reported in the following section. The Brookhaven and CERN design groups cooperated closely, with exchange of design data and personnel. As a result the machines have striking similarities. Intensive theoretical efforts at both places were concerned with the results of magnet misalignments and with beam behavior at “phase transition,” where the stable accelerating phase shifts from a rising to a falling location on the rf wave. At Brookhaven a decision was

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made to build first an electron analog to study the performance of AG focusing with electric fields and the phase transition phenomenon; this delayed the start of detailed design on the proton machine for some time. At CERN they went directly ahead on detailed design, justified when the Brookhaven “analog” results supported the most optimistic theoretical predictions. The CERN group was initially under the direction of 0. Dahl of Norway and F. Goward of England. Later, Dahl returned to Norway and, following Goward’s untimely death, direction of the design study passed to J. B. Adams of England and C. Schmelzer of Germany. The design group soon moved into the new laboratory site outside Geneva, and construction of the circular tunnel to house the magnets, and the other buildings of the complex, were started promptly. Their progress was reported in the CERN Symposium (106) of 1956, and in a sequence of CERN Reports and International Accelerator conferences (107). The CERN Proton Synchrotron (CPS) was brought into operation at 26 GeV in 1959 (Figs. 27 and 28). It has had a long and very impressive life as the major research

FIG. 27. Section of one type of pole face from CERN alternating gradient proton synchrotron. [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

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FIG. 28. Photograph of a short section of the CERN proton synchrotron tunnel. (CERN photo.) [From Livingston (39). Reprinted by permission of Harvard Univ. Press.]

instrument in the fields of nuclear physics and particle physics for scientists from the many countries in Europe which support CERN, as well as for visting scientists from the United States and the Soviet Union. The Brookhaven design group was headed by L. J. Haworth, G. K. Green, and J. P. Blewett. It was designed with slightly larger components and operates at energies up to 33 GeV. It was completed in July, 1960, and for many years held the record as the highest energy accelerator in operation and supported an important research program. In both laboratories the beam intensities have been increased by continuous development to far surpass the designer’s estimates. The use of multiple targets and emergent beams has broadened the capabilities and effectiveness for research support. The parameters of the Brookhaven and CERN proton synchrotrons are given in Table V. The most important application of AG focusing has been to these proton synchrotrons. The principle provided a major step upward in accelerator energy and was the stimulus for the rapid development of the field of high-energy particle physics. At CERN the CPS ultimately operated at 28 GeV, nearly three times the energy originally planned. At Brookhaven the 33-GeV AG synchrotron (AGS) gave a young laboratory the impetus to become agreat one. The U.S.S.R. followed the trend and in the late 1960s

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OF PARAMETERS

THE

TABLE V BROOKHAVEN A N D CERN AG PROTON SYNCHROTRONS

Pararneter Machine radius Injection energy Phase transition energy Final energy Aperture (vert. x horiz.) Number of superperiods Number of gradient reversals ( N ) Field index ( n ) Number of free oscillations in circumference

Brookhaven

CERN

421 ft 50 MeV 7 GeV 32 GeV 3 x 6 in.

100 rn (328 ft) 50 MeV 5 GeV 26 GeV E x 12crn

12 120

10

360

100 282

8.75

6.25

a laboratory at Serpukhov was built around an AGS of 70 GeV energy. The AG principle has completely changed the basic designs for the magnets used for synchrotrons. Particle orbits can be retained between much smaller magnet poles and within smaller vacuum chambers. It became economically practical to design synchrotrons for much higher energies. However, the major problem is still that, of cost, which is now nearly proportional to energy. Many working in the field of AG accelerators were aware that magnet systems could be built in which the magnet field did not vary with time but was fixed. Such systems are called fixed-field AG, or FFAG. The Brookhaven design group described above considered it briefly in 1952, but concentrated on pulsed-field concepts with which higher energies could be obtained. There were several who pursued the fixed-field ideas in universities in the American Middle West, and who were involved with a group called Midwestern Universities Research Association (MURA) whose function was to recommend an accelerator for scientists of that area. A paper (J08) by K. R. Symon, D. W. Kerst, and others described their early interests. A study group started as early as 1954 to consider FFAG systems, supported initially by the universities and later by Federal funds. By 1957 the MURA group occupied a laboratory in Madison, sponsored by the University of Wisconsin. An enthusiastic and talented group assembled and made many contributions to accelerator design, including a radial-sector FFAG synchrotron, a spiral-sector FFAG synchrotron, a spiral-sector AG cyclotron, an FFAG betatron, and several others. They were a major catalyst in initiating the Argonne 12.5-GeV ZGS project, 'described previously. Although this group did not itself build a major accelerator, they strongly influenced the accelerator field and added much to its technology. Their concepts were of most value in the types of accel-

