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Heavy ion injection for tandem accelerators
N U C L E A R I N S T R U M E N T S AND METHODS 146 (1977)
115-119 ; ©
N O R T H - H O L L A N D PUBLISHING CO.
HEAVY ION INJECTION FOR TANDEM ACCELERATORS KENNETH H. PURSER General lonex Corporation, Ipswich, Massachusetts,
U.S.A.
The advantages are reviewed for high energy (megavolt) injection of tandem accelerators during heavy ion operation. It is pointed out that beam transmission, loading, critical injected currents, total voltage effect and pulsed beam operation can all be significantly improved using high energy injection. The advent of reliable and long lived negative ion sources makes possible compact, pressurized injectors that allow any tandem facility to get the substantial advantage of high energy injection.
1. Introduction The availability of versatile, long lived, and reliable negative ion sputter sources of the Middleton type 1) make it possible to consider schemes for injecting heavy ions into tandem accelerators that would have been impractical a few years ago. For example, as part of a program to improve the heavy ion performance of the MP tandem facility at the University of Rochester and couple this machine to a large heavy ion cyclotron2), it has been proposed that a new injection system be constructed that would inject the tandem with negative ions of energy up to 3 MeV. In this scheme, negative ions would be extracted from a sputter source, capable of producing twelve selectable beam species, located in the terminal of a 3 MV pressurized Van de Graaff generator designed for fast access. The reliability of the system can be high and at Rochester it is anticipated that it will not be necessary to open the pressure vessel more often than once every few hundred hours, which is the typical period between service for injector systems presently in use. High energy tandem injection is not a novel concept and was an integral part of several proposals for new tandem arrangements suggested by Van de Graaff 3) in 1958. Experimentally, it has been found that whenever such schemes have been implemented, the performance and transmission of the two-stage tandem itself has been excellent; problems that have arisen are traceable to the ion source, the neutral beam drift section, or the injector. For tandems specializing in heavy ion physics there are a number of advantages to be gained by high energy injection: 1) Particle transmission efficiency can be close to 100%.
2) Beam induced loading phenomena are lessened allowing higher currents to be injected. 3) "Total voltage" effects can be reduced by diameter reductions and improvements in inclined field structure. 4) Good pulsed heavy ion beam performance is possible. 5) In ultra-high voltage machines the need for some intank optical elements can be eliminated allowing the machine to become more of a passive element.
2. Improved ion beam transmission efficiency Larson 4) has reviewed many of the techniques that can be used for tandem injection. One constraint that always appears to be present is that the negative ion beam must be focused to a waist at the terminal of the tandem accelerator. Several requirements lead to this constraint, the most important of which is that if a gas target is used for stripping, the canal must be long and of small diameter to prevent excessive flows of gas into the vacuum system. When foils are used, the increase in phase space area produced by small angle scattering is minimized by keeping the beam diameter small during charge exchange. To produce this beam waist some form of positive focusing element is needed in the low energy end of the accelerator. In earlier tandems the fringing field at the entrance to the first acceleration tube was commonly used to provide this focusing. Unfortunately, the focal strength of such fringing field lenses is usually far too high for convenient optics and leads to matching problems. Even more troublesome is that the focal strength varies strongly with terminal potential leading to variable transmission characteristics. These diffi1. TANDEM PHYSICS
116
K.H.
culties have been successfully eliminated by the High Voltage Engineering Corporation 5'6) by stretching a plane grid of fine mesh across the accelerator tube opening and providing the necessary focusing by means of a separate bipotential element. The grid eliminates the natural focusing fields by permitting the termination of the electrostatic fields within the tube at a pseudo plane rather than by the bulging field which is inherent in a free space solution of the Laplace equation. While the gridded lens solution works well and provides flat and high transmission characteristics for tandems over a large voltage range, it introduces several operational disadvantages: 1) A small amount of beam ( - 12%) is lost by interception at the grid. 2) Secondary electron emission and particles that are sputtered from the grid aggravates machine loading and instabilities. 3) The multiple facet lenses of the grid introduces aberations and emittance increases into the beam. The focal strength,.f, of an ungridded entrance lens is given approximately 7) by: f = 4V/F,
where V is the potential that is equivalent to the energy of the injected ion and F is the field gradient within the low energy tubes. It can be seen in the case of high energy injection, where the particles are injected with an energy of 2 MeV into a gradient of 20 kV/cm, that the focal length of an ungridded entrance lens is always much longer than the characteristic lengths of the optical system so that the tube entrance lens effect no longer dominates the optics of the whole accelerator. Because the focal lengths are so long gridding is unnecessary, making it possible to eliminate all undesirable characteristics. At injection energies of 2 MeV the trajectories within the accelerator become comparable to those of a non-field drift section. Thus, the focus that is needed at the stripper can be produced by an external lens at ground potential that is completely independent of the acceleration structure and the voltage across it. This lens can be a high quality, low aberration device whose focal strength needs to change only slightly with terminal voltage. Optically, high energy injection leads to constant optical properties over the whole energy range of the accelerator, high beam transmission and small increases in emittance due to aberrations.
