Three-cavity variable energy racetrack microtron with intra-sector beam focusing

NUCLEAR INSTRUMENTS

AND M E T H O D S

143 ( 1 9 7 7 )

473-486"

©

NORTH-HOLLAND

P U B L I S H I N G CO.

THREE-CAVITY VARIABLE ENERGY RACETRACK MICROTRON WITH INTRA-SECTOR BEAM FOCUSING H. R. FROELICH, A. S. THOMPSON*, D. S. EDMONDS Jr.t, J. J. M A N C A + J. W. McGOWAN, J. C. F. MacDONALD, J. BEARD and G. BEES

Department of Physics, and Centre .for Chemical Physics, University of Western Ontario, London, Ontario, Canada N6A 3K7 Received 11 January 1977 This paper describes the design and operation of a racetrack microtron in which the final energy of the pulsed electron beam can be varied, conveniently and continuously, from 1.5 to 18 MeV. In addition to its variable energy, the distinctive features of this machine are a multiple-cavity accelerating section and a two-sector magnetic guide field in which the necessary beam focusing is provided within the magnet sectors by shaping the pole-pieces in a simple way. Some features which make the racetrack type of microtron a very suitable accelerator for various research and commercial uses, in this energy range and at higher energies, are discussed.

1. Introduction The microtron, a fixed field cyclic electron accelerator, has been overshadowed from the beginning of its history by the much more intensively developed linear accelerator. Nevertheless there has been a continuing interest in the microtron since it was first proposed by Veksler 1) in 1944 and microtron design and development has been carried on by a number of workers in various parts of the world. The first microtron of the type suggested by Veksler was built in Canada2,3). Subsequent development of the conventional microtron not only as a tool for a variety of research applications but also as an injector for higher energy electron accelerators has been pursued most vigorously in the Soviet Union 4,5) and in Sweden6,7). In recent years, there has been a growing interest in the microtron as a commercial accelerator; commercial units using the conventional type of microtron have been developed in Sweden specifically for radiotherapy 8) and radiography 9) applications. Although it is not as well-known as the conventional microtron, the racetrack or split-magnet microtron has also seen considerable development. In comparison with the conventional microtron, the racetrack geometry has the advantage of smaller size and greater flexibility in design and operation. The possibility of applying the sectored * Present address: Atomic Energy of Canada Ltd., (Commercial Products), Ottawa, Canada, K2A 3W3. t Present address: Department of Physics, Boston University, Boston, Massachusetts, 02215, U.S.A. + Present address: Los Alamos Scientific Laboratory, Los AIamos, New Mexico, 87545, U.S.A.

magnet approach to microtrons was first suggested by Moroz 1°) and independently by Roberts ~) and various split-magnet guide field configurations have been devised~2,~3). In the two previous racetrack microtrons built at the University of Western Ontario, four-sector magnet designs incorporating in each case a single microwave accelerating cavity were usedl4). In contrast, the variable energy racetrack microtron described in this paper has two 180 ° bending magnets which are structured for focusing and a three-cavity linac accelerating section. With this arrangement, the energy of the extracted electron beam is continuously variable from 1.5 to 18 MeV. Brief descriptions of some aspects of this machine have been given previously 15'16) but this paper provides more comprehensive details of the design, construction, operation, and performance of the accelerator. To establish a framework for describing this machine, a brief discussion of the general principles and characteristics of racetrack microtrons is also included. The references quoted in this paper are intended only to point out the main areas of microtron development; they do not constitute a complete bibliography of microtrons. 2. General characteristics of racetrack microtrons In a conventional microtron, electrons move in circular orbits in a uniform magnetic field, which remains constant with time, and pass repeatedly through a single microwave cavity (usually Sband). During each traversal, the electrons acquire an additional amount of energy d E from the rf

474

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field in the cavity. Hence the orbit radius and the transit time for one complete orbit both increase with successive orbits. In any cyclic resonant accelerator, the electrons must return to the cavity after each orbit with the correct phase in order to be re-accelerated • In a microtron this synchronism between the electron beam and the microwave field is attained by making the transit times for successive orbits differ by an integral number of rf periods. For the conventional microtron, this requires that the following "resonance conditions" 3) be satisfied: 2re A E B = ec 2 n'c '

(1)

and E~,j n

12)

--- no~

AE

where B is the magnetic field strength, r is the period of the microwave field and Einj is the effective total injection energy, i.e. the electron rest energy plus any kinetic energy added by the injection system plus the difference between the energy gain during the first traversal of the cavity and the energy gain during subsequent traversals, The integers n and no specify the operating mode of the microtron; n is the difference between the number of rf periods required tbr successive orbits (usually n = 1 or n = 2) while no is determined by the injection energy such that n o + n is the number of rf periods required tbr the first orbit. Under these resonance conditions the transit time Tk for the kth orbit is: 2re Tk (Einj+kAE) = ( n o + n k ) r . (3) -

"

ec2B

In the racetrack microtron, the magnetic field is split into two or more sectors which are separated by regions with no magnetic field. Electron orbits in such a guide field are no longer circular but are elongated by straight sections in the field-free regions. The total length of each orbit in these drift spaces is then an additional parameter which can be used to control the time required to complete each orbit and thus to satisfy the resonance requirements. Eq. (1) remains valid for the racetrack microtron, but the second resonance condition be-

comeslT) n

Einj

AE

+ ~sk = no, vkr

(4)