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erators where high intensity is of major importance. For example, the spiral-sector cyclotron became the basis for most modern sector-focused cyclotrons, as will be described in a section to follow. C . Alternating Gradient Electron Synchrotrons

The first working accelerator to use the AG focusing principle was a 1-GeV electron synchrotron being constructed at Cornell University by R. R. Wilson at the time the AG principle was announced. Wilson chose to substitute pole tips designed for AG focusing for the replaceable pole tips originally planned. By late 1953 the Cornell machine was operating at 1 GeV energy. Although the use of AG focusing did not result in higher energies in this application, the reduced oscillation amplitudes provided the equivalent of larger beam aperture and resulted in somewhat higher intensities. The first sizable AG electron synchrotron designed specifically to utilize the small magnets and small apertures possible with strong focusing was the Cambridge Electron Accelerator (CEA) (109), a joint project of Harvard and M.I.T., supervised by the writer who was director of the project from 1956 to 1967. The CEA was designed for 6 GeV energy, and was supported by the AEC. During the planning years of 1952-1956, we took advantage of the newly discovered principle of focusing to design what was in those years a very high energy electron machine. Electrons were chosen because Brookhaven was building a record-making proton AG synchrotron, and it was located at Harvard University because the AEC had recently made a policy decision to support accelerators in some of the larger universities in addition to the national laboratories. The CEA was the first multi-GeV electron accelerator, and held the energy record for electrons from its completion in 1962 until the SLAC linac at Stanford came into operation in 1966. The CEA had many unique design features. Long before it was completed the growing interest in electron and photon physics made it the model for accelerator programs in other laboratories. It took full advantage of the small apertures possible with AG focusing, so its magnets were small, compact, and relatively inexpensive. The magnet cycling rate was 60 Hz, achieved with a unique resonant powering circuit at a modest power level. The magnet cores were laminated as in a transformer and bonded into block units for installation. An unusually high radio frequency was used for acceleration, driving a set of resonant cavities spaced around the orbit, each with its power oscillator unit mounted above. One of the problems of electron synchrotrons is the radiated power loss due to “synchrotron radiation;” the rf power supplies were

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adequate to provide these losses as well as to accelerate the electrons. The accelerated beams were reduced to such small dimensions by radiation damping that they could be ejected and focused onto external targets on a spot a few millimeters diameter, a tremendous advantage experimentally. The parameters of the CEA are given in Table VI.

PARAMETERS OF

THE

TABLE V1 CAMBRIDGE ELECTRONACCELERATOR

Energy Orbit diameter Vacuum chamber size Injection energy Magnetic field, maximum Magnet gap at orbit Number magnet units Core lamination thickness Radio frequency Volts/turn for acceleration Volts/turn for radiation loss

6.0 GeV 236. ft 1.5 x 6.0 in. x in. 25.0 MeV 7600 G 2.0 in. 48 0.014 in. 476 MHz 6.0 MeV/turn 4.5 MeV/turn

The official “letter of intent” contract between the AEC and Harvard University for funds to construct the accelerator was signed on April 2, 1956. This can be called the official starting date, although design studies had been going on for three years previously. The machine was completed and research activities started in late 1962 (Fig. 29). The total construction cost of machine and laboratory was under $12 million; which is just $0.002/volt. Other AG electron synchrotrons patterned on the CEA were 1. The h u t c h e s Elektronen Synchrotron (DESY) at Hamburg, designed for 7.0 GeV. They sent several staff members to CEA to learn by doing during the final stages of construction of CEA. DESY was completed in 1965 and has been the nucleus around which a major research laboratory has been developed. 2. The “NINA” machine at the Daresbury Laboratory (near Liverpool), which was designed for and operates at 4.5 GeV; it was completed in 1967. 3. The Physical Institute of the Armenian Academy of Sciences in Erevan built a 6.5-GeV machine based on the CEA called “ARUS,” which was completed in 1967. This laboratory also sent staff members to CEA to learn by doing.

