Recent Advances in Particle Accelerators

Recent Advances in Particle Accelerators JOHN P. BLEWETT Brookhaven National Laboratory. Upton. New York

I . Introduction . . . . . . . ............................................... 233 I1. Electron Ring Accele s .............................................. 234 I11. Heavy Ion Accelerators ................................................ 236 IV. Superconducting Accelerators ........................................... 238 A . Accelerators Using Superconducting Magnets .......................... 238 B . Accelerators Using Superconducting rf Cavities ........................ 240 V . Proton Synchrotons .................................................... 240 A . Serpukhov ......................................................... 240 B The National Accelerator Laboratory .................................. 241 C . The European 300-GeV Project ...... .............................. 243 D. Design Studies for Proton Synchroton ..................... E . Proton Synchroton Improvement Programs . . . . . . . . . . . . . . . . . . . VI . Electron Accelerators .................. ............................ 247 A . SLAC............................................................. 247 B. Cornell ............................................................ 247 C. Other Electron Synchrotrons ......................................... 248 VII . Storage Rings and Colliding Beams ...................................... 248 A . The CERN Intersecting Storage Rings ................................. 248 B . The Novosibirsk Proton-Antiproton Ring ............................. 249 C . Colliding Beam Studies at the National Accelerator Laboratory .......... 250 D . Electron Storage Rings .............................................. 250 V I E Cyclotrons ............................................................ 252 IX . Meson Factories ....................................................... 253 X . R.1.P ................................................................. 254 XI . Conclusion ........................................................... 255 References .......................................................... 256

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I . INTRODUCTION At national and international accelerator conferences held in 1967. 1968. and 1969 three new topics seemed to stand out in the interest and excitement that surrounded their discussion. First was the concept of the electron ring accelerator announced by the group of the late Veksler in Dubna and quickly adopted in several centers. This appears to be a successful culmination of many years of attempts to accelerate positive ions by the high electric fields that can be generated in bunches of electrons. Second. the idea that there 233

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may be islands of stability at masses much higher than those in the standard periodic table began to be accepted, and many centers have proposed a variety of heavy-ion accelerators to explore this possibility. Finally, there has been notable progress in the design and construction of superconducting magnets for pulsed operation at high fields. Speculation is now rife regarding the possible design of synchrotrons using these magnets. Parallel progress on superconducting cavities for use in linear accelerators gives promise of important improvement in the duty cycle of electron linacs. In the meantime, a number of new machines of more conventional design are under construction or have been completed. In this article, we shall attempt a review both of the new ideas and of progress on major accelerators throughout the world. RINGACCELERATORS rr. ELECTRON The minimum overall length of linear accelerators for high energies has, in the past, been set by the maximum electric field that could be maintained without breakdown between parts of the accelerating structure. It has long been evident that, in principle, much higher fields could be maintained within a dense bunch of electrons or ions; the unsolved problem was that of preventing the bunch from blowing up in its own fields. It now appears that Veksler and his associates have solved this problem. The solution was announced at the Sixth International Accelerator Conference in Cambridge, Massachusetts in 1967 ( I ) . The procedure proposed by the Dubna group is illustrated in Fig. 1. In a rather weak magnetic field, falling off with radius at a rate slower than I/r, a ring of electrons is injected at an energy of a few million electron volts. This ring is stable for the same reasons that the beam is stable in a betatron. The field is now increased by a factor of 20 or so and the ring shrinks both in major and minor radius. Due to the electric field induced by the changing

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magnetic flux, the electrons are accelerated at the same time. Provided only that enough electrons have been injected, the fields in the ring can now be much higher than can be maintained between electrodes. Typical parameters for this process might be: Initially--10'4 electrons at 4.5 MeV injected into a field of 750 G to produce a ring of 20-cm major radius and 5-mm minor radius. After compression by raising the field to 20 kG-the ring now has a major radius of 3.7 cm and a minor radius of 1 mm.

In the compressed ring the maximum electric field will be 1300 MVjm. This is to be compared with the maximum field of about I5 MV/m contemplated in the Stanford Linear Accelerator Center (SLAC) two-mile linac. With the increase in electron energy, the space-charge forces tending to blow the ring apart are decreased by the usual relativistic factor of (1 - p'). A burst of gas is now admitted into the chamber. The gas is ionized by the electrons and some ions are trapped inside the ring. The number of ions trapped should be small (perhaps 1 %) compared with the number of electrons in the ring. These ions will now make the ring completely stable even without the focusing action of the shaped magnetic field. As indicated in Fig. I , the magnetic field can now be varied in such a fashion as to allow the ring to drift along its axis into a region of smaller field. The circumferential momentum of the electrons will be partially converted into axial momentum; as the ring moves, it will carry the trapped ions with it and they will be accelerated to the same axial velocity as that achieved by the electrons. By this procedure it appears that proton energies of the order of 1 GeV should easily be attainable in a drift distance of a few meters. If still higher ion energies are desired, the electron ring can be accelerated in a radio frequency (rf) system like that in an electron linear accelerator. This procedure has its problems since only one large bunch is accelerated at a time. An alternative procedure has been proposed at Berkeley; acceleration here is accomplished by the fields, when a radial transmission line is discharged to provide a roughly square wave of accelerating field lasting a few nanoseconds. The Dubna proposal has been accepted with some enthusiasm at the Lawrence Radiation Laboratory, and an experimental program there is now well advanced. A symposium on electron ring accelerators was held at Berkeley in February of 1968. Its proceedings (2) are a useful reference on the subject. Work on electron ring accelerators is now in progress at several other centers including the University of Maryland, the Rutherford Laboratory, CERN, Munich, and Karlsruhe. At the 1969 International Accelerator Conference held in Erevan in the USSR (3), successful production and compression of electron rings was

