Studies for a 70 inch versatile cyclotron

Studies for a 70 inch versatile cyclotron

NUCLEAR INSTRU~IENTS AND~IETHODS 18,19 (1962) 114-119; NORTH-HOLLAND PUBLISHING CO. STUDIES FOR A 70 INCH VERSATILE CYCLOTRON J. D. LAWSON Rutherf...

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NUCLEAR INSTRU~IENTS AND~IETHODS 18,19 (1962) 114-119; NORTH-HOLLAND PUBLISHING CO.

STUDIES FOR A 70 INCH VERSATILE CYCLOTRON

J.

D. LAWSON

Rutherford High Energy Laboratory, Harwell, E1Zgland

Design studies are well advanced for a 70" cyclotron capable of accelerating protons of energy up to 50 ~IeV, and heavier ions to energies which depend on their charge to mass ratio. The cyclotron, if approved, will be built by the staff cf the Rutherford Laboratory at Harwell for use at the Atomic Energy Research Establishment mainly for studies in radio-chemistry, radiation chemistry, and radiation effects in solids. In basic concept the machine resembles the Oak Ridge ORIC and the Berkeley 88" machines. There are three ridges, providing a field \yhich makes an angle of 48° to the circumference at the maximum orbit radius. The field shape under different working conditions will be controlled by a combination of correcting windings and voids.

A tape controlled precision table of 24" X 30" traverse will be used with a pantograph to move the measuring head in field surveys. A 3: 1 frequency range for the dee voltage wiII be provided by an amplifier feeding a line with a variable shorting mechanism. Extraction presents a difficult problem, not yet solved. A septum at positive potential, (with leading edge shielded by a cooled earth plate) is contemplated; on the inner side, plates will shape the field to provide a regenerator. Consideration is being given to the injection of heavy ions through a hole along the pole axis.

1. History and Status

Atomic Energy Authority, it will be designed and built by staff of the Rutherford High Energy Laboratory, many of whom were with the Atomic Energy Authority before the Rutherford Laboratory was created.

The desirability of a cyclotron at the Atomic Energy Research Establishment, Harwell designed primarily for chemical studies has been recognised for many years. About four years ago, when some of the difficulties associated with large spiral ridge cyclotrons (in particular our proposed conversion to near continuous operation of the 110" synchrocyclotron) had become apparent, and the possibilities of intermediate energy cyclotrons capable of accelerating various ions over a range of energies were being explored at Oak Ridge, Berkeley, Colorado and elsewhere, the value of such a machine to the research programme of the Establishment came under active discussion. At this time a limited amount of study was given to the research programme appropriate to the machine and its overall cost and parameters. Early in 1961 it was decided to ask for Treasury approval "in principle" for the project. The present position is that we are waiting for final approval. Design studies for the machine were intensified early in 1961; by the end of the year the building was under intensive study, and preliminary engineering lay-outs of the major components were being started. If all goes well we should have the first usable beams early in 1965. Although the machine is being made for the

2. Specification of the Machine It is difficult to judge what the performance of a machine such as this is likely to be, nevertheless an outline specification of what is desired was drawn up in consultation with potential users in the fields of chemistry, metallurgy and solid state physics. The interests of nuclear physicists were borne in mind, but no special provision has been made for them at this stage. The specification calls for protons of maximum energy 50 :i\IeV, and deuterons and a-particles of maximum energy 12 MeV per nucleon. The proton energy is to be varied in steps of ratio about 0.7 down to 4 MeV, the energy of other ions is to be varied in steps of similar ratio down to 4 1\IeV per nucleon. For all these particles, over the whole energy range, currents of 100 JtA are required to be available in one of three shielded target rooms. For heavier ions, the specification is less precise. Energies as high as possible (but not exceeding 12 1\IeV/nucleon) are required, with total energies 114

STUDIES FOR A 70 INCH VeRSATILE CYCLOTRON

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of at least 100 :MeV for several ions. Several energies are again needed for each nuclear species though the details remain to be worked out. External currents exceeding 1 J1A of at least some of these ions are desirable.

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Fig. 3. Schematic layout of proposed building. The main wall (hatched) is of solid reinforced concrete. The side walls (crosshatched) are cast as blocks in position. so that they can be removed for later extensions. The walls between the vault and target rooms (chain-hatched) are built up of rernoveable shielding blocks. As shown. large sliding doors close either the central target room. or the two outer ones; additional shielding will be necessary however to shield against induced activity in the room not in one. The sector marked "probe access" is for probe targets for radiochemical studies.