PURSER
3. Reduction in beam induced loading Experience has shown that the interactions of heavy ion beams with the residual gas atoms in an acceleration tube can lead to a critical limitation in the injected and transmitted beam intensity 8) (the so-called "critical" current). These interactions not only strip the negative ions but also produce secondary particles which are themselves accelerated and increase the outgassing rate in the manner described by Assman et al.9). These results emphasize the importance of a good base vacuum and high pumping speeds. However if the interaction cross sections with the residual gas can be reduced then the upper limit on the injected beam current can be increased accordingly. Data presented by Rose and Galejs 1°) indicate that the loss cross section for negative ions in a gas is approximately propertional to E -½, where E is the particle energy. By injecting at 2 M e V rather than 200keV the secondary particles produced in the early part of the acceleration tube will be significantly reduced. 4. Total voltage effects Before the introduction of inclined field suppression 11-13) in the voltage tubes, the voltage capability of Van de Graaff accelerators was frequently limited by cooperative electron and ion loading processes which occur along the length of the tube. This maximum voltage which is often set by the total voltage across the tube rather than the sum of the maximum plane-to-plane voltages is known as the "total voltage effect". While the processes involved in the total voltage phenomenon are complex and not well understood, the effect is clearly dependent upon regenerative negative ion/positive ion cascades. A variety of suppression schemes have been used as cures for these effects. These include the tapered tube designs of Trumpt4), back biasing by McKibbentS), the use of alternating and rotating radial electric ~1,13) electric and magnetic fields and an ingenious design of the National Electrostatics Corporation ~6) which employs variable axial gradients and apertures to trap many of the particles not produced on axis. The effectiveness of any of the above suppression arrangements increases dramatically as the aperture of the acceleration tube is reduced. In practice the designer must always compromise between small aperture size, adequate pumping conductance and adequate beam admittance. Injection at energies - 2 MeV make possible a substantial
HEAVY
ION I N J E C T I O N
reduction in tube aperture. The reason for this is that for given source emittance the necessary geometrical acceptance of the accelerator changes as E0-I, where E0 is the injection energy. The increase in energy represents as much as a factor of three that it is possible to reduce the necessary apertures of the acceleration tubes if this is consistent with vacuum conductance requirements. Injection at energies of ~ 2 MeV make it possible to increase the effectiveness of an inclined field suppression structure in a completely different manner. In an inclined field acceleration tube, ions and electrons formed at low energy by the beam and other secondary processes are sweptoffaxis and intercepted by the electrode structure before traveling more than 30--60cm and before gaining substantial energy. In this way the sputter and X-ray, yields from these secondary particles are kept low. The radial components which produce suppression and remove the unwanted particles also cause the accelerated beam to suffer a series of small radial accelerations; ideally these arrange to average to zero over the length of the tube. Unfortunately for injection energy ~200 keV and gradients N20 kV/cm the beam particles do not have sufficient energy that the unwanted particles cannot be removed without also losing primary beam. For this reason all inclined field systems presently in use have an initial uniform field section within the accelerator of length 30-60 cm which accelerates the particles to an energy N 1 MeV before effective suppression begins. Over this length only magnetic suppression of electrons is possible. In practice this nonsuppressed region is the source of many of the secondary particles that ultimately limit the accelerator performance. An important advantage of injecting at energies ~ 2 M e V is that uniform field regions are no longer necessary and radial field suppression can be extended through the whole length of the accelerator. Such arrangements will improve the voltage holding ability of the machines, and increase the "critical" currents of heavy ions that can be accelerated as defined in section 3.