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announced by the Berkeley and Dubna groups. Also, at Dubna, a ring containing triply ionized nitrogen ions has been allowed to escape and expand; the nitrogen ions were accelerated to about 4 MeV per nucleon. Nothing has yet been done about acceleration of electron rings by applied rf or pulsed electric fields. A theoretical controversy has arisen about how radiation losses of energy of electron rings passing through acceleration systems vary with electron energy. Possibly, these losses will set an upper limit to acceleration. However, these losses may be reduced by a system of acceleration using a smooth, dielectric-loaded waveguide, as proposed by Schopper of Karlsruhe (3). Whether or not the electron ring accelerator will be a cheap multigiga electron-volt machine for proton acceleration is not yet clear. Its duty cycle will be very small, but for many applications this does not matter. Probably, it will be useful for the acceleration of heavy ions. Work carried out during 1970 and 1971 probably will lead to a better evaluation. In the meantime, the ideas of Veksler and his associates seem to be the most exciting contribution in the field of particle acceleration of the past decade. Review articles describing the present (late 1969) status of the electron ring accelerator will appear early in 1970 in a new journal Particle Accelerators. 111. HEAVYIONACCELERATORS

I n the past, research on transuranium elements has been concentrated in the laboratories of Seaborg and Ghiorso in Berkeley and of Flerov at Dubna. Recently, however, world-wide interest has flared up in the possible production of very heavy elements, which theoretical studies during the past decade ( 4 ) have indicated might possibly be stable. The goal of most of the enthusiasts is production of ions of all masses up to uranium with energies of 6 to 10 MeV:nucleon. Heavy ion accelerators are, of course, useful for many other purposes than production of transuranic elements. Experiments on nuclear reactions and Coulomb excitation of nuclei have been i n progress since about 1940, using the lighter heavy ions of such elements as carbon and nitrogen. These ions were accelerated in cylotrons, linear accelerators, or electrostatic accelerators. The catalog of heavy ion machines now in operation includes four linear accelerators capable of accelerating ions of mass up to argon (:gAr) up to 10 M.=V/nucleon,three classical cyclotrons of which the largest, at Dubna, can accelerate ions up to :GZn to over 6 MeVinucleon, four isochronous cyclotrons, and a large number of tandem Van de Graaff accelerators. These machines all have different mechanisms setting upper limits to the mass of the ion that can be accelerated to any desired energy. These limitations depend on the ratio of Q to A , the number of electronic charges borne by

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the ion and the weight of the nucleus in nucleons, respectively. The linear accelerator is designed to accelerate ions to a given velocity and, hence, to a constant energy per nucleon. As Q / A decreases from unity (for protons) to the attainable values in the neighborhood of 0.15 for very heavy ions, the accelerating field must be increased until limited by electrical breakdown. Cyclotrons are characterized by the relation : energy per nucleon

=

Tp( Q / A ) ’ ,

where T, is the peak energy to which protons can be accelerated. Thus, a proton cyclotron capable of proton acceleration to 300 MeV can accelerate heavy ions having Q / A of 0.15 to about 7.MeV/nucleon. Electrostatic accelerators have terminals at constant potential; if the ion source is in the terminal, ions will be accelerated to energies per nucleon of @ / A ) times the terminal voltage. In a tandem machine, negative ions with a single charge can be accelerated from ground to the terminal and there stripped in a gas jet or a foil to a high positive state of ionization; acceleration back to ground will yield ions of energy [(I + Q ) / A ] times the terminal voltage. The advantage of this method is that higher values of Q can be achieved by stripping at high energy than can be reached at low energy in an ion source. In most of the numerous proposals now under consideration in the United States, a cyclotron is proposed as the final stage. To provide a high state of ionization, a tandem Van de Graaff is proposed as the injector. At Oak Ridge, Argonne, the University of Rochester, and other centers it is felt that the Van de Graaff machine should have an energy as high as possible; it should be either the High Voltage Engineering Corporation’s MP tandem which has a guaranteed terminal voltage of 10 MV (16 MV has been reached) or its new TU tandem with a guaranteed terminal voltage of 16 MV (and a design voltage of 20 MV). The high injection energy is chosen because the degree of ionization by stripping increases with energy. The cyclotron for the final acceleration could then be of the order of 300(Q/A)2MeV/nucleon. A dissenting voice is heard from Michigan State University where it is felt that a smaller tandem and a cyclotron for 600(Q/A)2MeV/nucleon would be cheaper for the same final result. Elsewhere linear accelerators are favored. At the Lawrence Radiation Laboratory where the heavy ion linear accelerator (Hilac) has been in use in studies of transuranium elements since 1957 a “ Hilac improvement program ” is proposed to extend the capabilities of the machine (now capable of accelerating ions of mass up to 40) to make it possible to accelerate all ions up to uranium. Ions will be injected from a high-voltage machine of 2.7 MV with a Q / A of about 0.04, accelerated to 1 MeV/nucleon, and then stripped to a Q / A of 0.15. In a final linac section, the ions will be accelerated to an energy of 8.6 MeV/nucleon.

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In Germany, a project has been under study for some years under the direction of Schmelzer in Heidelberg. This project has resulted in initiation of construction in Darmstadt of a heavy ion linear accelerator. Several local institutions are collaborating in this effort. This machine will include three different types of linear accelerator, each peculiarly suited to the velocity range where it is used. The first stage will be a modified Sloan-Lawrence machine having its drift tubes connected alternately to the inner and outer conductor of a coaxial line. The second stage will be a conventional drift-tube accelerator. The final stage includes a series of independently powered accelerating cavities. An interesting parallel study was made at Frankfurt using a helix for the accelerating unit-this is the inverse of the conventional travelingwave tube. The Darmstadt machine will bring all ions to between 6 and 10 MeV/nucleon-it is known as the UNILAC because of its universal capability. Another scheme with intriguing possibilities has been proposed in Heidelberg by Hortig. This idea depends on the fact that a heavy ion passing through a foil stripper will emerge with a higher-charge-state than will be reached after passage through a gas stripper. Ions entering a tandem Van de Graaff are stripped in a foil and accelerated to the terminal of the tandem. Here, they pass through a gas stripper. This reduces the number of charges on the ion and as it proceeds to the other end of the tandem it loses less energy than it gained on its way to the terminal. The ion is now deflected in a magnet through an angle of 180" and returned to the tandem where the same process is repeated. After a rather large number of passes through the tandem, this process should result in the attainment of very high energies. There is some question as to how many ions would survive this multiple acceleration and no one has yet made an experimental attempt to verify Hortig's predictions. At Orsay, France, a cyclotron using a Sloan-Lawrence linear accelerator was in operation at the end of 1969. This machine will not reach the very heavy ion range; ions up to about mass 80 (krypton) are accelerated to 5 to 10 MeV/nucleon. In all, over 30 laboratories are studying, actively proposing, or constructing heavy ion machines. If there are islands of stability beyond the end of the periodic table, they should be discovered during the next decade. IV. SUPERCONDUCTING ACCELERATORS A . Accelerators Using Superconducting Magnets