where Sk and Ok are, respectively, the total drift space length and the speed of the electrons, in the kth orbit. The transit time for the kth orbit of a racetrack microtron is then 2~z Sic (5) Tk -- ec 2 B (E~,j + k A E ) + -vk- = (no + n k ) r . An important feature of the racetrack microtron is its compact size. In a conventional microtron, the energy gain A E is limited, partly by eq. (2) and ultimately by the fact that only a single cavity can be fitted into the circular orbits. This cavity must be short, moreover, to allow the small first orbit to get around. For A E = 1 MeV and S-band frequencies, eq. (1) requires magnetic fields of only 0.2 T even with n - 1 . Such relatively weak magnetic fields result in relatively large orbit radii and magnet dimensions for a given final energy. This restriction on A E is relaxed in a racetrack microtron because the accelerating cavity can be placed in one of the field-free spaces between the magnet sectors where it intercepts only a straight section of each orbit. With the length of the accelerating structure not limited by orbit curvature, a longer structure can be used. While this may take the form of only a slightly longer single cavity, the real advantage of the racetrack geometry is the fact that it permits the use of a chain of several cavities, thereby providing a substantial increase in the overall energy gain A E for each orbit. The correspondingly higher magnetic fields needed to satisfy eq. (1) result in considerably smaller orbit and magnet sizes. This becomes increasingly important at higher energies where practical difficulties arise in maintaining field homogeneity over the large magnet sizes required for conventional microtrons. The racetrack configuration also reduces the size of the gap needed between the magnet pole-pieces. In a conventional microtron the accelerating cavity must be placed within the magnetic field and this requires a magnet gap of about 10 cm for an S-band cavity. In a racetrack microtron, with its accelerating structure located between the magnet sectors, the gap between pole-pieces need only be wide enough to allow unobstructed circulation of the electron beam (about 1 cm). This difference in gap size gives even a single-cavity racetrack machine a significant size advantage over a conventional microtron of the same final energy. Other characteristics of racetrack microtrons include good energy resolution, ease of beam extrac-

RACETRACK

tion, and efficient use of rf power; the first two of these features are common to both types of microtron. The very small relative energy spread of the electrons in the beam results from the fact that electrons in a microtron are confined in a phase-stable region ~8) which has an energy width of less than 0.16 AE/n. The relative energy spread in the kth orbit is therefore less than O.16/kn. Because the orbits in a microtron are well separated in the region opposite the accelerating structure, beam extraction is straightforward and highly efficient (practically 100% for extraction from the final orbit). The efficient use of rf power is partly a result of the beam recirculation through the accelerating structure which occurs in any microtron, but it is further enhanced in a racetrack microtron because the accelerating structure itself can be designed for optimum efficiency without being constrained by the resonance conditions or by severe space limitations. Thus a racetrack microtron can, in fact, incorporate a very efficient short linac and make even more efficient use of the power supplied to the linac by passing the beam through it repeatedly. The most important feature of the racetrack microtron is its inherent design flexibility. The final beam energy can be changed, in a series of discrete steps by extracting from different orbits, or over a continuous range of energies by varying the energy gain per orbit, AE, or over a still wider continuous range by varying both AE and the operating mode (given by n). Because of the ratio Sk/Ok in eq. (4), continuous energy variation in a racetrack microtron does not-require compensating changes in E~,j. Alternatively, a racetrack machine can be designed for a specific final beam energy and this can be achieved to suit the particular application, with a high AE and high magnetic fields to give a very compact size, or with a low AE and low magnetic fields to give a larger machine which has a reduced energy spread because of its greater number of orbits, or with the most suitable compromise between these two extremes. The magnetic field in a racetrack microtron is not limited by resonance requirements but only by saturation effects in the magnetic materials. Furthermore, a specified energy gain per orbit can be obtained either with a long accelerating structure and low power microwave tube or with a short structure and a high power tube. Here the choice is determined mainly by the current requirements of the particular application. This feature is of special in-

MICROTRON

475

terest because it opens the possibility of designing CW microtrons with room temperature linac sections. By using long accelerating structures of high shunt impedance in conjunction with CW klystrons of only a few hundred kW, relatively high energy gains can be obtained without resorting to superconducting accelerating sections. The splitmagnet type of guide field also provides flexibility in beam focusing methods. Focusing can be provided either by including suitable features as an integral part in the design of the magnet sectors or by adopting a separated-function guide field arrangement with special focusing elements for each orbit in the drift spaces between magnet sectors. These drift spaces can also accommodate a variety of other design options including beam monitoring devices, injection systems of varying complexity (with pre-bunching cavities if necessary), and also inflector systems for injecting a pre-accelerated beam into the return portion of an orbit. 3. Description of the microtron The characteristics described in the previous section qualify the racetrack microtron as a potentially very useful accelerator for both research and commercial purposes. The University of Western Ontario (UWO) multi-cavity variable energy racetrack microtron was built primarily as a test machine to devise and evaluate simple solutions for various microtron design problems, especially in the areas of injection, accelerating structures, and guide fields, which might be applicable in the development of commercial machines for energies up to about 40 MeV. The immediate design objective was to build a racetrack microtron with the final beam energy variable from about 1.5 to 18 MeV and with pulsed current levels in the extracted beam ranging from about 20 mA at the highest energies to 50 mA at 8 MeV. A secondary consideration was to build the machine in such a way that it could be readily adapted for more general research uses after its initial developmental functions had been served; this was quite consistent with the test machine requirements for a flexible, easy to modify design. The main parts of the microtron are shown schematically in fig. 1. Two 180° bending magnets are separated by a variable length drift space which contains the accelerating structure, the electron gun, and beam monitors. Shields in front of the magnet sectors keep the drift space free from stray magnetic fields. After the final pass through