5. Heavy ion pulsing For many experiments and heavy ion booster applications, nanosecond beam pulsing is essential. For example, the superconducting cyclotron proposed by the University of Rochester has frequencies that lie between 18 and 36 MHz and requires
117
TABLE 1 Beam characteristics at exit from tandem using gas stripper plus V3/4 foil stripper configuration%. Beam
Iodine
Injection energy Terminal voltage Straggling in gas stripper Charge state Energy loss in foil Charge state Straggling in foil Thickness variations (1096) Terminal instability (2 kV) Final ion energy Total energy spread Inverse velocity of ion Flight time beyond tandem AE/E At due to post-acceleration drift Time spreads from pre-acceleration buncher Total At
1.5 MeV 11.5 MV 0.5 keV 5+ 400 keV 13 + 22 keV 40 keV 22 keV 139 MeV 50 keV 68 ns/m 3060 ns 3.6× 10-4 - 0 . 6 ns _<0.8 ns _< 1.0 ns
beam bursts having widths and phase stability of N0.5 ns. In practice these specifications represent an impossible criterion to meet if all of the beam pulsing is done before the tandem accelerator. Small changes in the injector voltage, tandem terminal voltage, or voltage gradients produce substantial variations in the transit time of ions through the machine. A very sensitive region is the injector drift spaces where the velocity of the particles is slow and the transit times are long. For example, iodine injected at 2 MeV with an energy stability of _+0.5 keV, has time fluctuations at the target of +_ 1.3 ns. Fortunately, these time shifts occur on a scale which is of the order of milliseconds so that phase correction is possible. In practice, the pulse widths and time stabilities needed require the use of a post acceleration buncher. Such a device can be constructed using a resonator similar to those developed by Dick and Shepard ~7) and by Stokes and coworkers at Los Alamos~8). Such devices operate in the frequency range 90-150 MHz and have a time acceptance of - 3 ns. This acceptance is matched to the burst widths that can be produced by a well designed pre-acceleration bunching system. A basic limitation to the pulse width available after tandem acceleration is the energy spread in the beam. This energy inhomogeneity depends primarily upon straggling in the strippers, thickness variations in the foils, and the constancy of the terminal voltage. Table 1 ~9) lists the magnitude of various contributions to the 1. T A N D E M
PHYSICS
118
K.H.
PURSER
energy spread expected for an iodine beam, assuming 1.5 MeV injection energy. For the purpose of the present discussions we will assume that the pre-acceleration velocity modulator is a simple two-gap device. Optically, each gap is an equi-diameter two-cylinder immersion lens whose strength is not fixed but varies with the rf phase. Clearly, the modulator can increase the effective emittance of the beam and because of this, it is necessary that the modulator be located at a beam waist so that the introduced optical distortions enter only in angle variable and do not affect the radial size at beam waists. Clearly, the magnitude of these undesirable effects can be minimized by keeping the diameter of the modulating cylinders as large as possible. In practice, however, the diameters that can be used are quite limited because the particles should remain within the modulating field region for only a small rf phase angle. In this regard it is interesting to note that a 2.5 MeV uranium ion takes 8.3 ns to cross a 1 cm region. This is approximately the effective field length of a pair of 1 cm equi-diameter cylinders. For a buncher operating at 20 MHz, this time corresponds to a phase angle of ~60 °. This phase angle is large and emphasizes the importance of bunching at high energies to minimize transit time effects. It can be readily demonstrated 2°) that to a high approximation the conditions for producing a cornUNIFORM FIELD x 720 INFLECTION MAGNET -7 / /
DOUBLET/STEERER/1
~
QUAOROP E --7 MATCHING GAP LENS
.loo°
I/
,\\\.~"'~"
dt
where E0 is the energy of an ion having a mass m. The same formalism can be applied to a tandem accelerator and it can be shown that an equivalent L can be readily calculated. It is found that the drift space between the buncher and the low energy acceleration tube makes the most significant contribution to the equivalent L with the rest of the tandem acceleration system making a comparable contribution. For the system proposed by the University of Rochester, shown in fig. 1, the equivalent L for high energy injection energy is approximately 13.4m so that for iodine at 1.5 MeV
dE/dt = 340 eV/ns. The necessary motlulator voltages are easy to achieve above 4 MHz. It is important that dE~dr be moderately large for all ions. First, (dE/dt)xAT, where AT is the desired time resolution, should be much larger than the natural energy spreads that are present in the ions leaving the source. Secondly, and in practice more importantly, is the related phenomena
~~::°'°0°°~
EXISTING INJECTOR /
ANOAPERTURE(A)
CHOPPER WITH RETRACE ELIMINATION FARADAYCUP
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//
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I
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,
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I
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I L-/ / "
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ENERGY FEEDBACK SLITS
1
ELECTROSTATIC
pletely bunched beam at a final plane located at a drift distance, L, away from the center of the buncher is that the energy of the ions must change at the rate of
\ LPRE-ACCELERATIONTUBE \ ~__ SECONDARy ELECTRON
SUPRESS~N APERTURE
L--%ANENT MAGNET ELECTRON SUPPRESSION VELOCITY SELECTOR - - SPUTTER-CONE
Fig. 1. Experimental set-up proposed by the University of Rochester.
t ,
~
SGNAL FROM CYCLOTRON -PHASE STABILIZER AND FREQUENCY SYNTHESIZER
~
MP-TANDEM
.......