For more than half a century, Nature has alternately held out promises of major applications of superconductivity, and then hastily retracted them. At first, superconducting magnets for high fields could not be built because superconductivity is destroyed in a high magnetic field. With the advent,

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a decade ago, of type I1 superconductors, high-field magnets again appeared possible. However, these magnets were plagued with instabilities associated with losses when the field was turned on or changed. Parts of the magnet would “go normal” and the stored energy in the field would be dissipated in the normal region, often with destructive results, This was cured by dilution of the superconductor with such large quantities of copper that the conductor could still carry the current without instability when the superconductor became unstable. This worked nicely for large magnets for large bubble chambers, but for magnets with small apertures suitable for use in synchrotrons, the current density had been so drastically reduced in the process of stabilization that small magnets could not be designed for high fields. The probable solution of this dilemma was presented at a 6-week “ Summer Study on Superconducting Devices and Accelerators ” (5) held at Brookhaven during 1968. Smith, of the Rutherford Laboratory, building on the work of many earlier workers, proposed that cables including many strands of superconductor each having a diameter of 5 to 10 p , and with the strands transposed as in a Litz cable, should be inherently stable and should show quite tolerable losses when used under pulsed or ac conditions. The latter half of 1968 and all of 1969 have been spent in verifying these predictionsthey appear to be correct both qualitatively and quantitatively. No unpleasant new surprises have been experienced, and work is now turning toward the design of synchrotrons for energies of several hundred giga electron volts using magnets with dipole fields of 60 kG or higher. This work is in progress at Brookhaven, Berkeley, the Rutherford Laboratory, and many other centers, but it will be several years before the electrical and mechanical design problems are solved. At the present writing, however, all of these problems appear to be soluble, and it is to be expected that the next decade will see the construction of synchrotrons having superconducting bending magnets operating at fields of 60-80 kG. In a parallel, low-temperature program, studies have been made of the properties and behavior of very pure metals. Although these are not superconducting metals, this program is mentioned here since it parallels, in many ways, the program on superconducting magnets. Of the various metals suitable for use in magnets, aluminum seems to be the most promising. Aluminum, pure to about one part in lo6, has a resistivity at 4°K that is lower by a factor of about 15,000 than its resistivity at room temperature. In a magnetic field, a magnetoresistivity component appears, but saturates at reasonable fields to give an increase by a factor of about three in coil resistance. Taking refrigerator efficiency into account, an optimum operating temperature for aluminum coils appears to be between 15 and 20”K, where cooling with liquid hydrogen is possible (6). Losses in aluminum coils are, of course, purely resistive, whereas losses in

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superconducting coils are hysteretic in character (loss per cycle is constant, relatively independent of frequency). Hence, for very rapid cycling, aluminum may have advantages. For the slower cycles appropriate for very high energy machines, probably superconducting coils will be preferable. B. Accelerators Using Superconducting rf Cavities

Linear accelerators operated at room temperature require such enormous quantities of rf power, simply to make up for losses in the walls of the accelerating cavity, that they can be operated only in short pulses. The low dutycycle is very undesirable for many applications, and attempts have been made, by cooling the walls, to reduce losses. Superconducting walls now appear very promising. Two superconductors are outstanding because of their high critical magnetic fields-the rf magnetic field at the wall is proportional to the accelerating electric field. In lead, the critical field is about 800 G ; in niobium, the field is over 1600 G. In a TMolo cavity these correspond to accelerating fields of 50 and 100 MV/m. Losses are very small, but are still large enough at 4°K that it is not possible to provide adequate cooling by boiling helium outside of the cavity. Hence, it appears necessary to drop the temperature to less than 2"K, at which temperatures, it is possible to make use of the desirable properties of superfluid helium as a coolant. The leading role in development of superconducting cavities has been taken by the group under the direction of Fairbank and Schwettman at the High Energy Physics Laboratory of Stanford University. In the laboratory, this group has achieved Q of 10" factors at low levels. Electric fields as high as 70 MV/m have been observed in niobium cavities with attendant Q's of 8 x l o 9 . These figures are more than adequate to encourage the construction of an accelerator-an electron machine 150 m in length with a design energy of 2 GeV is now well advanced. Less ambitious projects using superconducting cavities will be found at Brookhaven, Karlsruhe, CERN, the University of Illinois, and a number of other centers. V. PROTON SYNCHROTRONS

A . Serpukhov In 1967, two new records were set at Serpukhov in the USSR. In June of that year, the world's largest proton linear accelerator was brought into satisfactory operation. This is the 100-MeV linac injector for the "70-GeV" proton synchrotron at Serpukhov. Four months later, on October 13th, protons were accelerated in the synchrotron to a peak energy of 76 GeVmore than twice the previously highest energy in the alternating-gradient synchrotron (AGS) at Brookhaven.