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Fig. 1. Schematic diagram of the racetrack microtron. (1) Electron gun, (2) accelerating structure, (3) magnet pole-pieces, (4) magnetic shields, (5) shimming magnets, (6) beam current monitor, (7) vacuum chamber, (8) extraction pipe. the accelerating structure, the electrons are deflected once more in the first magnet sector and then removed from the machine via an extraction pipe located beside the second magnet sector. The size of the accelerator in the orbit plane is indicated by the scale of fig. 1. In combining these main parts into an actual working accelerator, a demountable modular design was chosen to allow easy access to any internal part and to make it possible to assemble, test, and modify each subsystem separately. The basic accelerator consists of three modules, a central module and two magnet modules, supported on a rigid frame. These three modules form the vacuum chamber for the microtron; they were constructed from 1" thick stainless steel plates which were ground on both sides, welded together to make vacuum tight joints all around, and then machined to tolerance on the outside surfaces, The central module, with dimensions 35 c m × 35 c m × 30 cm, is fixed to the support frame. The accelerating structure and the electron gun are both attached to the cover plate on a port on one side of this module, while beam monitoring devices are introduced into the v a c u u m through a port on the opposite side. The magnet modules are bolted to the two ends of the central module by means of O-ring sealed v a c u u m flanges. Because each one weighs about 350 kg, the magnet modules are supported on the

main flame by means of specially made sliding mounts so that each entire magnet module can easily be moved along the frame out of the way of the central module. The vacuum system was designed, not to provide the ultimate in vacuum capability, but only to ensure a satisfactory vacuum for machine operation while maintaining fast and easy demountability. Since frequent opening of the vacuum chamber to make alterations to some parts of the machine was required at some stages, an ion pump is not included in the present design. An oil diffusion pump (NRC model VHS-4, rated at 500 l/s with liquid nitrogen t r a p ) u s i n g Santovac 5 oil is attached to the bottom of the microtron's central module and a Welch mechanical pump (model 1376B) is used as fore-pump. This system provides a reliable vacuum of (2 or 3)× 10 -7 torr when the microtron is in full operation. Each magnet module contains all the components necessary for one 180 ° deflecting sector of the magnetic guide field. Each module is really a C-shaped electromagnet which is constructed in such a way that its pole-pieces are situated inside the microtron vacuum chamber while the yoke with a pair of coils is outside the vacuum. This is done by mounting the pole-pieces in a specially shaped vacuum compartment (fig. 2). This compartment has parallel sides, but the top and bottom walls are tapered together in a truncated V-shape which

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is closed at its narrow end. The rectangular mouth, which is open, is fitted with a vacuum flange. The pole-piece compartment of each magnet module is then sealed against the appropriate end of the central module to complete the microtron vacuum chamber. The yoke of each magnet module fits around the pole-piece vacuum compartment as shown in fig. 2. Magnetic contact between the pole-pieces and the parts of the magnetic circuit that are outside the ,vacuum chamber is provided via mild steel plates which are welded into the sloping walls of the pole-piece compartment; the yoke is bolted to these plates. Each magnet module can produce an induction of up to

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0.6 T across a 1 cm gap, over an area large enough to accommodate six orbits of the racetrack microtron operating in the n = 1 mode. The drift space length is adjusted by moving the pole-pieces within each magnet module. Mild steel wedges bolted to the inside of the sloping top and bottom walls of the vacuum compartment provide a pair of horizontal plane parallel surfaces for mounting the pole-piece assembly. Details of the assembly (fig. 3 ) a r e the same for both magnet modules. The two pole-pieces, each one fastened to a carriage plate, are held at the correct spacing and alignment by means of two separation plates which connect the pair of carriage plates. This pole-piece assembly is positioned between the wedges by means of small wheels (in the form of thin single-row ball bearings)attached to the sides of the carriage plates; these wheels run in shallow grooves in the parallel faces of the two wedges. This arrangement facilitates pole-piece alignment and allows simple and accurate movement for drift space length adjustment. It also makes it possible to test various two-sector guide field designs without extensive modification of the whole accelerator, since only the pole-pieces themselves need to be replaced. The pole-piece position is adjustable from outside the vacuum by a drive tube which passes through a vacuum feedthrough in the end wall of the pole-piece compartment and this adjustment can be remotely controlled while the

tl. R. FItOELICtt

478

microtron is operating. The drive tube also contains a rotating-coil gaussmeter probe which moves with the pole-piece assembly; thus the field in each magnet sector can be monitored continuously, The one adverse effect of the modular design chosen for this test machine is the fact that the larger than normal separation between the coils on the magnet yoke and the pole-pieces leads to stray magnetic fields in the drift space region. This problem is easily eliminated, however, by providing magnetic shields around the drift space. All of this shielding is attached to, and moves with, the pole-piece assemblies. A soft iron shield with holes to pass the various orbits is mounted in front of each magnet sector (fig. 1) and reduces the field to zero at a distance of 1.5 cm from the sector edge. Shielding above and below the orbit plane is provided by plates which extend forward

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ct ai.

from the pole-piece assemblies; the plates from the two opposite magnet modules are interleaved to make a continuous shield above and below the drift space while still allowing the pole-pieces to be moved. Additional stationary shielding is provided around the accelerating structure. In designing the UWO racetrack microtron, the separated function type of magnetic guide field was avoided because it is too complex for a compact low energy machine with variable energy. Beam focusing is therefore provided within the deflecting magnet sectors. For this purpose, the pole-pieces of each 180 ° sector are shaped to give three distinct uniform field regions, with different gap widths and hence different magnetic field strengths. Focusing occurs in the transition fields between these different uniform field regions. The three regions, shown in fig. 4 as ridge, plateau, and valley, have gap widths of 7.00, 8.75, and 13.13 m m respectively between the pole-pieces. All orbits except the first pass through this three-level field and experience axial focusing at the transition between the valley and the ridge. Radial focusing in these orbits is produced both by the transition field between plateau and valley and by the 180 ° deflection of the orbit. Because it has an average radius of only about 2 cm within the magnet sector, the first orbit undergoes quite strong axial defocusing in the fringe field outside the magnet. Axial focusing to compensate for this cannot be achieved in the same way as in the other orbits, however, since the first orbit does not penetrate far enough into the magnet sector. First orbit focusing is therefore obtained by making the ridge narrow and parallel-sided in the first orbit region. Strong axial focusing then occurs at the transition between the ridge and the lower field plateau region, while radial focusing is still maintained by the fringe field outside the magnet sector and by the 180 ° deflection of the orbit. A more detailed explanation of this type of focusing method and the calculations required to determine a suitable pole-piece shape will be given in a separate paper~9). Because of the long drift space and narrow magnet gaps, even small inaccuracies in pole-piece machining and magnet assembly can produce large enough radial deviations of the electron beam to prevent orbit closure. Fine trimming