ZI~ l ....
\\
-
L>CCELERAT,ON
"% L-LENS
FOCUSS,NG
HEAVY lON I N J E C T I O N
that (dE/dt)×zlT should be large compared to fast power supply fluctuations in the injector that are of a speed or of frequency such that they cannot be corrected for by phase stabilizer circuitry. High frequency ripples and fast voltage fluctuations of the order of many hundreds of volts are common in existing injector systems and in practice are difficult to avoid. The only practical solution which can be expected to operate reliably on a day-to-day basis is to keep dE/dt high. Both these considerations and the transit time effects mentioned earlier indicate the importance of high energy injection.
6. Injector system design The major features of the injection system proposed by Rochester are shown in fig. 1. After extraction, negative ions produced by the sputter source will be focused by a single unipotential lens through a velocity selector to a crossover at the first defining aperture. The velocity selector is necessary to eliminate the intense negative oxygen ions, carbon ions, and electrons that are always produced by sputter sources. Because of the h a zards from accelerating electrons, a permanent magnet solenoid is also placed around the electrostatic lens structure. This solenoid has weak focal properties for heavy ions but is a strong element for electrons and strongly overfocuses them preventing acceleration. Matching into the injector is accomplished using a standard gap lens arrangement. After acceleration the beam is focused by an electrostatic alternating gradient lens and magnet combination and then momentum analyzed. The velocity modulator would be located close to the momentum rejection aperture. The beam will be focused to the stripper of the MP tandem by a single quadrupole lens. The optics of this coupling stage produce a slight demagnification between the injection point, where the buncher is located, and the stripper in the terminal of the tandem. Close to 100% transmission is expected for rather high intensities of heavy ion beams. 7. Conclusion It has been pointed out by Larson4), that during recent years there has been a substantial shift in
119
design emphasis which has transformed tandem accelerators from largely passive elements to active parts of the total beam transport system. This involves the installation of many intank optical elements. While this shift in emphasis, and its consequent requirement of higher reliability, is inevitable with the advent of much larger machines, high energy injection can reduce the need for some of the complexity. High energy injection leads to substantially improved particle transmission efficiency, reduced loading and reductions in the total voltage effects of the accelerator. High energy injection allows the possibility of higher critical current levels and substantially improved pulsed beam performance. The availability of versatile and reliable negative ion sources make it possible to design pressurized injectors with fast access which make it possible to take advantage of these features.
References I) R. Middleton, Nucl. Instr. and Meth. 122 (1974) 35. 2) University of Rochester, Heavy ion post accelerator proposal, UR-NSRL-106 (1976)~ 3) R. J. Van de Graaff, Nucl, Instr. and Meth. 8 (1960) 195. 4) j. D. Larson, Nucl. Instr. and Meth. 122 (1974) 53. 5) K. H. Purser, U. S. Patent 3, 731, 211 (1973). 6) Brooks et al., IEEE Trans. Nucl. Sci. NS-12 (1965) 313. 7) C. J. Davisson and C. J. Calbick, Phys. Rev., 42 (1932) 580. 8) T. R. Ophel et al., Nucl. Instr. and Meth. 122 (1974) 227. 9) Assman et al., Nucl. Instr. and Meth. 122 (1974) 191. 10) p. H. Rose and A. Galejs, Progress in nuclear techniques and instrumentation (ed. F. J. M. Farley; North-Holland, Publ. Co., Amsterdam, 1967). ll) R. J. Van de Graaff et al., Nature 195 (1962) 1292. 12) K. H. Purser et al., Rev. Sci. Instr. 36 (1965) 453. t3) W. D, Allen, Rutherford Laboratory Report, N1RL/R21 (1962). 14) j. G. Trump and R. W. Cloud, U.S. Patent 2, 521, 426 (1950). 15) j. L. McKibben, Proc. 1st Int. Conf. on Insulation o f high voltage in vacuum (1964) p. 337. 16) R. G. Herb, Nucl, Instr. and Meth. 122 (1974) 267., 17) G. J. Dick and K. W. Shepard, Appl. Phys. Lett. 24 (1974) 40. 18) D, D. Armstrong et al., to be published in Part. Acc. 19) Some data obtained from: Apache - A proposal for physics and chemistry of heavy elements, Oak Ridge National Laboratory (1969). 2o) K. H. Purser et al., 1EEE Trans. Nucl. Sci. NS-12 (1965) 415.