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The Serpukhov machine is conservatively designed, well built, and can be expected to continue to give a very impressive performance. It is almost 1.5 km in circumference. Its magnet system, which includes 20,000 tons of steel and 700 tons of aluminum coils, requires a peak input power of 100 MW. Its experimental areas are enormous ; they are distributed around a central hall 90 x 150 m in dimensions. By 1969, the Serpukhov accelerator was running with a peak intensity of over 10” protons/pulse with 5-10 pulses/min. A full-blown experimental program was in full swing, including several collaborative efforts with outside groups. Perhaps the most interesting cooperative arrangement is between Serpukhov and CERN. CERN has undertaken to provide a fast beam-extraction system for the 70-GeV accelerator. Also, CERN will provide rf particle separators to be used at Serpukhov for at least 10 yr. In return, CERN scientists will be allowed to take an active part in the 70-GeV experimental program, generally as members of joint CERN-Soviet teams. France also has its own collaborative arrangement to allow French scientists to join the new experimental program. In return she is providing a 6000-liter liquid hydrogen bubble chamber known as “ Mirabelle.” Mirabelle is under construction at Saclay, and received her first test in July 1969, when photographs of cosmicray showers were taken. B. The National Accelerator Laboratory The rather flamboyant search for a site for the US national 200-GeV accelerator finally was terminated in December of 1966, when the Atomic Energy Commission announced that it had chosen an area to the west of Chicago, rather close both to the Argonne Laboratory and to O’Hare Airport. The Universities Research Association, which is to manage the project, was already in being, and early in 1967, it chose Robert R. Wilson to be director of the new National Accelerator Laboratory (NAL). A design group was established near the site, and set to work in June of 1967. It soon was announced that the accelerator would be designed in such a fashion that it could later be extended to 400 GeV and, possibly, to 500 GeV. Its completion is planned for July of 1972. Ground was broken for the first building, the injector housing, in December of 1967, and construction of conventional buildings began immediately. During 1968, design and construction of prototypes proceeded apace and, by the end of 1969, many of the machine components were on order. A landmark in 1969 was acceleration of a proton beam to 10 MeV in the prototype first section of the injector. In contrast to the Brookhaven and CERN synchrotrons, where the same magnet system serves both to bend and focus the beam during acceleration, “

”

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the NAL synchrotron will have a " separated function " magnet ring. Bending of the beam into its circular orbit will be accomplished by dipole magnets with uniform fields, whereas focusing will be by separate quadrupole magnets. This idea was considered as early as 1952 for use in the Brookhaven AGS, but it has not previously been used in a major accelerator. Injection will be accomplished in three stages. A 750-keV CockcroftWalton preinjector will provide a beam for a 200-MeV linear accelerator very similar to the one under construction at Brookhaven. The linac, in turn, will be the injector for a " booster " synchrotron which will raise the proton energy to about 8 GeV before transfer into the main ring. The arrangement of the various components is shown in the plan of the National Accelerator Laboratory shown in Fig. 2. WEST CHICAGO

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It is too early to predict whether this major project can be completed within its rather ambitious schedule. However, by the end of 1969, its progress is very impressive.

C . The European 300-Ge V Project The efforts preparatory to initiation of the American 200-GeV project have been paralleled in the European CERN community of nations. A design study for a 300-GeV accelerator was organized at CERN ; this group had presented a comprehensive report and cost estimate at the end of 1964. The machine described was, in general, similar to that resulting from the American studies, except that its magnet system combined the functions of bending and focusing in the conventional fashion. In 1963, a “ European Committee for Future Accelerators (ECFA) was organized under the chairmanship of Professor Amaldi of Rome. By 1967, ECFA has surveyed the whole European program in high energy physics and presented a set of recommendations, both for international programs and for development of regional facilities. It recommended the construction of a 300-GeV accelerator “ with the least possible delay.” With the assistance of the accelerator experts at CERN, an extensive study was made of possible sites for the 300-GeV machine. Over 100 sites were investigated in 1 I European countries. Then, by the simple procedure of allowing each country to submit only one site, the number of sites was cut down to nine (some CERN countries did not submit site recommendations). A committee of “three wise men” from three of the CERN countries that did not submit sites made a careful study of the nine sites and gave them graded ratings under several related headings; the final choice was not made by this committee, however, and this was still the status at the end of 1969. Two important events occurred during 1968. In June, for financial reasons, the United Kingdom withdrew from the 300-GeV project. The decision of the British government was by no means popular with Britain’s scientists, who have made great efforts to have this decision reversed. In December of that year, the CERN Council chose a director-general for the 300-GeV program; their choice was John B. Adams, a British scientist who had directed the construction of CERN’s 28-GeV synchrotron and who, for a short time, had been Director-General of CERN. A few months later, Adams moved back to CERN and took up direction of the 300-GeV program which, by now, was somewhat attenuated as it waited for European approval. In April of 1970, the future of this project was still not clear. The members of the CERN council still had not agreed on a site for the machine, and the international debate was continuing at the ministerial level. ”

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D. Design Studies f o r Proton Synchrotrons By 1967, three design studies had been completed for proton synchrotrons in the 40- to 60-GeV range. One, in Tokyo, had receivedinterim approval by the Japanese government. Since then, it has fallen into political difficulties and, in 1969, its future was still doubtful. A second project, in France, was in the hands of a capable group at Saclay and a third, German project was centered in the Nuclear Physics Intitute at Karlsruhe. The French and German projects were uncomfortably close to each other and questions were raised in the European community about the wisdom of such parallel expenditures. In 1969, the situation was resolved by France’s fiscal troubles; the French program has been set aside indefinitely. At Karlsruhe, much enthusiasm still remains. What will become of the German machine probably depends, t o a considerable extent, on the clarification of the 300-GeV situation. Two more speculative study programs are aimed at very high energies. At the Radio Technical Institute in Moscow a study was inspired in 1960 by an abortive attempt to organize a program for an “ intercontinental accelerator ” to have an energy in the 300- to 1000-GeV range. Discussions between Soviet and American study groups were to have been held in 1961. The Radio Technical Institute study resulted in proposals for a “cybernetic accelerator ” (7) in which information derived from the proton beam was to be used automatically to apply magnetic corrections in order to restore the beam to its correct orbit. It was claimed that this could result in important reduction of machine aperture and consequently in its cost. A I-GeV model of this machine was constructed and is now in operation, performing essentially as predicted. Since the idea of more or less automatic correction of beam position from pickup electrode information is now generally accepted and since most other design groups have used this procedure to make reductions in aperture, this scheme does not, by now, appear particularly novel. The Radio Technical Institute will terminate its model study in 1970; Mints, Director of the Institute, continues to be enthusiastic regarding the practicability of a 1000-GeV accelerator and, in 1969 ( 3 ) proposed again that consideration be given to collaboration in its construction between the Soviet Union and the United States. The American effort in the abortive 1960 and 1961 intercontinental studies was centered at Brookhaven, where also it was concluded that 1000-GeV synchrotrons are practical. Low keyed efforts have continued at Brookhaven and, by 1969, became concentrated on the use of superconducting or cryogenic magnets. As has been discussed i n Section IV, problems connected with pulsing superconducting magnets have largely been solved. Bending magnets for peak fields of 60 kG or higher are under design and should be in test operation during 1970.