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FIG.29. Cambridge electron accelerator installed in underground tunnel. Note that the two shapes of AG pole faces alternate from sector to sector. Also note the waveguides and rf cavities for acceleration. [From Livingston (39).Reprinted by permission of Harvard Univ. Press.]

4. A large-orbit, somewhat simplified AG synchrotron rated for 10

GeV was built at Cornell University and brought into operation in 1967.

So the CEA has been the prototype for a series of electron synchrotrons throughout the world. Its staff members have also earned credit from the accelerator fraternity by developing one of the first systems to accelerate positrons simultaneously (the other way around) in the synchrotron, and to bring out both beams into an interaction region for colliding beam studies. The use of strong focusing quadrupole lenses in these emergent beams to provide a “low beta” pinch to focus the beams to small size at the interaction region, was another first which has been copied and amplified elsewhere. Unfortunately, failure of AEC funding due to government policy changes in 1968 forced the CEA to close. In accordance with the original contract the machine has been dismantled and its components distributed to other laboratories.

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D. Sector-Focused Cyclotrons Bethe and Rose ( I 10) in 1937 first described the upper limit of energy to be expected in the standard cyclotron due to the relativistic increase in mass of the ions with increasing energy. The increased mass causes a decrease in the resonance frequency, and the ions drop out of phase with the accelerating rf field. In order to compensate, the magnetic field would have to increase in the outer regions, which would result in axial defocusing of the ions. Bethe and Rose estimated the energy limit using certain assumptions for the applied D-voltage (much too low), to be 12 MeV for protons and 17 MeV for deuterons. This paper drew a strong reaction from Lawrence at Berkeley, who was at that time planning a cyclotron of much larger size (184411. pole face) with an expected energy far above Bethe’s estimate of the limit, and considered it a threat to his plans for promotion of the new venture. However, before the 184411. cyclotron was completed World War I1 intervened and caused a postponement. At the end of the war McMillan discovered the principle of synchronous stability. So the 184411. was completed as a synchrocyclotron which compensates for the relativistic energy by modulation of frequency. This was a fortunate resolution of Lawrence’s dilemma. In fact, larger standard cyclotrons were built in later years using higher power sources of radio frequency and producing much higher Dvoltages, and energies were reached higher than those predicted by Bethe and Rose. However, there is a practical upper energy limit to the standard cyclotron due to this phenomenon. None have exceeded about 25 MeV for protons or 30 MeV for deuterons. Another response to Bethe and Rose’s article was a proposal by L. H. Thomas (111) suggesting a technique for correcting the defocusing effect on particles that exceed the relativistic energy limit, thus allowing higher energies to be attained. Thomas’s concept used radial sectors of iron on the pole tips which produce alternate higher and lower magnetic fields around a circular orbit. These alternations provide additional axial focusing forces, as shown in Thomas’s paper. This paper was not appreciated by accelerator designers in those days, who had their hands full of other technical problems. The concept was revived ten years later by scientists working in the war effort, as a technique for producing highenergy, high-intensity particle beams, but the work done was kept secret for security reasons at that time. As World War I1 ended and scientists returned to their laboratories new ideas were available to accelerator designers. McMillan (63) and also