of the mag-

netic field to ensure closed orbits can be clone e i t h e r by m e c h a n i c a l

shimming

of the pole-pieces

with soft steel foils in appropriate places or by in-

R A C EI-R, A C K

cluding small trimming electromagnets in each orbit. For the present test machine, the latter method is more convenient since the trimming magnets can then be used with any guide field configuration or pole-piece shape that is being tested and repeated lengthy shimming procedures can be avoided. Five small trimming magnets are attached to the magnetic shield in front of one magnet sector (fig. 1). They are located at the openings for the return portions of the first five orbits, on the inside of the drift space shielding enclosure, to supply small amounts of radial steering which may be required in the individual orbits, The accelerating structure is a side-coupled standing wave linear accelerator with three accelerating cavities and two coupling cavities; it operates in the ½n mode. The basic design of the linac is derived from the Los Alamos design2°), but some modifications were made to satisfy specific microtron requirements which are not usually encountered in linear accelerator design. For example, the lengths of the accelerating cavities could not be optimized for a specific energy gain or for a single value of initial energy. Instead, it was necessary to find a compromise cavity shape which is capable of accelerating electrons with initial speeds of 0.3c in the first traversal and c in the higher orbits, for any energy gain in the range from 0.5 to 1.0 MeV per cavity. Moreover, this shape must produce an adequate effective shunt impedance per unit length" Ls~ 2 fo B(Z) exp ( ircNz /Lst) dz Zefr = , (6) Ps~L~ where e(z)is the electric field amplitude along the axis of the accelerating structure, Pst is the power dissipated in the structure, N is the number of accelerating cavities, and Lst is the length of the structure. The shunt impedance that can be obtained is limited by the small size of the first orbit. With the energy gain per orbit ranging from 1.5 to 3.0 MeV, the distance between the axis of the accelerating structure and the return portion of the first orbit in the n = 1 mode varies from 3.8 to 4.2 cm. This distance is less than the outer radius required for an S-band Los Alamos type cavity of maximum shunt impedance, The three accelerating cavities in the structure are all identical in shape. Each cavity has an outer radius of 3.4 cm and a length L specified by

MICROTRON

479

/3 = 2 L / 2 =0.91, where /l is the free space wavelength of the 2812 MHz microwave field. The choice of cavity length as somewhat less than ½2 solves the problem of accelerating electrons with widely different energies, while the choice of radius is small enough to allow the return path of the first orbit to pass through a groove in the outer part of the cavity assembly. Since the small outer radius has the effect of reducing the cavity inductance, a larger capacitance must be provided in the equivalent circuit of the cavity in order to maintain the correct cavity frequency. This is done by shaping the cavity to have a comparatively large re-entrant cone (fig. 5). The shunt impedance for this cavity design, although appreciably less than the maximum value possible in the Los Alamos design, is still quite high; the theoretical effective shunt impedance per unit length of the accelerating structure is 60.5 M,O/m for a theoretical Q of 12 900. The cavities were machined in sections from OFHC copper and were individually pre-tuned to the operating frequency by contouring the re-entrant cones until the desired resonance was obtained. Final tuning was done after assembly and brazing of the complete accelerating structure. The unloaded Q for the whole linac was measured to be 9900 and thus the actual effective shunt impedance per unit length is 46 M.Q/m. The entire linac assembly, including cooling water tubes and also the rf waveguide connection to the middle accelerating cavity, is mounted on the removable cover plate for the vacuum port on one side of the microtron's central module (fig. 1). The rf power source is a 3.5 MW tunable magnetron (Raytheon QKH-327) driven by a radar modulator. The magnetron provides a 2 ps pulse at a repetition rate of 300 Hz. The waveguide feed from the magnetron to the accelerating structure is filled with Freon-12 vapor and is sealed from the vacu u m in the cavities by means of a high-power window at the cavity input. A 20 dB ferrite isolator at the magnetron output protects the magnetron from waveguide malfunction, while a 10 dB ferrite isolator at the cavity input protects the waveguide against excessive standing waves when the magnetron is not properly tuned to the linac frequency. The waveguide also contains a variable power divider 2~) which controls the fraction of the magnetron output power that is delivered to the cavities, a phase shifter between the power divider and the cavities, and directional couplers for for-

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ward and reverse power measurements. Even though the accelerating structure is water-cooled, drifting of the resonant frequency does occur and therefore the rf system includes an automatic frequency control to lock the magnetron to the linac frequency. Electrons are injected into the first accelerating cavity by an annular in-line electron gun 22) attached to the accelerating structure (fig.l). The electron beam passes through the hollow centre of the gun before re-entering the cavities for each subsequent acceleration. The gun (fig. 6)consists of an anode, a focusing electrode, and a bifilar emitting filament which is mounted in a circular groove in the focusing electrode; the bifilar winding ensures that there is no net magnetic field due to the heating current to deflect the emitted electrons. The gun voltage is pulsed at the rate of 300 Hz with a variable pulse length and a variable delay between the gun pulse and the rf pulse in the cavities. The peak gun voltage can be varied up to 50 kV and the maximum pulse current output from this gun is 1 A. This gun design is more convenient and more efficient than the off-axis injection system with electrostatic deflection that was used initially in this microtron]5). Beam current measurements inside the microtron were made originally with a movable Faraday cup which was inserted into the drift space, through a vacuum seal in the side of the central module, to intercept one orbit at a time. This de-

vice has been replaced by an array of inductive detectors which allows continuous monitoring of both the amplitude and the shape of the current pulse in any orbit except the first, without disrupting the electron beam23). This monitor unit is mounted on a vacuum port cover plate and is left permanently inside the drift space (fig. 1). It is now being used in conjunction with a switching circuit which displays the current pulses in all of the last five orbits simultaneously on an oscilloscope screen. The information about beam current in the different orbits is useful not only for adjusting the microtron parameters to satisfy resonant .-___q