1. TANDEM PHYSICS
115-119 ; ©
N O R T H - H O L L A N D PUBLISHING CO.
HEAVY ION INJECTION FOR TANDEM ACCELERATORS KENNETH H. PURSER General lonex Corporation, Ipswich, Massachusetts,
U.S.A.
The advantages are reviewed for high energy (megavolt) injection of tandem accelerators during heavy ion operation. It is pointed out that beam transmission, loading, critical injected currents, total voltage effect and pulsed beam operation can all be significantly improved using high energy injection. The advent of reliable and long lived negative ion sources makes possible compact, pressurized injectors that allow any tandem facility to get the substantial advantage of high energy injection.
1. Introduction The availability of versatile, long lived, and reliable negative ion sputter sources of the Middleton type 1) make it possible to consider schemes for injecting heavy ions into tandem accelerators that would have been impractical a few years ago. For example, as part of a program to improve the heavy ion performance of the MP tandem facility at the University of Rochester and couple this machine to a large heavy ion cyclotron2), it has been proposed that a new injection system be constructed that would inject the tandem with negative ions of energy up to 3 MeV. In this scheme, negative ions would be extracted from a sputter source, capable of producing twelve selectable beam species, located in the terminal of a 3 MV pressurized Van de Graaff generator designed for fast access. The reliability of the system can be high and at Rochester it is anticipated that it will not be necessary to open the pressure vessel more often than once every few hundred hours, which is the typical period between service for injector systems presently in use. High energy tandem injection is not a novel concept and was an integral part of several proposals for new tandem arrangements suggested by Van de Graaff 3) in 1958. Experimentally, it has been found that whenever such schemes have been implemented, the performance and transmission of the two-stage tandem itself has been excellent; problems that have arisen are traceable to the ion source, the neutral beam drift section, or the injector. For tandems specializing in heavy ion physics there are a number of advantages to be gained by high energy injection: 1) Particle transmission efficiency can be close to 100%.
2) Beam induced loading phenomena are lessened allowing higher currents to be injected. 3) "Total voltage" effects can be reduced by diameter reductions and improvements in inclined field structure. 4) Good pulsed heavy ion beam performance is possible. 5) In ultra-high voltage machines the need for some intank optical elements can be eliminated allowing the machine to become more of a passive element.
2. Improved ion beam transmission efficiency Larson 4) has reviewed many of the techniques that can be used for tandem injection. One constraint that always appears to be present is that the negative ion beam must be focused to a waist at the terminal of the tandem accelerator. Several requirements lead to this constraint, the most important of which is that if a gas target is used for stripping, the canal must be long and of small diameter to prevent excessive flows of gas into the vacuum system. When foils are used, the increase in phase space area produced by small angle scattering is minimized by keeping the beam diameter small during charge exchange. To produce this beam waist some form of positive focusing element is needed in the low energy end of the accelerator. In earlier tandems the fringing field at the entrance to the first acceleration tube was commonly used to provide this focusing. Unfortunately, the focal strength of such fringing field lenses is usually far too high for convenient optics and leads to matching problems. Even more troublesome is that the focal strength varies strongly with terminal potential leading to variable transmission characteristics. These diffi1. TANDEM PHYSICS
116
K.H.
culties have been successfully eliminated by the High Voltage Engineering Corporation 5'6) by stretching a plane grid of fine mesh across the accelerator tube opening and providing the necessary focusing by means of a separate bipotential element. The grid eliminates the natural focusing fields by permitting the termination of the electrostatic fields within the tube at a pseudo plane rather than by the bulging field which is inherent in a free space solution of the Laplace equation. While the gridded lens solution works well and provides flat and high transmission characteristics for tandems over a large voltage range, it introduces several operational disadvantages: 1) A small amount of beam ( - 12%) is lost by interception at the grid. 2) Secondary electron emission and particles that are sputtered from the grid aggravates machine loading and instabilities. 3) The multiple facet lenses of the grid introduces aberations and emittance increases into the beam. The focal strength,.f, of an ungridded entrance lens is given approximately 7) by: f = 4V/F,
where V is the potential that is equivalent to the energy of the injected ion and F is the field gradient within the low energy tubes. It can be seen in the case of high energy injection, where the particles are injected with an energy of 2 MeV into a gradient of 20 kV/cm, that the focal length of an ungridded entrance lens is always much longer than the characteristic lengths of the optical system so that the tube entrance lens effect no longer dominates the optics of the whole accelerator. Because the focal lengths are so long gridding is unnecessary, making it possible to eliminate all undesirable characteristics. At injection energies of 2 MeV the trajectories within the accelerator become comparable to those of a non-field drift section. Thus, the focus that is needed at the stripper can be produced by an external lens at ground potential that is completely independent of the acceleration structure and the voltage across it. This lens can be a high quality, low aberration device whose focal strength needs to change only slightly with terminal voltage. Optically, high energy injection leads to constant optical properties over the whole energy range of the accelerator, high beam transmission and small increases in emittance due to aberrations.
