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The acceleration of deuterons by the university of Manitoba variable energy cyclotron
NUCLEAR INSTRUMENTS AND METHODS I36 (I976) 393-394;
© NORTH-HOLLAND PUBLISHING CO.
LETTERS TO T H E E D I T O R THE A C C E L E R A T I O N OF D E U T E R O N S BY THE U N I V E R S I T Y OF M A N I T O B A VARIABLE E N E R G Y C Y C L O T R O N I. G U S D A L , G. K N O T E , A. M c I L W A I N , J. S. C. M c K E E , S. O H a n d H . W . U Z A T
Cyclotron Laboratory, Department of Physics, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 Received 18 M a y 1976 D e u t e r o n b e a m s o f energies up to 21 MeV have been successfully extracted f r o m the University of M a n i t o b a cyclotron, a n d 24 M e V will shortly become available.
The success of the axial injection program recently undertaken at the University of Manitoba Cyclotron Laboratory1.2) has enabled rapid progress to be made toward the immediate goal of polarised beam production from the accelerator. As polarised sources will in general only operate reliably in a region of negligible stray magnetic field, an ion source hall has been constructed at ground level, two stories above the cyclotron vault, and three separate sources are shortly to be installed in this new area. The conventional duoplasmatron source of unpolarised ions which is already operational will shortly be moved to its new site above ground. A Sona source of polarised protons, which is in fact a modified version of the University of Alberta prototype of the T R I U M F source, is expected to be installed in the Hall in early summer and provide a polarised beam for experiments by early fall in 1976. In addition, a tailor-made Lamb-shift nuclear spinfilter source of the Los Alamos type is in the final design stage, and construction is expected to be completed and the facility to become operational, late in 1977 or early in 1978. This latter source will provide polarised D - in addition to H - ions. In order to proceed as rapidly as possible with the projected polarised deuteron program, it became essential to establish that the University of Manitoba spiral ridge machine was, in fact, capable of accelerating D - ions and extracting a deuteron beam. On the only previous occasion on which such acceleration was attempted, some nine years ago, most of the accelerated beam was lost at 10" radius, violent internal sparking was observed and the differential coupling rods used in horizontal adjustment of the dees vaporised, depositing zinc over large areas of the vacuum tank and insulators. Although the dees succeeded in holding voltage, problems of excessive local heating were not overcome at that time. It was therefore important to demonstrate that the problems encountered and identified in those
early days were not insurmountable, and indeed that most of them had since been overcome. In particular the improved design of shorting bar was expected to reduce the earlier problem of overheating, significantly. D - acceleration was therefore studied in some detail in March, 1976. In order to accelerate deuterons, the frequency of the master oscillator and predriver was switched from 28.48 M H z to 14.24 M H z and the input pi-network and the coil of the tuned circuit in the final stage of the rf drive chain, replaced. The resonant frequency of the cyclotron was altered by adjustment of the shorting bar. The dee-tips-inflector assembly, shown in detail in a previous paper2), is specifically designed for the axial injection of protons of 11 keV energy, and is thus suitable for acceleration of 5.5 keV deuterons at half the normal dee voltage, namely around 14 kV. It was however anticipated that the injection of deuterons of 11 keV energy would in the end be desirable and the question as to whether or not the dees could hold 27 kV at 14.24 MHz, was one requiring an immediate answer. In summary, the results of the present investigation have been most encouraging. Beam was injected, accelerated and several hundred nA of beam at 15 MeV energy were extracted. As there was no beam buncher available for injected beam during these tests, a factor of five increase in extracted current at this energy can therefore be anticipated under normal operating conditions. During these tests, beam was extracted with 100% efficiency for energies up to 21 MeV corresponding to 19.5" radius in the machine. At this point all beam was lost due to the lack of isochronism of the magnetic field at large radius. The highest energy deuterons, being less relativistic than the 50 MeV protons, require a flat field profile in this region, and it was found impossible to correct for field imperfections using temperature controlled invar blocks 3) which