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~aCL]~ACK MIC~O]~O~ operation, but also for evaluating the performance of the various microtron subsystems, 4. Operation and performance All power supplies, controls, meters, and indicator panels are grouped around a central control console outside the small room which houses the microtron itself and provides radiation shielding for the surrounding area. All aspects of microtron operation, including adjustments of injection energy, injected current level, rf field in the cavities, drift space length, and magnetic field in the magnet sectors, are controlled and monitored at this control console, An important feature of the racetrack design is the fact that the position of the extraction port remains fixed for all values of final beam energy, This is possible because the microtron is operated in different modes, specified by the integer n, to cover the design energy range from 1.5 to 18 MeV. Each mode requires a different number of traversals of the accelerating structure to bring the beam, at final energy, to the extraction port position. The energy gain per traversal, AE, can be varied from 1.5 to 3.0MeV. In the n = 1 mode, the beam traverses the cavities six times and the final energy varies from 9.0 to 18.0 MeV. In the n = 2 mode, three traversals produce a final energy range from 4.5 to 9.0 MeV, while the n = 3 mode has two traversals and final energies from 3.0 to 6.0 MeV. A single traversal of the cavities with n = 6 allows the beam to be extracted with energies from 1.5 to 3.0 MeV. During the first few months that the microtron was operated, energy gains of 3 MeV per orbit were readily obtained and the accelerator was in fact operated at an energy of slightly more than 18 MeV after six orbits. However, vacuum problems have caused some deterioration of the cavity re-entrant cones, with the result that excessive field emission and secondary emission of electrons now occur in the cavities for energy gains greater than about 2.6 MeV. As a result, the microtron is operated at the present time at final energies up to only 15.5 MeV. A four-cavity linac has been designed, but not yet built, to replace the present three-cavity accelerating structure when higher energies (up to 20 MeV) are required. To obtain a particular final energy, various machine parameters must be adjusted so that the requirements for resonance are satisfied. In the n = 1 mode, it is neither feasible nor necessary to have

481

strict resonance, i.e. strict synchronism between the phase of the electrons as they re-enter the accelerating structure and the phase of the rf field at that instant. Although the transit time for each orbit can be made an exact integral multiple of the rf period for any energy gain AE by choosing sk and B appropriately in eq. (5), very complicated magnetic field distributions and drift space adjusting mechanisms would be required to obtain strict resonance over the entire range of AE for all five closed orbits of the n = 1 mode simultaneously. Moreover, such strict resonance is not necessary because there exists in fact a range of phases over which phase-stable operation of a racetrack microtron is possiblelS). Not only does this mean that electrons with an initial distribution of phases over a certain range can be injected and accelerated to final energy, but it also means that during the acceleration process the phase of the beam can be allowed to shift, within a limited range, relative to the phase required for exact synchronism in the various orbits~7). As long as the shifting phase is kept within this limited range of the exact resonance phase, phase-stable acceleration to full energy is possible. This situation makes it possible to operate the microtron over the whole range of energy gains with relatively simple adjustments of drift space length and magnetic field which satisfy only approximately the resonance requirements for the individual orbits. It also means, of course, that the required parametric relationships between final energy and energy gain, drift space length, and magnetic field strength are not determined by a direct application of eqs. (1) and (4), but are derived from a detailed analysis of the electron orbits from injection to final energy. The calculations necessary to determine these parametric relationships will be described as part of a more general analysis of synchronism and phase stability in racetrack microtrons24). For the present electron gun, accelerating structure, and guide field configuration, maximum acceptance in the n = 1 mode is achieved if the following approximate relations are satisfied for the distance D, between the pole-piece edges of the two 180 ° magnet sectors, and the nominal energy gain 1~ AE = e J o ~(z) exp(ircNz/L~d dz , with parameters for AE defined as in eq. (6): D = (3.129-0.7875 Bo/Bp) 2, (7a)

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,

(7b)

where B0= 2 r C m o c / ( e 2 ) , Bp is the magnetic induction in the plateau regions of the magnet sectors, and 2 is the free space wavelength of the microwave field. In the n = 2 mode there are only two closed orbits; the beam is extracted in mid-orbit after the third traversal of the cavities. The two parameters D and Bp are sufficient to adjust both orbit times to be integral multiples of the rf period and thus strict resonance is possible in this mode. In practice, however, the microtron is operated in quasiresonant fashion for n = 2 as well as for n = 1 because the phase acceptance of the accelerator in the n = 2 mode is increased by relaxing the synchronism. The relations for machine operation in the n = 2 mode, as derived from orbit calculations, are: D = (3.129-0.7000Bo/Bp) 2, (8a)

n=2

24

=

(2.048Bv/Bo

0.029 B p2/ B o 2)

-

mo

c2 •

(8b)

For both the n = 1 and n = 2 modes, the final energy of the beam, Er, can be expressed in terms of the distance D and the induction Bp as: Ef = [(11.674-1.905

D/2) (Bp/So)

-

(9)

1] m o c 2 •

The range of a d j u s t m e n t for each of the microtron parameters is shown in table 1, which lists the values of D, Bp, and A E required for the maxi m u m and m i n i m u m energies as well as an intermediate energy, in the n = 1 and n = 2 modes. For n = 3 operation, there is no unique set of values for D, Bp, and A E corresponding to a given energy. Because there is only one orbit time to adjust, exact synchronism for a particular energy in the n = 3 mode can be obtained with a n u m b e r of difTABLE 1

Ranges of microtron parameters for n = 1 and n - 2

modes.