PURSER
3. Reduction in beam induced loading Experience has shown that the interactions of heavy ion beams with the residual gas atoms in an acceleration tube can lead to a critical limitation in the injected and transmitted beam intensity 8) (the so-called "critical" current). These interactions not only strip the negative ions but also produce secondary particles which are themselves accelerated and increase the outgassing rate in the manner described by Assman et al.9). These results emphasize the importance of a good base vacuum and high pumping speeds. However if the interaction cross sections with the residual gas can be reduced then the upper limit on the injected beam current can be increased accordingly. Data presented by Rose and Galejs 1°) indicate that the loss cross section for negative ions in a gas is approximately propertional to E -½, where E is the particle energy. By injecting at 2 M e V rather than 200keV the secondary particles produced in the early part of the acceleration tube will be significantly reduced. 4. Total voltage effects Before the introduction of inclined field suppression 11-13) in the voltage tubes, the voltage capability of Van de Graaff accelerators was frequently limited by cooperative electron and ion loading processes which occur along the length of the tube. This maximum voltage which is often set by the total voltage across the tube rather than the sum of the maximum plane-to-plane voltages is known as the "total voltage effect". While the processes involved in the total voltage phenomenon are complex and not well understood, the effect is clearly dependent upon regenerative negative ion/positive ion cascades. A variety of suppression schemes have been used as cures for these effects. These include the tapered tube designs of Trumpt4), back biasing by McKibbentS), the use of alternating and rotating radial electric ~1,13) electric and magnetic fields and an ingenious design of the National Electrostatics Corporation ~6) which employs variable axial gradients and apertures to trap many of the particles not produced on axis. The effectiveness of any of the above suppression arrangements increases dramatically as the aperture of the acceleration tube is reduced. In practice the designer must always compromise between small aperture size, adequate pumping conductance and adequate beam admittance. Injection at energies - 2 MeV make possible a substantial
HEAVY
ION I N J E C T I O N
reduction in tube aperture. The reason for this is that for given source emittance the necessary geometrical acceptance of the accelerator changes as E0-I, where E0 is the injection energy. The increase in energy represents as much as a factor of three that it is possible to reduce the necessary apertures of the acceleration tubes if this is consistent with vacuum conductance requirements. Injection at energies of ~ 2 MeV make it possible to increase the effectiveness of an inclined field suppression structure in a completely different manner. In an inclined field acceleration tube, ions and electrons formed at low energy by the beam and other secondary processes are sweptoffaxis and intercepted by the electrode structure before traveling more than 30--60cm and before gaining substantial energy. In this way the sputter and X-ray, yields from these secondary particles are kept low. The radial components which produce suppression and remove the unwanted particles also cause the accelerated beam to suffer a series of small radial accelerations; ideally these arrange to average to zero over the length of the tube. Unfortunately for injection energy ~200 keV and gradients N20 kV/cm the beam particles do not have sufficient energy that the unwanted particles cannot be removed without also losing primary beam. For this reason all inclined field systems presently in use have an initial uniform field section within the accelerator of length 30-60 cm which accelerates the particles to an energy N 1 MeV before effective suppression begins. Over this length only magnetic suppression of electrons is possible. In practice this nonsuppressed region is the source of many of the secondary particles that ultimately limit the accelerator performance. An important advantage of injecting at energies ~ 2 M e V is that uniform field regions are no longer necessary and radial field suppression can be extended through the whole length of the accelerator. Such arrangements will improve the voltage holding ability of the machines, and increase the "critical" currents of heavy ions that can be accelerated as defined in section 3.