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Veksler (64) discovered the principle of synchronous acceleration, and new technical devices such as high-frequency power sources were awaiting application. Much of this story has been told in the earlier sections. McMillan, and others, started building synchrotrons. Alvarez applied higher radio frequency to linacs. And Richardson and others in the Berkeley Radiation Laboratory experimented with the old 37-in. cyclotron to test the principles of frequency modulation on the cyclotron. This last result made it possible to complete the 184-in. magnet as a synchrocyclotron and to produce much higher energy particles. To be sure, the output beam intensities of synchrotrons and synchrocyclotrons were low, seldom exceeding 1 p A average. But the much higher particle energies available more than compensated for the low intensity in research laboratories. So there was little or no pressure to revive Thomas's suggestion for extending the energy range of the standard cyclotron. The azimuthally varying field concept was revived in 1949 at the wartime laboratories of the University of California and also at the Oak Ridge National Laboratory, under the stimulus of potential applications of high-intensity, high-energy particle beams for the production of fissionable material. The useful feature was the very much higher beam intensity possible with continuous operation of a cyclotron, as much as 1000 times greater than frequency-modulated synchrocyclotrons were able to produce. These studies were carried out under security restrictions, and progress reports were classified. They became known to other laboratories only after declassification of this field of investigation in 1955. The first report published in the open literature was by Kelly, Pyle, Thornton, Richardson, and Wright (112) of the University of California Radiation Laboratory, in 1956. This paper described two electron analogs built to test the principles of focusing and resonance stability. In following years a sequence of sector-focused cyclotrons were built, both at Berkeley and at Oak Ridge. As the techniques of the FFAG spiral sector cyclotron developed at MURA were merged with the radial sector Thomas cyclotron, the latter was recognized as an independent development of a special case of the same FFAG principle. A result is that all modern isochronous cyclotrons are equipped with spiral sector pole faces to provide the focusing. Several new scientific research fields for both light ions and heavy ions, for which high-intensity beams are important, have utilized these machines. A sequence of conferences on sectorfocused cyclotrons was organized by the Oak Ridge staff starting at Sea Island, Georgia, in 1959, and reported in a National Academy of Sciences publication (113). Successive conferences in this field were held in 1962, 1963, and 1966.

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E . Separated Function Proton Synchrotrons The 500-GeV AG proton synchrotron at the Fermi National Accelerator Laboratory in Batavia, Illinois (near Chicago), and the 400-GeV SPS (Super Proton Synchrotron) at the CERN Laboratory near Geneva, are examples of the latest developments of the proton synchrotron. They are AG machines, but do not use successive magnet sectors with alternating gradients like the 33-GeV AGS at Brookhaven or the 28-GeV PS at CERN. Rather, the orbits are filled with two types of magnet, one which has a dipole field and provides the bending, and another which consists of quadrupole lenses to provide the focusing. This separation of function is possible because the energies are so great, and the orbits so large, that the deflection in traversing one magnet unit is almost negligible. The simplified function of the bending magnets means that they have parallel poles (with preshaped faces to give the maximum width of uniform field) and can have a small cross section and be of simple construction. The quadrupole magnets can also have small cross section and be built to a simple and uniform pattern. The concept of separated function magnets provided, almost demanded, a large leap upward in energy, by nearly a factor of 10. Such a large step in energy had seldom been achieved during the earlier history of accelerator development. The orbit circle for the Fermilab machine is nearly 1s miles in diameter, and to a person in the machine tunnel, it seems almost straight. Many of the machine features are more like a linac than a synchrotron, with the exception of the resonance feature in which the particles make thousands of revolutions to achieve maximum energy. The transverse dimensions of the magnets, vacuum chamber, and so on do not depend on energy, but are essentially constant, as in a linac. So cost varies essentially linearly with the size of the orbit, or with the maximum energy. Strong focusing is provided by the alternating gradients of the quadrupole lenses, spaced around the orbit in a pattern which limits particle oscillation amplitudes and provides momentum compaction. At the Fermilab the orbit has six long, straight sections for injection, rf acceleration, and ejection of beams. So the AG pattern is arranged in six superperiods each of which is matched to parallel beams at each straight section. Within each superperiod a pattern of focusing (Fj, defocusing (D), and bending magnet lengths (0) is designed to produce the desired amplitudes and directions at crucial points around the closed orbit. For example, F D F triplets between 0 sectors make the beams narrow in the vertical and wider in the horizontal plane, which is reversed for DFD triplets. So alternate bending magnets have either short (and wide) gaps or longer (and narrower) gaps. This re-