In addition to the external beams, adequate access by probes to the internal beam at all radii is to be provided. Of special importance in the study of time and temperature dependent phenomena is the requirement that the beam modulation (at frequencies below 1 "1.Icjs) should be low. 3. Basic Concepts By the time it became necessary to settle our basic parameters, a considerable amount of thought had already gone into this type of machine, particu-

larly by the groups at Oak Ridge, Berkeley and Colorado, and we are very grateful for the help and advice which we received from them both directly, and through their publications. In finally deciding our method of approach, many local factors had to be taken into consideration. The particular experience of the staff on the job, and even the accounting system used for costing the project, by its allocation of overheads, can have an important influence on the design. A general idea of our proposed machine can be got from the sectioned elevation and plan, figs. 1 and 2, though these should not be taken as final. Fig. 3 shows our plans for building layout. Details of the various components are discussed in the following sections. 4. Magnet The magnetic field will be provided by a 180 ton magnet with horizontal gap. The azimuthally varying field is provided by three ridges. These are flat, the edges being bounded by arcs of circles. The depth of the ridges is 4t in. and the minimum gap is 7-1 in. The locus of the maximum of the field makes an angle of 48° to the circumference at the maximum orbit radius. The central field will be 13 kG for protons, deuterons and alpha particles, and 16-17 kG for heavier ions. The copper coils will consume at most 275 kW, for an excitation of about 560000 ampere turns. Some care was taken to optimize the yoke design, and this resulted in a pole cylinder in which the diameter is tapered from 84 in. to 70 in. in two steps '). The effect of this is to reduce the effects of saturation, which are normally bad at the root of the pole cylinder, yielding a high efficiency and reducing the variation of field with radius at different field levels. The spacing between the two coils is 23 in., allowing space for the d. line and plenty of access round the edge of the pole. Field corrections and variations will be provided by an array of concentric windings, the details of which have not yet been worked out. A "rosette" winding which passes round the edge of the pole and is reentrant into the valleys is contemplated for provi1) J. H. Coupland and K. 18, 19 (1962) 148.

J. Howard. Kuci. Instr. and Meth.

STUDiES FOR A 70 INCH VERSATILE CYCLOTRON

ding the increased flutter and field required at the larger radii for protons. Field plots for a range of field levels with no correction are shown in fig. 4. These will be improved by the use of voids in the backing plate, and possibly an edge shim in the valleys at large radius.

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A one third scale model magnet is planned, on which ridge and coil designs can be finalized. This model will also be used for establishing field survey routines. To move the probe head we shall use a tape controlled machine tool table, which uses an optical fringe-counting system as a reference, and can be set to an accuracy of at least 0.001 in. This table has a 24 in. by 30 in. traverse; it will be used with a rigid arm on the t scale model, and with a pantograph on the full size magnet. Instructions for an r - 0 grid, with arbitrary intervals and

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periodic checks at the centre to provide data for drift correction, can easily be provided on the input control tape.

5. RF System A single dee system, supported on the end of the inner of a coaxial line, will be fed with power sufficient to excite a dee to ground voltage of 100 kV over the frequency range from 6.5-21 nle/s. The power will be fed in from a power amplifier through a capacitive plate coupler, and the system will be tuned by means ofa moving short mechanism. The amplifier will probably be a commercial design intended for a radio transmitter, which feeds the dee line via a balance-to-unbalance transformer. The dee line will have outer and inner diameters of about 6 and 4 feet, and the maximum distance from short to the machine centre will be 24 ft. The expected power consumption varies from 240 kW at 21 nle/s to 80 kW at 6.5 nIe/s. This includes allowance for beam loading and ion loading at the machine centre. The effect of line and dee geometry on tuning range has been studied with the aid of 116 scale models, and tests on the feasibility of capacitive feeding have been carried out on the dee of our 4 nleV cyclotron"). In order to study the best form of contact between the shorting plate and the dee line, a test resonator line with a contact capable of running at a somewhat greater value of current per inch than is required in the machine has been constructed. This will soon be undergoing tests. The open circuit end of the line terminates as flat box section, which simulates a dee and can if necessary be used in an existing magnet for breakdown tests. The line will also be used for further tests on the feeding problem. 6. Ion Injection Little thought has yet been given to the problems of the ion source, at present we plan to be able to insert a standard hooded ion source either from the side of the vacuum tank, or through a hole in the yoke along the pole axis. Studies of conditions at 2) 11. Snowden, "Sector Focused Cyclotron" conference proceedings, 1959, p. 47.