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Because of the possibility that the NAL 200-GeV accelerator may be extended to 400 GeV or higher, the Brookhaven study has been extended upward toward 2000 GeV. With 60-kG magnets, a 2000-GeV synchrotron would have a radius of about 1.5 km, only 50% larger than the 200-GeV accelerator. It is hoped that this reduction in size will result in a corresponding reduction in cost. To what extent this is true will be shown by the Brookhaven studies during 1970 and 1971.

E. Proton Synchrotron Improvement Programs All proton synchrotrons seem either to be in the process of being improved or preparation and submitting of such proposals is under way. Here we mention only the two major programs now in progress. Both CERN and Brookhaven are in the midst of programs aimed at increasing the intensity of their proton synchrotrons (PS). Both are raising intensity per pulse by increasing the energy of their injectors. At Brookhaven, the present 50-MeV linac injector will be replaced by a new 200-MeV linac. At CERN, injection energy will be increased by inclusion of a booster synchrotron that raises proton energy from the present 50-MeV linac t o 800 MeV. For reasons connected with the fact that the PS beam is to be used to inject into a colliding beam system (see Section VII), the booster consists of four synchrotrons stacked one above the other and each having a radius exactly one quarter of that of the PS. This rather complicated system will be located adjacent to the PS and will be half in France and half in Switzerland (see Fig. 3). Average intensity at the two machines is to be increased by increasing the cycling rate. This calls for larger power supplies for the ring magnets and for increased power for the rf accelerating stations. In both cases, the cycling rate can be approximately doubled or, alternatively, the field can be held at its peak value for long periods while the beam is slowly extracted. Both machines now have both fast and slow extracted beams, the former referring to the process of extraction during one revolution of the proton beam, the latter to extraction during periods of tens or hundreds of milliseconds. Slow extraction is the more difficult and has advanced rather slowly; however, both machines now have available slow beams with extraction efficiencies between 80 and 90 %. At both machines and at most other high energy accelerators, computers play increasingly important roles in the control room. Beam operations of great complexity can now be monitored and controlled. The difficult process of analyzing and correcting beam-position errors is now carried on at high speed with computer assistance. More and more, the computer is in " on-line " use both in machine operation and in the experimental programs.

FIG.3. CERN geography. Legend: ( I ) SC, (2) PS, (3) North Hall, (4) South Hall, (5) East Hall, ( 6 ) neutrino area, (7) ISR ring, ( 8 ) transfer tunnels, (9) interaction regions, (10) switchyard, (1 1) West Hall, (12) BEBC buildings, (13) booster, (14) central computing.

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VI. ELECTRON ACCELERATORS

A . SLAC In January of 1967, the latest in a series of historic electron linear accelerators was brought into full operation at the Stanford Linear Accelerator Center (SLAC). This is the " two-mile accelerator " designed for initial operation at 20 GeV, later to be pushed upward toward 40 GeV by increases in rf power levels. The two-mile machine was completed on schedule and within its cost estimate. This performance was capped by the preparation of a monograph of more than 1000 pages (8) detailing the history and organization of the project and describing all important details of construction. The accelerator, like previous Stanford linacs, uses iris-loaded waveguide excited at 2856 MHz. Power is supplied by 245 klystron amplifiers each capable of a pulsed output of 24 MW. The SLAC accelerator supplies electrons of the highest energy available in any laboratory in the world. Accelerated beams pass through a beam switchyard where they can be directed to an experimental area housing three spectrometers rated at 1.6, 8, and 20 GeV, to a second experimental area designed for study of secondary particles, or to a large bubble chamber. SLAC is not resting on its laurels. Close collaboration is maintained with the High Energy Physics Laboratory at Stanford where work on superconducting linacs is far advanced (see Section 1V.B). Plans are already in preparation for converting the two-mile machine to a superconducting linac. It is expected that its energy could thus be raised from 20 to 100 GeV and its duty cycle from 0.07 to 100% at 25 GeV or 6 % at 100 GeV. A further flight of fancy involves deflecting the accelerated beam, returning it to the injection point and reaccelerating to 200 GeV. Possibly, this process could be repeated more than once to give 300 or even 400 GeV. B. Cornell

Before leaving Cornell to become the Director of the NAL, R. R. Wilson presided over the completion of a 10-GeV electron synchrotron. Preliminary tests were in progress at that time. Finally, in March of 1968, the machine operated at its full energy. To keep energy loss by synchrotron radiation within controllable limits, this machine has a very large radius of 100 m. For this reason, the peak magnetic field is only 3.3 kG; this field can be provided with rather meager magnets. The machine has no vacuum chamber. The whole magnet structure including its coils is enclosed in a stainless-steel skin, which serves as the

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vacuum chamber. The accelerator is cycled at 60 Hz as is the smaller Cambridge Electron Accelerator (CEA). It should be possible, by adding to the rf accelerating system, to raise the energy of this machine t o about 15 GeV. C. Other Electron Synchrotrons

Two machines inspired by the CEA and operating in the 6- to 7-GeV range are now in full operation. The Deutsches Electronen Synchrotron (DESY) in Hamburg was first operated in 1964. Its construction was directed by W. K. Jentschke. The other machine is in the laboratory of Alikhanian i n Erevan in Soviet Armenia. Its first operation was in the summer of 1967. Both machines are, in general, similar in concept and construction to the Cambridge accelerator. A British electron synchrotron, nicknamed “ NINA,” was brought into operation at its design energy of 4.5 GeV in December of 1966. Consideration has been given to using NINA as an injector for a much larger ring to increase energy to 15 to 20 GeV. At Bonn, an electron synchrotron for 500 MeV was started in 1953 and completed in 1958 to be the first alternating-gradient synchrotron in Europe. This was replaced in 1967 by a new electron synchrotron for 2.5 GeV. The later accelerator is distinguished by the fact that its magnet was entirely designed by a computer.