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normally enable such corrections to the magnetic field to be made. In a series of further tests however, it was found that by increasing the current through the cyclotron main magnet to ~ 3 4 0 0 A (an increase o f 12%), the difference between the field value at the centre and at large radius could be reduced by over 30 G. This is believed to be a sufficient change to enable the invar controllers to correct the remaining discrepancy, but necessitates a change of approximately 100 k H z in the master oscillator frequency for deuteron acceleration. T h r o u g h o u t the tests of deuteron acceleration the rf remained stable for voltages up to 22 kV over periods of 24 h. A few bright spots were observed on the dee tips, but these were believed due to particle deposits burning off at the unaccustomed frequency o f 14.28 MHz. The machine vacuum pressure was around 2.5 x 10-6 mm and improving. As a result of these tests it appears that deuterons can indeed be accelerated to maximum energy without additional shimming of the main magnetic field of the cyclotron. For injection at 1 l keV energy, a new dee-tip assembly will clearly be required for deuterons but whether or not a universal dee-tip assembly suitable
for both protons and deuterons can or cannot be designed, is a question that thus far is without an unequivocal answer. The fact that the D - beam can be accelerated with reasonable overall transmission efficiency up to an energy of 15 MeV, indicates that the magnetic field configuration of the cyclotron is not far removed from an optimum condition. It is now our intention to complete a series of detailed field mappings and computer studies, designed to determine the necessary corrections which should be made to the field. When this has been done and the corrections made we believe that the University of Manitoba cyclotron can provide 50 nA of polarised D - beam of energies up to 24 MeV, in addition to the present H beam of energy up to and including 50 MeV.
References 1) A. Mcllwain and S. Oh, Proc. 7th Int. Conf. on Cyclotrons, Zurich (1975) p. 394. 2) R . A . Batten, J. Bruckshaw, I. Gusdal, G. Knote, A. Mcllwain, J. S. C. M c K e e and S. Oh, Nucl. Instr. and Meth. 136 (1976) 6. 3) K. G. Standing, J. J. Burgeron and F. K o n o p a s e k , Nucl. Instr. and Meth. 18 (1962) 111.
© NORTH-HOLLAND PUBLISHING CO.
LETTERS TO T H E E D I T O R THE A C C E L E R A T I O N OF D E U T E R O N S BY THE U N I V E R S I T Y OF M A N I T O B A VARIABLE E N E R G Y C Y C L O T R O N I. G U S D A L , G. K N O T E , A. M c I L W A I N , J. S. C. M c K E E , S. O H a n d H . W . U Z A T
Cyclotron Laboratory, Department of Physics, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 Received 18 M a y 1976 D e u t e r o n b e a m s o f energies up to 21 MeV have been successfully extracted f r o m the University of M a n i t o b a cyclotron, a n d 24 M e V will shortly become available.
The success of the axial injection program recently undertaken at the University of Manitoba Cyclotron Laboratory1.2) has enabled rapid progress to be made toward the immediate goal of polarised beam production from the accelerator. As polarised sources will in general only operate reliably in a region of negligible stray magnetic field, an ion source hall has been constructed at ground level, two stories above the cyclotron vault, and three separate sources are shortly to be installed in this new area. The conventional duoplasmatron source of unpolarised ions which is already operational will shortly be moved to its new site above ground. A Sona source of polarised protons, which is in fact a modified version of the University of Alberta prototype of the T R I U M F source, is expected to be installed in the Hall in early summer and provide a polarised beam for experiments by early fall in 1976. In addition, a tailor-made Lamb-shift nuclear spinfilter source of the Los Alamos type is in the final design stage, and construction is expected to be completed and the facility to become operational, late in 1977 or early in 1978. This latter source will provide polarised D - in addition to H - ions. In order to proceed as rapidly as possible with the projected polarised deuteron program, it became essential to establish that the University of Manitoba spiral ridge machine was, in fact, capable of accelerating D - ions and extracting a deuteron beam. On the only previous occasion on which such acceleration was attempted, some nine years ago, most of the accelerated beam was lost at 10" radius, violent internal sparking was observed and the differential coupling rods used in horizontal adjustment of the dees vaporised, depositing zinc over large areas of the vacuum tank and insulators. Although the dees succeeded in holding voltage, problems of excessive local heating were not overcome at that time. It was therefore important to demonstrate that the problems encountered and identified in those