5. Heavy ion pulsing For many experiments and heavy ion booster applications, nanosecond beam pulsing is essential. For example, the superconducting cyclotron proposed by the University of Rochester has frequencies that lie between 18 and 36 MHz and requires
117
TABLE 1 Beam characteristics at exit from tandem using gas stripper plus V3/4 foil stripper configuration%. Beam
Iodine
Injection energy Terminal voltage Straggling in gas stripper Charge state Energy loss in foil Charge state Straggling in foil Thickness variations (1096) Terminal instability (2 kV) Final ion energy Total energy spread Inverse velocity of ion Flight time beyond tandem AE/E At due to post-acceleration drift Time spreads from pre-acceleration buncher Total At
1.5 MeV 11.5 MV 0.5 keV 5+ 400 keV 13 + 22 keV 40 keV 22 keV 139 MeV 50 keV 68 ns/m 3060 ns 3.6× 10-4 - 0 . 6 ns _<0.8 ns _< 1.0 ns
beam bursts having widths and phase stability of N0.5 ns. In practice these specifications represent an impossible criterion to meet if all of the beam pulsing is done before the tandem accelerator. Small changes in the injector voltage, tandem terminal voltage, or voltage gradients produce substantial variations in the transit time of ions through the machine. A very sensitive region is the injector drift spaces where the velocity of the particles is slow and the transit times are long. For example, iodine injected at 2 MeV with an energy stability of _+0.5 keV, has time fluctuations at the target of +_ 1.3 ns. Fortunately, these time shifts occur on a scale which is of the order of milliseconds so that phase correction is possible. In practice, the pulse widths and time stabilities needed require the use of a post acceleration buncher. Such a device can be constructed using a resonator similar to those developed by Dick and Shepard ~7) and by Stokes and coworkers at Los Alamos~8). Such devices operate in the frequency range 90-150 MHz and have a time acceptance of - 3 ns. This acceptance is matched to the burst widths that can be produced by a well designed pre-acceleration bunching system. A basic limitation to the pulse width available after tandem acceleration is the energy spread in the beam. This energy inhomogeneity depends primarily upon straggling in the strippers, thickness variations in the foils, and the constancy of the terminal voltage. Table 1 ~9) lists the magnitude of various contributions to the 1. T A N D E M
PHYSICS
118
K.H.
PURSER
energy spread expected for an iodine beam, assuming 1.5 MeV injection energy. For the purpose of the present discussions we will assume that the pre-acceleration velocity modulator is a simple two-gap device. Optically, each gap is an equi-diameter two-cylinder immersion lens whose strength is not fixed but varies with the rf phase. Clearly, the modulator can increase the effective emittance of the beam and because of this, it is necessary that the modulator be located at a beam waist so that the introduced optical distortions enter only in angle variable and do not affect the radial size at beam waists. Clearly, the magnitude of these undesirable effects can be minimized by keeping the diameter of the modulating cylinders as large as possible. In practice, however, the diameters that can be used are quite limited because the particles should remain within the modulating field region for only a small rf phase angle. In this regard it is interesting to note that a 2.5 MeV uranium ion takes 8.3 ns to cross a 1 cm region. This is approximately the effective field length of a pair of 1 cm equi-diameter cylinders. For a buncher operating at 20 MHz, this time corresponds to a phase angle of ~60 °. This phase angle is large and emphasizes the importance of bunching at high energies to minimize transit time effects. It can be readily demonstrated 2°) that to a high approximation the conditions for producing a cornUNIFORM FIELD x 720 INFLECTION MAGNET -7 / /
DOUBLET/STEERER/1
~
QUAOROP E --7 MATCHING GAP LENS
.loo°
I/
,\\\.~"'~"
dt
where E0 is the energy of an ion having a mass m. The same formalism can be applied to a tandem accelerator and it can be shown that an equivalent L can be readily calculated. It is found that the drift space between the buncher and the low energy acceleration tube makes the most significant contribution to the equivalent L with the rest of the tandem acceleration system making a comparable contribution. For the system proposed by the University of Rochester, shown in fig. 1, the equivalent L for high energy injection energy is approximately 13.4m so that for iodine at 1.5 MeV
dE/dt = 340 eV/ns. The necessary motlulator voltages are easy to achieve above 4 MHz. It is important that dE~dr be moderately large for all ions. First, (dE/dt)xAT, where AT is the desired time resolution, should be much larger than the natural energy spreads that are present in the ions leaving the source. Secondly, and in practice more importantly, is the related phenomena
~~::°'°0°°~
EXISTING INJECTOR /
ANOAPERTURE(A)
CHOPPER WITH RETRACE ELIMINATION FARADAYCUP
/
//
~ ' '
I
\ \
r-~l
~
,
~
~
I
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pletely bunched beam at a final plane located at a drift distance, L, away from the center of the buncher is that the energy of the ions must change at the rate of
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that (dE/dt)×zlT should be large compared to fast power supply fluctuations in the injector that are of a speed or of frequency such that they cannot be corrected for by phase stabilizer circuitry. High frequency ripples and fast voltage fluctuations of the order of many hundreds of volts are common in existing injector systems and in practice are difficult to avoid. The only practical solution which can be expected to operate reliably on a day-to-day basis is to keep dE/dt high. Both these considerations and the transit time effects mentioned earlier indicate the importance of high energy injection.