VII. STORAGE RINGSAND COLLIDING BEAMS There are many serious problems associated with the construction, operation, and use of colliding beam systems. To avoid beam loss, pressures must be below lo-’ Torr. To achieve usable collision rates, circulating currents must be of the order of amperes, yet such high circulating currents are plagued by a host of instabilities. In electron or positron storage rings, the stacking process is aided materially by radiation damping. Several electron-electron and electron-positron systems have been built and operated. As yet, however, there have been no experiments using protons. Two systems using protons are under construction, one at CERN for protonproton collisions and one at Novosibirsk for proton-antiproton collisions. In the following paragraphs we review the proton projects and then turn to electron rings in operation, under construction, and proposed.

A . The CERN Intersecting Storage Rings It has been evident for many years that colliding beam experiments are, in principle, possible and that probably they should be undertaken with beams either from the 33-GeV AGS at Brookhaven or from 28-GeV CERN PS.

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Two colliding beams at energies of the order of 30 GeV will make available for particle production as much energy as could be yielded from 2000-GeV protons impinging on protons at rest. After much soul searching at Brookhaven and CERN, the decision was made to proceed with construction at CERN. At Brookhaven, it was felt that duplication of the CERN effort was not advisable. Construction was approved at CERN in 1965 and the project is now well advanced. Completion is expected at the end of 1971. The intersecting storage ring (ISR) system shown in Fig. 3 consists of two concentric rings distorted in such a fashion that they intersect at eight points. They are 300 m in diameter, 100 m larger than the PS. At the intersection points, the two counter-rotating protons beams cross each other at an angle of 15”. It is expected that circulating beams of 20 A of protons can be built up by multiturn injection from the PS. Experimentation with the ISR is complicated by the fact that the primary interactions cannot be observed directly ; information must be derived from the secondary particles. If intermediate bosons or quarks are produced, however, they should readily be detectable. Intense preparations are in progress so that experimentation can begin as soon as the machine is in operation. B. The Novosibirsk Proton-Antiproton Ring

In many ways, collisions between protons and antiprotons are even more interesting than proton-proton collisions. Antiprotons are produced in some quantity by present accelerators in the 30-GeV range, but their angular distributions are sufficiently broad that it has not appeared possible to produce a sufficiently intense beam for experimentation in a colliding beam system. However, Professor Budker and his associates at Novosibirsk have pointed out the fact that, if coaxial beams of antiprotons and electrons of the same velocity are maintained, the radial deviations of the antiproton beam will gradually be transferred to the electrons and a concentrated antiproton beam can result. This process is now referred to as “electron cooling.” It will probably be tested in one of the smaller machines at Novosibirsk. There seems to be no theoretical reason to doubt that it will be successful. Construction has been started at Novosibirsk on a ring for studies of 25-GeV colliding beams of protons and antiprotons. Protons will initially be accelerated in an injector synchrotron to about 3 GeV. Several pulses from the injector will build up the beam in the main 25-GeV ring to lOI4 protons. This beam is accelerated in the main ring to 25 GeV and allowed to strike a target. The antiprotons produced are transferred to a third ring about the same size as the injector synchrotron. In the straight sections of this ring they will be electron cooled, transferred back to the main ring and there accelerated

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simultaneously with a new charge of protons. It is hoped that collisions can be observed between circulating beams of l O I 4 protons and 10’ antiprotons. It is hoped that construction of this very ingenious system can be completed during 1971. It is known as VAPP-4.

C. Colliding Beam Studies at the National Accelerator Laboratory At the NAL it was felt that a study should be made of possible colliding beam storage rings before the 200-GeV accelerator was too far advanced, since future colliding-beam programs might call for modifications in the machine itself. This study took place during the summer and fall of 1968 and is described in a report published at the end of 1968 (9). It was concluded that storage rings for the full energy of the machine would be prohibitively expensive and the design study concentrated on 100-100-GeV colliding beams. The storage rings could be located in a convenient location adjacent t o the main ring, as indicated in Fig. 2. It is hoped that, at some later date, developments of superconducting magnets can lead to replacement of the storage ring magnets by superconducting magnets yielding much higher fields and that, by this means, the colliding beam energies can be raised to 200 GeV or higher. D . Electron Storage Rings

Almost all colliding-beam systems involving electrons are designed for studies of collisions between electrons and positrons. At Orsay, France, a 500-MeV ring (known as “ACO”) has been operating since 1966. It is supplied with electrons and positrons by the Orsay linear accelerator. At the Frascati laboratory in Italy, a larger ring for energies up to 1.5 GeV (called “ADONE”) came into operation at the end of 1967. It is capable of maintaining circulating beams of the order of I A. At Novosibirsk, VEPP-2, a 700-MeV ring, has been in use since 1966. It is about to be superseded by VEPP-3, a 3.5-GeV ring. It is this ring whose straight sections will be used for a test of electron cooling of a beam of protons. To a visitor to the Soviet Union, the laboratory in Novosibirsk is one of the most interesting. Some of the most ingenious and resourceful accelerator physicists in the Soviet Union have gathered there and the visitor is continually amazed at the unconventional techniques that have been invented and made to work. The laboratory has one deficiency, however. When confronted with the success of many of their projects, the laboratory managers have realized that they do not have enough high energy physicists to utilize the working machines. To rectify this situation, they have made an agreement with Japan t o allow Japanese physicists to come to Novosibirsk and to help to organize an experimental program, and several Japanese physicists already are in residence.