Final energy of beam Ef(MeV)

Pole-piece separation D(m)

Induction in plateau region Bp(T)

Nominal energy gain AE(MeV)

n=l

9.0 14.0 18.0

0.3055 0.3157 0.3197

0.3009 0.4730 0.6107

1.599 2.485 3.178

n=2

4.5 7.0 9.0

0.2841 0.3022 0.3093

0.1517 0.2392 0.3092

1.546 2.407 3.080

,

i

r

I

i

n"l

k-3

k-6

20 o

(cm)

k-5

16 k=2

k=4

k-3

k:l

k:2

! k-1

0

t

4

AE

al.

t

i

8

i

i

i

12

16 Ef

(MeV)

Fig. 7. Distance d between the axis of the accelerating structure and the straight section of the kth orbit return as a function of final beam energy El.

ferent combinations of the three machine parameters within the ranges shown in table 1. It should be noted that the values of Bp in table 1 are the measured field strengths in the plateau region of each magnet sector. The m a x i m u m field strength is 1.25 Bp in the ridge region. The magnetic field strength is controlled by a regulated current supply for the magnet coils, while A E is adjusted by means of the variable power splitter in the rf line. To adjust the distance D between the two m a g n e t sectors, both pole-piece assemblies are moved. The one in the m a g n e t module at the extraction end of the microtron is set at either of two positions, which are 1 cm apart, corresponding to n = 1 or n = 2 operation, while the one at the opposite end of the microtron is continuously adjustable to span the energy range of the selected mode. Because the field in each sector extends outside the sector edge, the total drift space length in each orbit is less than 2D. To operate the microtron at any selected final energy, the required values of the machine parameters are read from graphical or tabular forms of either eq. (7a) or (8a) and eq. (9) and the controls are adjusted to these values to bring the beam to the extraction port. Fine tuning adjustments of the magnet trim coils or of the resonance

RACETRACK

parameters themselves can be made to optimize the current in each orbit in succession. Complete adjustment of the machine for maximum extracted current at a different final energy requires only a few minutes. The energy of the extracted beam can be determined ( + 1%) from the settings of the microtron controls, using eq. (9). Although the position of the extraction port remains fixed for all energies, there is a small variation of the actual beam position within the 3 cm wide extraction aperture as the energy is varied. Because of the way that D and Bp are used jointly to adjust the orbit times for each energy, the radius of each orbit decreases with increasing final energy. This variation is shown in fig. 7 where the distance d between the beam axis of the accelerating structure and the straight section of the kth orbit return is plotted against final beam energy El. Over the 1.5-18 MeV energy range, the total lateral variation in the position of the extracted beam is about 15 mm. A ferrite-induction beam position detector 23) has been developed for applications which require position measurements to be made in a way that does not interrupt the beam. This device is suitable for use in beam steering systems within evacuated beam transport lines. Current profiles of the circulating beam in the drift space region are shown in fig. 8 for the n = 1 and n = 2 modes. Each profile was obtained by

3 (a)

Iret 2

0

4

0

8

12

16

d

20

(cm)

24

3-

(b) I re[ 2-

1-

0

!

0

'

;.

'

8

'

1'2

'

1'6

d

,

20(

Fig. 8. Typical current profiles, measured by scanning a Faraday cup across the drift space region, showing orbit positions and relative orbit currents for (a) 12 MeV in the n - - 1 mode and (b) 6 MeV in n - - 2 .

483

"MICROTRON

' /946 h,v (b) .,-

' ~' . . . .

/~

),2o7'8M.v

(a)

0.8

~= 06

4.B95 MeV--

N .--

~ 4.991 MeV

12.049 MeV~

--12.104MeV

o z

oo

'

~.8

s.o

si2

' ~'

'

,,18

Energy

~2.o

~z2

( MeV )

Fig. 9. Typical measured energy spectra: (a) nominal beam energy of 12.1 MeV in n = 1 mode, f w h m = 55 keV, (b) nominal beam energy of 4.9 MeV in n = 2 mode, f w h m = 93 keV.

making a continuous scan with a Faraday cup across the orbits, in the orbit plane, while the microtron was operating at a fixed final energy. Thus, these profiles show the positions of the various orbits at specific energies and the relative orbit currents, but they do not provide a direct measure of beam size because of the broadening produced by the finite aperture of the scanning Faraday cup. Even with this broadening, the orbit peaks in fig. 8 are narrower than the widths of the orbit openings in the magnetic shields and therefore the profiles do demonstrate the characteristically good orbit separation of a microtron. A significant feature of the 12 MeV profile is the fact that there is no appreciable loss of beam current after the third orbit in the n - 1 mode. The microtron has been operated over the entire design range of energies. Maximum pulse currents obtained in the extracted beam are 30, 60, 20, and 30 mA at energies of 4.5, 7.5, 9.0, and 15 MeV respectively, under normal 1/~s pulse operation. Pulse currents as high as 80 mA for energies in the n = 2 mode and 40 mA for n = 1 have been obtained with shorter pulse lengths. In either type of operation, the current is of course limited by the power that is available to the beam. For the normal 1 ~zs pulse conditions, this limit is simply the maximum power that the rf source can deliver to the accelerating structure while maintaining stable operation. This is determined by the coupling of the waveguide to the cavities; at the present time, the coupling coefficient is 1.6, overcoupled. The 1/zs pulse length for this type of operation results from the fact that approximately the first half of the 2/zs magnetron pulse is spent in building up the rf field in the cavities. Under

484

H R.