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quires two sets of bending magnets with slightly different pole face shapes. The simplicity of construction of the magnets helps reduce costs. The steel is in laminated sheets punched to the desired pole-face shape, assembled on a mandrel, and welded in strips along the entire length of the magnet unit (20 ft) so it is self-supporting. Excitation coils are of stranded wire, wound in forms and embedded in plastic with cooling coils interspersed. Quadrupoles are formed in similar fashion, with steel cores assembled first in two halves, then the coils are inserted and the units are welded. The magnet excitation cycle has a basic Csec rise time during which the particles are injected and accelerated, a variable flattop during which beams are extracted over an extended period, and a 2-sec fall. The power requirement at maximum field is so large that no electrical storage system such as the flywheels used at Brookhaven can be used. The power comes directly from the electrical power grid covering some six states, and the unused portion of the stored energy in the magnets is restored to the grid during the 2-sec fall. Three accelerators are used in sequence for preacceleration to obtain the 7 GeV energy needed for injection into the large magnet ring. The first is a commercial Cockcroft -Walton voltage multiplier which accelerates a long pulse of protons from an ion source to 0.75 MeV. The second is a 200-MeV Alvarez-type linac about 400 ft long, consisting of six tanks formed of copper-plated steel enclosing copper cylinders of increasing length. The output from the linac is injected into a fast-cycling synchrotron which accelerates the beam to 7 GeV. About 12 successive cycles from this “booster” synchrotron are injected into the main ring, so they fill the entire circle end-to-end during one turn. Filling the full 44 mile orbit of the large machine makes it possible to store very high beam intensities for acceleration. It is hard to overstate the scientific importance of the results obtained with these giant machines. A new dimension in knowledge of the structure of matter has been reached and surpassed. The opportunities to do research with this powerful machine have attracted scientists as users from all over the United States and from many foreign countries including the U.S.S.R. The work involved in an experiment is tremendous and must be split among many scientists. One of the consequences is that many papers are published with 10,20, or even 30 scientific collaboraors on the title page. The number of users is great, but the scientific results are even greater. A major new development in technology, occurring during the years while the Fermilab machine was being designed and built, is the use of su-

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perconductivity to produce higher magnetic fields. The Fermilab bending magnets will produce fields up to 21,000 G, or 2.1 T in the new terminology, which is sufficient to contain 500-GeV protons in the orbit. Developments of superconducting magnets have resulted in twice this field strength, which will allow the Fermilab orbit (if such superconducting magnets are used) to contain protons of 1000 GeV. An iron enclosure is used around these new magnets to contain the flux and prevent stray fields from causing interference with other beams or equipment. These units are located just beneath the existing magnet units, threading through the supports. A sector containing 25 such units has been installed and tested. This “energy doubler” is one of the long-range goals of the planners at Fermilab. The story of Fermilab would not be complete without describing the importance to the development of the director, Robert R. Wilson. A professor of physics at Cornell, Wilson had a considerable record of accomplishments to his credit before being chosen to head the National Accelerator Laboratory (NAL). One aspect of his experience had been to build a very high energy electron synchrotron at Cornell with a minimum of money, using designs that simplified construction, reduced size, and lowered costs. He took this experience with him to NAL, where his original assignment (and the AEC allocation) was aimed at construction of a 200-GeV proton synchrotron. The simplified magnets and other components designed and built under his leadership were much less costly than preliminary concepts. The machine was completed ahead of schedule, operating at 200 GeV by 1973, and was well within budget. Furthermore, the simple construction also provided a large engineering factor of safety, so when suitable changes were made in the powering system and in other components, it was capable of operations at 300 GeV, then 400 GeV, and ultimately at 500 MeV. This astonishing result was due largely to the inspiring leadership of the director, Bob Wilson. IX. STORAGE RINGSA N D COLLIDING BEAMS The wave of the future in the field of particle physics is in the use of colliding beams of particles contained in storage rings. Managements of laboratories with the biggest fixed-target accelerators are all planning colliding beam projects, both for electron-positron systems and for proton-proton systems. Reasons for this change in emphasis are not hard to find. The most massive new particle yet discovered, the upsilon with a mass of over 9 proton masses, was found at the Fermilab using 400-GeV protons on a fixed target. Only a few months later, the staff at the DESY