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At the DESY electron synchrotron in Hamburg, colliding beam rings for electrons and positrons were authorized in 1969 and should be operating by 1973. In this case, the electrons and positrons will not be stored in a single ring; an electron ring and a separate positron ring will be located one above the other. The primary reason for this is the observation that interactions between the two beams can often lead to instabilities that blow up the minor diameter of one or both beams. In the Hamburg system, the beams will collide at only one point. The scheme has the further advantage that it can also be used to study electron-electron collisions. It is expected that 1 A of circulating current can be stored at 3 GeV in each ring. At 1.5 GeV it should be possible to store 10 A in each ring. There are no colliding beam systems operating in the US. In 1964 proposals for electron-positron colliding beams of about 3-GeV energy were submitted by the CEA staff and by the staff of the SLAC. A panel set up by the Atomic Energy Commission recommended that the SLAC proposal be implemented, but since then, neither proposal has received support. In the meantime, the CEA group has invented a clever procedure for modifying the electron accelerator so that it can be used as a storage ring for electrons and positrons. To increase the probability of collisions, a bypass has been introduced at one point in the ring, including focusing lenses that reduce the beam cross-section at the point where collisions will be observed. The vacuum chambers in the remainder of the ring have been replaced with ceramic sections in which the necessary high vacuum can be attained. It is expected that the system can be tested during 1970. Circulating beams of electrons and positrons, provided by a new combination of injector linacs, should reach intensities of about 100 mA in each beam. The SLAC proposal was modified and resubmitted in 1969. It calls for two, separate, somewhat distorted rings intersecting at two points. The maximum energy is now 2 GeV at which it is expected that circulating beams of 500 mA can be maintained. The project now bears the name “SPEAR” (Stanford positron-electron asymmetric rings). An unusual storage ring project is to be found at the Physical Sciences Laboratory of the University of Wisconsin (formerly the MURA laboratory). Here, a 240-MeV electron storage ring has been in operation since 1967. Its injector is a 50-MeV FFAG accelerator built by the MURA group to study the FFAG principle. Colliding beam experiments have never been planned for this storage ring. Originally it was built to study instabilities and other phenomena in storage rings. Recently, however, much interest has arisen in possible applications of the spectrum of synchrotron radiation emitted by the circulating electrons. The ultraviolet region of the spectrum has found a number of applications in research programs on solid state physics and atomic physics. The machine now operates continuously as a research tool in these programs.

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VIII. CYCLOTRONS The old-fashioned cyclotron of the type built by Lawrence and Livingston has now been twice superseded. The postwar invention of the synchrocyclotron led to the construction of a number of machines, many of which have had very honorable histories. Notable among these are the 180-in. machine at Berkeley, the 600-MeV SC at CERN, and the 680-MeV synchrocyclotron at Dubna. In November of 1967, the largest synchrocyclotron ever built came into operation at Gatchina, near Leningrad. The design energy of this machine is 1 GeV; its magnet weighs 8500 tons. Probably the Gatchina synchrocyclotron will be the last major synchrocyclotron ever to be built, since the present trend is toward isochronous cyclotrons that can operate continuously, rather than in the pulsed fashion of the synchrocyclotron. These cyclotrons depend for focusing on azimuthal variation of the magnetic field, which yields focusing in the same fashion as is used in alternating gradient systems. With this focusing, the average field need not fall off with radius, as is necessary in conventional cyclotrons, and it is now possible to increase field with radius until the period of an orbit becomes independent of radius. This can be extended into the relativistic range of energies and appears practical for energies up to about 1 GeV and possibly to still higher energies.

, ,

FIG.4. Indiana cyclotron

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This principle has been applied in a rather extreme design illustrated in Fig. 4. This design has been evolved at the University of Indiana by Rickey, Sampson, and their associates. The magnet has been separated into four quite separate sectors, narrow enough so that only about half of the orbit lies between magnet poles. The space between poles is now available for injection and extraction of beams and, if desired, for localized rf accelerating systems. Injection is from another cyclotron which is a scaled-down version of the main machine. Construction of this machine was well advanced at the University of Indiana by the end of 1969. The Indiana cyclotron has aroused great enthusiasm among cyclotron designers. In particular, many of the proposals for heavy ion accelerators include either four-sector or six-sector versions of the Indiana design. Probably most cyclotrons to be built in the near future will be of this type. IX. MESONFACTORIES

In 1965, four groups in the US were actively studying machines of high intensity to produce proton beams in the range between 500 MeV and 1 GeV for the purpose of generating intense beams of pions, muons, and other secondary particles. These were inspired by intense interest in the many interesting experiments in meson physics and nuclear structure that could be made possible, provided that beams of sufficient intensity could be produced. At Yale University, a linear accelerator was proposed; later this became a joint Yale-Brookhaven project. A similar linac was under study at Los Alamos. At Oak Ridge, two possibilities excited interest, a conventional proton cyclotron and a “ separated-orbit cyclotron,” a cyclotron with separated magnet sectors and so high a rate of acceleration that the machine became a sort of spiral linear accelerator. At the University of California at Los Angeles, negative hydrogen ions were to be accelerated in a cyclotron; their charge would be reversed in a stripping foil and they then would be extracted easily. An “Ad Hoc Panel on Meson Factories ” chaired by Professor Bethe had recommended to the Office of Science and Technology in 1964 that one, but only one, meson factory be built. Finally, in July of 1967, the “Los Alamos Meson Physics Facility ” (LAMPF) was chosen and authorized. Construction began almost immediately and, in 1969, was about half completed. LAMPF will be a drift tube linac from its preinjector energy of 750 keV to 100 MeV. At 100 MeV, the beam will pass from the 200-MHz drift tube section into an 800-MHz iris-loaded waveguide-system in which it will be accelerated to 800 MeV. Average current at 800 MeV is to be 1 mA. The duty cycle will be 6%, possibly to be increased later to 12%. Both sections of the accelerator have new and ingenious modifications from previous linear