(b)

1.0-

FROEL1CH

(a)

Ire[ 0.5-

0.0

2~1

3'0

D

3'1 (cm)

3'2

Fig. 10. Relative output current measured as a function of pole-piece separation D, with magnetic field strength and rf power held constant at their values required for maximum current at (a) 14 MeV, n = 1 and (b) 6 MeV, n = 2.

the_shortened pulse conditions of operation, the beam is accelerated not only by acquiring energy at the rate that it is supplied by the rf source but also by actually depleting the energy that is stored in the rf field in the cavities. This increases temporarily the effective power available to the beam, and hence the peak output current, but it also reduces the pulse length by shortening the time interval in which ttie rf field in the cavities is high enough to sustain phase-stable acceleration. Typical pulse lengths under these conditions are a few hundred nanoseconds. This short pulse type of operation is triggered by increasing both the injected current and the delay time between the gun high voltage pulse and the magnetron pulse relative to their values for normal 1 ~zs operation at the same final energy. Measurements of the spectral distribution of energy in the extracted beam have confirmed the narrow spread of electron energies that is theoretically predicted 18) for a microtron beam. Typical

1.0 Iret

0.5

0.0

0.16

|

0.~0

|

/~,

0.2'~~0;,2 •

i

Bp

0.~6

i

(T)

o.go

Fig. 11. Relative output current measured as a function of magnetic field strength Bp in the plateau regions of the magnet sectors, with pole-piece separation and rf power held constant at their values required for maximum current at (a) 14 MeV, n - 1 and (b) 6MeV, n = 2 .

et al.

measured spectra are shown in fig. 9 for nominal energies of (a) 12.1 MeV in the n - 1 mode and (b) 4.9MeV in the n - 2 mode. The full widths of these spectra at half maximum intensity are 55 keV for (a) and 93 keV for (b), corresponding to relative energy spreads of 0.46% and 1.9% respectively. The relative energy spread is larger in the n = 2 mode because the beam makes only three traversals of the accelerating structure in that mode. The curves in figs. 10 and 11 illustrate how the microtron's output current is affected by changes in the resonance parameters. Fig. 10 shows the variation of extracted current as the pole-piece separation is varied over a small interval around the values required for (a) 14 MeV in the n - - 1 mode and (b)6 MeV in the n - 2 mode. Each curve was obtained by first adjusting the pole-piece separation, the magnetic field in the pole-pieces, and the rf power in the cavities to give the maximum output current at the selected energy. The output current was then measured as the pole-piece separation was varied about its initial value while the other parameters were kept fixed. Similarly, the curves in fig. 11 show the relative current as the magnetic field is varied while the other parameters remain fixed at the values corresponding to peak output current at (a) 14MeV for n - 1 and (b) 6 M e V for n=-2. Curves of the type shown in figs.. 10 and 11 have been measured at 1 MeV intervals for n - - 2 and at 2 MeV intervals for n - 1 . In addition, similar curves have been measured as the rf power level in the cavities was changed while the magnetic field and pole-piece separation were held constant. All of these curves show resonance-type peaks. As one of the microtron tuning parameters is changed from its required value for a given energy, the phase acceptance of the accelerator is also changed. This results not only in a reduction in the number of electrons accelerated to final energy, but also in a slight shift of the final beam energy relative to its initial value. Each measured curve of relative output current has a well-defined maximum and a width that is large compared to the smallest practical adjustment of the resonance parameter involved. For example, the magnetic field is controlled to within 10-3T, while the widths of the peaks in fig. 11 are appreciably larger than this even for h - 2 . These relatively broad peaks account for the comparative ease with which the microtron is tuned for a particular en-

RACETRACK

ergy during routine operation. By setting the controis to the values specified by eq. (7) or (8) and eq. (9), some output current is obtained; small adjustments of the controls then bring the output current level to its well-defined maximum, 5. Discussion Although many of the characteristics of racetrack microtrons indicated in section 2 have been recognized for some time, the machine described in this paper is, to our knowledge, the first practical demonstration of (1)microtronic recirculation through a multiple-cavity accelerating structure, (2) continuous energy variability over a wide range, and (3) satisfactory beam focusing in a twosector design without auxiliary focusing magnets, While perhaps the most striking features of the UWO racetrack microtron are its small size and its convenience of operation, it is really the overall performance of the machine since its initial operation in late 1972 that has shown that the racetrack microtron is not just an interesting laboratory curiosity but is in fact a versatile and dependable machine. Because of its inherent adaptability, it should be seriously considered as the accelerator-of-choice for various applications, both in the present energy range and at higher energies, In a recent discussion 25) of the microtron and its possible applications in nuclear physics research, gamma and neutron activation analysis, radiotherapy, and non-destructive testing, only a brief reference was made to the racetrack form of the microtron, apparently because of its relative unfamiliarity. Nevertheless the racetrack microtron is at least as well-suited as the conventional microtron for any application and, in fact, is frequently more suitable because of the size and flexibility advantages. Some specific conclusions about the advantages of a racetrack design for radiotherapy, based on the results obtained with our present variable energy machine, have already been reported16, 26). The features of the racetrack microtron are also of particular value in nuclear safeguards27). For this application the low energy spread of the electron beam is a major requirement. Compact size is an additional important advantage for portable monitoring units and variable beam energy is desirable to allow a single unit to test a broad variety of nuclear materials. The ease of operation and the dependability and reproducibility experienced with the machine described here, provide practical incentives for the design of a racetrack microtron