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laboratory operating the electron-positron colliding beam machine DORIS, beefed up some of the magnets with improved units so they could operate at 10 GeV energy, and also observed the upsilon particle. This illustrates the energy advantage gained by using head-on collisions of equal-mass particles rather than fixed targets. Future studies of supermassive particles will surely come from such colliding beam systems. The purpose of this article -to describe early particle acceleratorshas already been accomplished. It is not proper to extend the discussion into the many forms, shapes, and purposes of modern-day accelerators. But with this start it seems only reasonable to give a brief view of the most significant and largest scale recent development, that of storage rings and colliding beams. Earlier, we made reference to the greatly increased energy available for nuclear excitation using the target system of head-on collisions rather than fixed targets. It seems that a brief description of the origins and some of the present developments in this field is justified. An electron -positron collision is a matter-antimatter annihilation process, in which the total energy including the mass-energy can turn into anything having a mass up to this maximum value if it follows fundamental laws. This fact has been known to physicists for years. The first installation built to study the process was “ADONE” in Frascati, started in 1967. (No one remembers for long what such acronyms stand for, but each laboratory finds it useful to coin some nickname for their project.) ADONE did some pioneering work on very high vacuum techniques, including metal seals and bake-out of the vacuum chambers at high temperature, and on other problems involved in making a storage ring operate with a long beam lifetime. Their magnet ring was designed for 3- to 4-GeV electrons and positrons. Other labs joined in the development. At Harvard, the CEA was adapted to produce positrons with the electron beam from an injection linac and to accelerate them simultaneously with electrons in the opposite direction in the same vacuum chamber. Development continued until both beams were extracted and focused to intersect each other in a “bypass” external to the orbit. Detection equipment could be assembled around the intersection. A second generation, of much improved design, started in the early 1970s and included DORIS at Hamburg, for 7-GeV electrons and positrons, contained in a separate storage ring. A similar 6-GeV ring was built at Erevan in the U.S.S.R. to use the 6-GeV electrons produced in their AG electron synchrotron. Another project is SPEAR at Stanford, in which electrons and positrons are produced in a target about one-third the way down the SLAC 20-GeV linac, producing electrons and positrons of 2-4 GeV each, which go two ways around a single storage ring. Now under development is the third generation of electron-positron

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storage rings for colliding beams. At Hamburg, DORIS is to be superseded by PETRA, for 20 GeV per beam. And at SLAC, PEP is to produce 20 GeV per beam in a single storage ring being developed for an injection energy of 8 GeV per beam. Studies for a very large electron-positron (LEP) machine began at CERN in 1976, along with a higher energy proton-proton collider. Interest eventually concentrated on the electron-positron option, with an energy of 100 GeV per beam. The European Committee for Future Accelerators is recommending this as the prime candidate for a major European project for the 1980s. The total cost of the first stage of the LEP project is estimated as 1050 million Swiss francs. The location is not decided, with a fair possibility that it will not be at CERN. Proton -proton storage rings also required much development, primarily to achieve adequate high vacuums for long beam lifetimes. An important part of this development was done at CERN, starting in about 1968, where Kjell Johnsen headed a group building the Intersecting Storage Rings (IRS). Two identical rings of AG magnets arranged to intersect at two points were fed protons from the 28-GeV proton synchrotron, at energies up to 18 GeV. The beams circulate in opposite directions around the two rings and intersect at a small angle at six locations, around which the detection instrumentation is assembled. This machine started operation in 1971 and has had a long and profitable life as a research tool; the staff recently celebrated its 1000th run for experiments. The biggest proton -proton storage ring project actually under way is Brookhaven’s ISABELLE, designed for 400 GeV-400 GeV protons. The rings will be filled with protons from the AGS at 33 GeV energy, and then the ring will be operated as a synchrotron using rf drivers to raise the energy with increasing magnet excitation, to raise the stored beam energies to maximum, at which the beams will be made to intersect for experiments. The magnet ring will be formed of six sectors, with six straight sections where beam interactions can be studied. The estimated cost of the project is $275 million, and construction started in February 1979. It is obviously desirable to accelerate and store antiprotons, so they can be used in matter-antimatter interactions. The difficulties are tremendous, and success is not just around the corner. The problem is to make a large number of antiprotons and store them in an orbit at a common energy, and to collect them into a condensed beam. An early experimental approach toward condensing the beam was “electron cooling” suggested by G. I. Budker of the Novosibrisk laboratory of the Soviet Academy of Sciences. This technique sends a collimated beam of electrons along a parallel path which causes mingling with the antiproton beam; mutual interactions reduce transverse amplitudes of the anti-

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