I. OPERATI);G <;HARACTERISTICS A);D DESIG); FEATURES

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D. LAWSON

the centre, and the use of puller electrodes and defining slits will be made on our existing 4 1\IeV cyclotron fitted with new ridges. As a later development we intend to study the problem of injecting multiply charged ions down the hole in the yoke. In this way we hope to be able to make use of a large and more powerful source than can be situated at the centre of the machine. 7. Extraction It is planned to extract the beam between the dee line and the magnet yoke, at an angle of about 15° to the dee line. The beam will then be bent fairly soon in a direction at right angles to the axis of the line, and passed to a switching magnet to be deflected into one of three target rooms. The extractor is without doubt the most worrying feature of the machine, part of the difficulty being that it is not possible to do satisfactory calculations before the precise nature of the fringing field and its variation with the level and shape of the field in the gap is known. \Ve expect to use an electrostatic deflector, but to enhance the turns separation with the aid of a regenerator and perhaps a peeler also. An idea for a regenerator (similar to one described by Blosserl) which looks promising is shown in fig. 5. The septum is maintained at a potential of + 60 kV with respect to the outer plate, which is earthed. Inside the septum is a further plate with a slot on the

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median plane of the machine, shaped to give a roughly linear fall of field inward for a few inches from the septum. The leading edge of the septum 3) H . G. Blosser, Ibid, p. 241.

would be protected by placing a cooled earthed plate which "casts a shadow" on it. Such a system can in principle be made to work without a peeler only in a region of falling field it is likely therefore that a peeler will be needed also. If this turns out to be the case a field "bump" can be provided by coils mounted inside the dee, or in a valley. It must be emphasised that these are only preliminary ideas, and no detailed calculations have been made. Such calculations as have been done refer to the radial oscillation build up in a linear system with an extended regenerator, or to the path of particles through the extractor and fringing field. These calculations show that in the case considered the extraction angle is a minimum when the septum is placed just after the maximum of the flutter field. Dispersion due to variation of angle, radius and momentum at entry to the deflector has also been estimated. Whether this can be reduced by modifying the fringing field with iron or coils remains to be seen. 8. Building The design of the building, shown in fig. 3, is based on the firm request by the future users of the machine for three independent shielded target rooms, and access round the machine for probe target work. The cyclotron vault is of solid concrete construction with 13 ft headroom. A large sliding plug door at one end will allow entry of the machine and bending magnets, a smaller door at the side will be used for personnel and equipment when the machine is assembled. This door is designed so that a small shielded truck can be driven into the vault from the workshop area. In order to allow insertion of the ion source, and later. possibly of an ion beam through the hole in the yoke, a 7 ft x 3 ft hole will be left in the vault roof. Above this will be a laboratory, shielded by sliding blocks which cover the hole, and blocks placed between the top of the magnet and the roof of the vault. The target room walls will be built up partly of moveable blocks, to allow some flexibility and the possibility of expansion. Since the emphasis of the work of this machine will be on radiation damage studies rather than nuclear chemistry, the main

STUDIES FOR A 70 INCH VERSATILE CYCLOTRON

requirement is for a uniform and steady beam on a small target. It is not anticipated that the beam layout will be complicated or will need frequent change. No very sophisticated shielding calculations have been carried out. The necessary wall thickness in the vault was estimated by application of the neutron attenuation curves given by R. H. Thomas") to data (experimental and theoretical) on the neutron yield and energy spectrum for the two worst conditions of a 1 rnA beam of 50 :MeV protons striking a heavy target, and almA beam of 24 lIIeV deuterons striking a beryllium target. Ducts are being designed according to the rules established for reactors, the thermal neutron flux being assumed equal to the fast flux at the walls.

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9. Concluding Remarks Many details of the machine have yet to be fixed; in particular, conditions at the centre for injection and at the edge for extraction need further study. It is not expected however that the basic layout of machine or building will be substantially altered. A number of people have contributed to those studies. In addition to those whose work is reported elsewhere in this volume, T. C. Randle carried out the extraction studies, E. J. Jones and H. E. Payne are designing the d. system, D. H. Trew is responsible for the field shaping, A. G. Hewitt for overall engineering, and P. T. nI. Clee and W. Holland for mechanical design. 4} R. H. Thomas, National Institute for Research in Nuclear Science, Harwell, Report N.I.R.L.fl\IfI3 (1961).

DISCUSSION

Speaker addressed:

J. D.

LAWSON

(Harwell)

Question by 'V. ~I. BROBECK: You have a positive high voltage on the electrode; that usually results in electron osciltion. Do you have some way to avoid that?

Answer: No. This is a point we have not considered; perhaps we should. Comment by J. S. ALLEN [Urbana}: We have just finished making some of the largest Philips ion gauges in existence (laughter). 'Ve needed a deflector with a positive electrode;

the voltage would start off at 10 kV. then drop to 1 kV and just stay there, with a very large current. It seems to be impossible to work this way.

Comment by R. S. LIVINGSTON (ORNL): I can't resist making one remark about holding positive voltage in a magnetic field in vacuum. There were many acres of positive-voltage calutrons in operation at Oak Ridge in 1944-45. All you have to do is protect the insulators with suitable electron dumps, and to work at a reasonable vacuum. Otherwise, it will tear up the insulators something furious.

I. OPERATIXG CHARACTERISTICS AXD DESIGX FEATURES