6. Injector system design The major features of the injection system proposed by Rochester are shown in fig. 1. After extraction, negative ions produced by the sputter source will be focused by a single unipotential lens through a velocity selector to a crossover at the first defining aperture. The velocity selector is necessary to eliminate the intense negative oxygen ions, carbon ions, and electrons that are always produced by sputter sources. Because of the h a zards from accelerating electrons, a permanent magnet solenoid is also placed around the electrostatic lens structure. This solenoid has weak focal properties for heavy ions but is a strong element for electrons and strongly overfocuses them preventing acceleration. Matching into the injector is accomplished using a standard gap lens arrangement. After acceleration the beam is focused by an electrostatic alternating gradient lens and magnet combination and then momentum analyzed. The velocity modulator would be located close to the momentum rejection aperture. The beam will be focused to the stripper of the MP tandem by a single quadrupole lens. The optics of this coupling stage produce a slight demagnification between the injection point, where the buncher is located, and the stripper in the terminal of the tandem. Close to 100% transmission is expected for rather high intensities of heavy ion beams. 7. Conclusion It has been pointed out by Larson4), that during recent years there has been a substantial shift in
119
design emphasis which has transformed tandem accelerators from largely passive elements to active parts of the total beam transport system. This involves the installation of many intank optical elements. While this shift in emphasis, and its consequent requirement of higher reliability, is inevitable with the advent of much larger machines, high energy injection can reduce the need for some of the complexity. High energy injection leads to substantially improved particle transmission efficiency, reduced loading and reductions in the total voltage effects of the accelerator. High energy injection allows the possibility of higher critical current levels and substantially improved pulsed beam performance. The availability of versatile and reliable negative ion sources make it possible to design pressurized injectors with fast access which make it possible to take advantage of these features.
References I) R. Middleton, Nucl. Instr. and Meth. 122 (1974) 35. 2) University of Rochester, Heavy ion post accelerator proposal, UR-NSRL-106 (1976)~ 3) R. J. Van de Graaff, Nucl, Instr. and Meth. 8 (1960) 195. 4) j. D. Larson, Nucl. Instr. and Meth. 122 (1974) 53. 5) K. H. Purser, U. S. Patent 3, 731, 211 (1973). 6) Brooks et al., IEEE Trans. Nucl. Sci. NS-12 (1965) 313. 7) C. J. Davisson and C. J. Calbick, Phys. Rev., 42 (1932) 580. 8) T. R. Ophel et al., Nucl. Instr. and Meth. 122 (1974) 227. 9) Assman et al., Nucl. Instr. and Meth. 122 (1974) 191. 10) p. H. Rose and A. Galejs, Progress in nuclear techniques and instrumentation (ed. F. J. M. Farley; North-Holland, Publ. Co., Amsterdam, 1967). ll) R. J. Van de Graaff et al., Nature 195 (1962) 1292. 12) K. H. Purser et al., Rev. Sci. Instr. 36 (1965) 453. t3) W. D, Allen, Rutherford Laboratory Report, N1RL/R21 (1962). 14) j. G. Trump and R. W. Cloud, U.S. Patent 2, 521, 426 (1950). 15) j. L. McKibben, Proc. 1st Int. Conf. on Insulation o f high voltage in vacuum (1964) p. 337. 16) R. G. Herb, Nucl, Instr. and Meth. 122 (1974) 267., 17) G. J. Dick and K. W. Shepard, Appl. Phys. Lett. 24 (1974) 40. 18) D, D. Armstrong et al., to be published in Part. Acc. 19) Some data obtained from: Apache - A proposal for physics and chemistry of heavy elements, Oak Ridge National Laboratory (1969). 2o) K. H. Purser et al., 1EEE Trans. Nucl. Sci. NS-12 (1965) 415.
1. TANDEM PHYSICS