MICROTRON

485

that is tailored to specific safeguards requirements. The racetrack microtron also has potential usefulness as a CW accelerator, especially for nuclear physics applications at energies of 100 MeV or more. At these energies, the power requirements for CW operation of a single-pass linear accelerator become unreasonably large and the more efficient use of rf power, that is characteristic of a cyclic accelerator such as the microtron, becomes a very important feature. CW operation of a conventional microtron 5'28) is not very practical because of difficulties in supplying adequate cooling capacity for the cavity in the limited space that is available. Proposals for high duty factor microtrons using high power microwave tubes 29) also entail considerable cooling problems because of the high peak powers and long pulse lengths. For this reason, actual development of CW racetrack microtrons has hitherto made use of superconducting accelerating structures 3°'3~) with greatly reduced power requirements. However, a CW racetrack microtron with a long linac section operating at ordinary temperatures seems to be feasible. With a high shunt impedance linac design, adequate energy gains for a 100 MeV machine of reasonable size can be obtained with CW klystrons of only a few hundred kW. The most practical approach for a machine with low energy injection would be to design for n - - 2 operation. This makes the first orbit large enough so that the linac shunt impedance is not limited by space constraints as it is in the present UWO machine. It also provides space for a bulky linac assembly, thereby facilitating cooling of the structure. While cooling requirements would still be quite severe, a 100 MeV CW racetrack microtron which does not require superconducting systems appears to be within the realm of practical possibility. Such a machine could be used either directly for experiments or, as proposed previously for pulsed operation~4), as an injector for a still higher energy CW racetrack microtron. A recent design proposal 32) for an 820 MeV CW racetrack microtron system makes use of two microtrons and a pre-injection accelerator to reach an energy of 100 MeV, but it seems to us that a single stage racetrack microtron with low energy injection merits investigation as a reasonable alternative for obtaining a 100 MeV CW beam. The authors wish to thank all the technical staff who have participated in the Microtron Project

486

H.R.

F R O E L I C H et al.

and to acknowledge especially the contribution made by Mr. I. Schmidt and his assistants in their careful fabrication of many parts of the microtron. We also thank Profs. A. Watson and S. H. AbuSitta of the Faculty of Engineering Science and Prof. D. J. Dawson of the Ontario Cancer Foundation London Clinic for their interest and cooperation, and Mrs. M. Anderson for her help in typing the manuscript. This work was supported by Atomic Energy of Canada Ltd. (Commercial Products Division), Ottawa, Canada.

References

1)

V. I. Veksler, Dokl. Akad. Nauk SSSR 43 (1944) 329; [transl. J. Phys. USSR 9 (1945) 1531. 2) W. J. Henderson, H. LeCaine and R. Montalbetti, Nature 162 (1948) 699. 3) p. A. Redhead, H. LeCaine and W. J. Henderson, Can. J. Res. 28A (1948) 699. 4) A. P. Grinberg, Sov. Phys. Uspekhi 4 (1962) 857. 5) S. P. Kapitza and V. N. Melekhin, The microtron ( " N a u k a " ,Publ. House, Moscow, 1969). 6) O. Wernholm, Ark. Fysik 26 (1964) 527. 7) O. Wernholm; S. Rosander and M. Sedlacek, private communication. 8) D. Reistad and A. Brahme, Phys. Med. Biol. 17 (1972) 692. 9) A. Junghem, J. Osterberg and J. Weitman, Brit. J. Nondestructive Testing 16 (1974) 1. 10) E. M. Moroz, Dokl. Akad. Nauk. SSSR 115 (1957) 78; Sov. Phys. Doklady 2 (1958) 311. 11) A. Roberts, Ann. Phys. 4 (1958) 115. 12) H. Froelich and E. Brannen, IEEE Trans. Microwave Theory Tech. MTT-11 (1963) 288.

13) H. BaNd and M. Sedlacek, Nucl. Instr. and Meth. 56 (1967) 170. 14) H. R. Froelich and E. Brannen, IEEE Trans Nucl. Sci. NS14 (1967) 756. 15) H. R. Froelich, A. S. Thompson, D. S. Edmonds, Jr., and J. J. Manca, IEEE Trans. Nucl. Sci. NS-20 (1973) 260. 16) H. R. Froelich and J. J. Manca, IEEE Trans. Nucl. Sci. NS-22 (1975) 1758. 17) H. R. Froelich, P h . D . Thesis (Univ. of Western Ontario, 1962). 18) C. Henderson, F. F. Heymann and R. E. Jennings, Proc. Phys. Soc. (London) 66B (1953) 41. 19) H. R. Froelich, to be published. 20) E. A. Knapp, B. C. Knapp and J. M. Potter, Rev. Sci. Instr. 39 (1968) 979. 21) W. L. Teeter and K. R. Bushore, IRE Trans. Microwave Theory Tech. MTT-5 (1957) 227. 22~) j. j. Manca, D. S. Edmonds, Jr. and H. R. Froelich, Rev. Sci. Instr. 47 (1976) 1148. 23) j. j. Manca and H. R. Froelich, Nucl. Instr. and Meth. 136 (1976) 249. 24) H. R. Froelich, to be published. 25) G. Baciu, Proc. Intern. Conf. on Photonuclear reactions and applications, Pacific Grove, California, vol. 2 (1973) p. 1225. 26) j. C. F. MacDonald and H. R. Froelich, Proc. 3rd Conf. on Applications oj small accelerators, Denton, Texas (1974) p. 334. 27) T. Dragnev, At. Energy Rev. 11 (1973) 341. 28) p. p. Wintersteiner and D. S., Edmonds, Jr., IEEE Trans. Nucl. Sci. NS-14 (1967) 749. 29) B. H. Wiik and P. B. Wilson, Nucl. Instr. and Meth. 56 (1967) 197. 30) D. C. Sutton, A. O. Hanson, D. Jamnik, C.S. Robinson and P. Axel, IEEE Trans. Nucl. Sci. NS-I6 (1969) 985. 31) p. Axel, A. O. Hanson, J. R. Harlan, R. A. Hoffswell, D. Jamnik, D. C. Sutton and L. M. Young, IEEE Trans. Nucl. Sci. NS-22 (1975) 1176. 32) H. Herminghaus, A. Feder, K. H. Kaiser, W. Manz and H.v.d. Schmitt, Nucl. Instr. and Meth. 138 (1976) 1.