early days were not insurmountable, and indeed that most of them had since been overcome. In particular the improved design of shorting bar was expected to reduce the earlier problem of overheating, significantly. D - acceleration was therefore studied in some detail in March, 1976. In order to accelerate deuterons, the frequency of the master oscillator and predriver was switched from 28.48 M H z to 14.24 M H z and the input pi-network and the coil of the tuned circuit in the final stage of the rf drive chain, replaced. The resonant frequency of the cyclotron was altered by adjustment of the shorting bar. The dee-tips-inflector assembly, shown in detail in a previous paper2), is specifically designed for the axial injection of protons of 11 keV energy, and is thus suitable for acceleration of 5.5 keV deuterons at half the normal dee voltage, namely around 14 kV. It was however anticipated that the injection of deuterons of 11 keV energy would in the end be desirable and the question as to whether or not the dees could hold 27 kV at 14.24 MHz, was one requiring an immediate answer. In summary, the results of the present investigation have been most encouraging. Beam was injected, accelerated and several hundred nA of beam at 15 MeV energy were extracted. As there was no beam buncher available for injected beam during these tests, a factor of five increase in extracted current at this energy can therefore be anticipated under normal operating conditions. During these tests, beam was extracted with 100% efficiency for energies up to 21 MeV corresponding to 19.5" radius in the machine. At this point all beam was lost due to the lack of isochronism of the magnetic field at large radius. The highest energy deuterons, being less relativistic than the 50 MeV protons, require a flat field profile in this region, and it was found impossible to correct for field imperfections using temperature controlled invar blocks 3) which
394
I. GUSDAL et al.
normally enable such corrections to the magnetic field to be made. In a series of further tests however, it was found that by increasing the current through the cyclotron main magnet to ~ 3 4 0 0 A (an increase o f 12%), the difference between the field value at the centre and at large radius could be reduced by over 30 G. This is believed to be a sufficient change to enable the invar controllers to correct the remaining discrepancy, but necessitates a change of approximately 100 k H z in the master oscillator frequency for deuteron acceleration. T h r o u g h o u t the tests of deuteron acceleration the rf remained stable for voltages up to 22 kV over periods of 24 h. A few bright spots were observed on the dee tips, but these were believed due to particle deposits burning off at the unaccustomed frequency o f 14.28 MHz. The machine vacuum pressure was around 2.5 x 10-6 mm and improving. As a result of these tests it appears that deuterons can indeed be accelerated to maximum energy without additional shimming of the main magnetic field of the cyclotron. For injection at 1 l keV energy, a new dee-tip assembly will clearly be required for deuterons but whether or not a universal dee-tip assembly suitable
for both protons and deuterons can or cannot be designed, is a question that thus far is without an unequivocal answer. The fact that the D - beam can be accelerated with reasonable overall transmission efficiency up to an energy of 15 MeV, indicates that the magnetic field configuration of the cyclotron is not far removed from an optimum condition. It is now our intention to complete a series of detailed field mappings and computer studies, designed to determine the necessary corrections which should be made to the field. When this has been done and the corrections made we believe that the University of Manitoba cyclotron can provide 50 nA of polarised D - beam of energies up to 24 MeV, in addition to the present H beam of energy up to and including 50 MeV.
References 1) A. Mcllwain and S. Oh, Proc. 7th Int. Conf. on Cyclotrons, Zurich (1975) p. 394. 2) R . A . Batten, J. Bruckshaw, I. Gusdal, G. Knote, A. Mcllwain, J. S. C. M c K e e and S. Oh, Nucl. Instr. and Meth. 136 (1976) 6. 3) K. G. Standing, J. J. Burgeron and F. K o n o p a s e k , Nucl. Instr. and Meth. 18 (1962) 111.