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accelerators; unfortunately these features are somewhat too esoteric for description in this brief review. Completion is scheduled for 1972. While the complex situation in the US was being resolved, two meson factories were started elsewhere in the world. At the Eidgenossische Technische Hochschule in Zurich, studies on possible meson factories led to the choice of a complex of two isochronous cyclotrons to yield a beam of 100 FA of 500-MeV protons. The injector cyclotron is a four-sector machine which yields 70-MeV protons. With so high an injection energy, it is possible to leave out the central section of the main cyclotron since the protons are injected on an orbit of radius 2.1 m. Acceleration brings the protons to a final energy of slightly more than 500 MeV on an orbit of radius 4.5 m. The cyclotron magnet consists of eight completely separate sectors between which there is room for four rf accelerating cavities each capable of adding 500 keV to the proton energy. This makes for a clear separation between orbits and allows the extraction efficiency to approach 100%. It is expected that other ions (deuterons, alpha particles, etc.) will be used in this machine. Heavy ions can be accelerated to 10 MeV/nucleon. A site for this machine has been chosen at Villigen, near Zurich. Here will arise the “ Schweitzerisches Institut fur Nuklearforschung ” which has been abbreviated to “SIN.” Construction of the accelerator will begin in 1971 and it is expected that the machine will be finished by 1974. The second meson factory outside of the US is a joint project of four western-Canadian universities. Initially, three universities were involved, the University of British Columbia, the University of Victoria, and Simon Fraser University, and the project was named the “ TRI-University Meson Facility ” or “T R I U M F ” for short. Later, the University of Alberta joined the three British Columbia universities, but the original name has been retained. TRIUMF will be an isochronous, sector-focused cyclotron with six sectors. In it, negative hydrogen ions will be accelerated; they will be stripped at extraction. As protons, they will be deflected outward by the magnetic field. This is the same scheme as was proposed by Richardson for the nonapproved cyclotron meson factory at Los Angeles. Indeed, Richardson is one of the more important consultants on the TRIUMF program. Since negative hydrogen ions tend to lose their electrons when deflected by too strong a magnetic field, it has been necessary to reduce the field to less than 6 kG. For 500-MeV protons the orbit diameter in this field is about 16 m; the total magnet weight is about 3500 tons. The cyclotron will be built on the campus of the University of British Columbia in Vancouver. Completion is expected to be in 1974. X. R.I.P. During recent years, a number of older machines have been turned off and several proposed projects have been canceled. Four of these are particu-

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larly notable. On December 30, 1966, the Brookhaven Cosmotron, the first machine to accelerate charged particles to energies over 1 GeV, was shut down after a productive lifetime of 14 yr. The machine has now been completely dismantled. In January of 1969, the 68-MeV proton linear accelerator at the University of Minnesota was closed down. its lifetime also was 14 yr. Until the 100-MeV linac injector at Serpukhov began operating in 1967, the Minnesota linac provided protons of the highest energy attained in a linear accelerator. The other two machines died before they were born. Both were bold extrapolations of accelerator technology, and it seems sad that they failed to be supported. First is the “ Omnitron of the University of California. This was a combination of a synchrotron and a storage ring to be used in acceleration of heavy ions. By transferring ions back and forth between the synchrotron and the storage ring passing through appropriate strippers, heavy ions could have been accelerated to several hundred million electron volts per nucleon. The Omnitron proposal was withdrawn in 1968. The second unsuccessful project was the “ Intense Neutron Generator ” (ING) proposed by the Chalk River Laboratory in Canada. It was to be a long linear accelerator to be used in continuous operation accelerating a current of 65 mA of protons to l GeV. Its target was to be a stream of molten lead and bismuth. The neutron flux derived from this process would have been higher than that yet achieved at any reactor. Unfortunately, it would have been very expensive (over $150 million) and, although it was approved by the Canadian Science Council, the Canadian government canceled it in 1968. These few sad demises fade into insignificance, however, when viewed against the many new accelerator projects described in the preceding sections of this summary. It would appear that acceleration of charged particles is still a vigorous and exciting endeavor. Much remains to be done in the study of the nucleus and its components. ”

XI. CONCLUSION

No claim for completeness can be made for the preceding presentation of recent highlights in accelerator development. In the last five years, thousands of pages of proceedings of accelerator conferences have appeared describing interesting developments far too numerous for inclusion in so brief a summary as this one. For example, no mention has been made of the conversion of Columbia’s Nevis synchrocyclotron to higher energy and intensity operation. Also neglected or barely mentioned are the new linac injector and pressurized preinjector for the 3-GeV proton synchrotron at Saclay, the high intensity electron linac at MIT, the splendid new tandem accelerators developed by High Voltage Engineering, the Dynamitron accelerators developed

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by Radiation Dynamics, various high current electron accelerators for use as flash X-ray tubes, and many proposals for modification and improvement of existing accelerators. The reader who wishes more detailed information is referred to the proceedings of the International Conferences on High Energy Accelerators held in 1965 ( I O ) , 1967 ( I ) , and 1969 (3), and to the proceedings of the American Accelerator Conferences held in 1965 ( I I ) , 1967 (12), and 1969 (13). A less technical but very readable source of information about current accelerator developments is the CERN Courier published monthly in French and English by CERN’s Public Information Office.

REFERENCES 1. V. I. Veksler er al., in “Proceedings of the Sixth International Conference on High Energy Accelerators, 1967” (R. A. Mack, ed.), p. 289, CEAL-2000, Cambridge Electron Accelerator, Cambridge, Massachusetts, 1967. (Available from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia.) 2. “ Symposium on Electron Ring Accelerators,” UCRL-18103, Lawrence Radiation Laboratory, Berkeley, California, 1968. (Available from Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia.) 3. “ Proceedings of the Seventh International Conference on High Energy Accelerators, 1969,” Erevan, USSR (to be published). 4 . G. T. Seaborg, Ann. Rev.Nucl. Sci. 18, 53 (1968). 5. A. G. Prodell and H. Hahn, eds., “Proceedings of the 1968 Summer Study on Superconducting Devices and Accelerators,” BNL 5015 5 , Brookhaven National Laboratory, Upton, New York. (Available from Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia.) 6 . G. T. Danby, in “Proceedings of the 1968 Summer Study on Superconducting Devices and Accelerators ” (A. G. Prodell and H. Hahn, eds.), p. 1 115. BNL 501 5 5 , Brookhaven National Laboratory, Upton, New York. (Available from Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia.) 7. A. A. Vasil’ev, ed., “ 1000 GeV Cybernetic Proton Accelerator.” English translation issued by the US Atomic Energy Commission, AEC-tr-6949, 1968, and available from Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia. 8. R. B. Neal, ed., “The Stanford Two-Mile Accelerator.” Benjamin, New York, 1968. 9. “ Design Study for Proton-Proton Colliding-Beam Storage Rings for the National Accelerator Laboratory,” National Accelerator Laboratory, Batavia, Illinois, 1968. 10. M. Grilli, ed., “Proceedings of the Fifth International Conference on High Energy Accelerators, Frascati, 1965,” Comitato Nazionale per I’Energia Nucleare, Rome, 1966. 11. IEEE Trans. Nucl. Sci. NS-12 No. 3 (1965). 12. IEEE Trans. Nucl. Sci. NS-14 No. 3 (1967). 13. IEEE Trans. Nitcl. Sci. NS-16 No. 3 (1969).