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RFQ ion accelerators with variable energy
Nuclear Instruments and Methods in Physics Research B40/41 North-Holland, Amsterdam
(1989) 937-942
RFQ ION ACCELERATORS
ENERGY
WITH VARIABLE
937
A. SCHEMPP Institut ftir Angewandte
Physik, J. W. Goethe Universitiit, Postfach I 11932, D-6000 Frpzkfurt,
FRG
RFQs are low-energy rf accelerator structures which can efficiently transport and accelerate high-current ion beams. Whilst usually RFQs, like all rf accelerators, have a fixed velocity profile and output energy per amu, attempts have been made to vary the ion beam energy by changing the resonator frequency. The 4-rod X/2-RFQ resonator developed in Frankfurt for heavy-ion acceleration is well suited for such an application. Results of the work on variable-energy VE-RFQs are presented and the status of the variable-energy cluster accelerator project is discussed.
1. Introduction RFQs are favourable injector structures for high-current beams as well as for low-current beams, e.g. heavy ions. The concept of spatial homogeneous strong fucusing proposed by Kapchinskij and Tepliakov [l] has closed the low-velocity gap of high-frequency accelerators. The work triggered a large number of research activities starting with the work at Los Alamos [2-41. While the first aim was the improvement of injectors for high-energy accelerators, the possibility of an application for heavy ions and polarized beams was seen early too [5]. The capability of conventional accelerators is limited in respect to low-energy and high-current beams. For low-current accelerators - “low current” means a negligible influence of beam space-charge forces on beam dynamics - the choice of a lower operating frequency for which the rf-defocusing is weaker and longer drift tube magnets are possible can improve the acceptance. For beams with significant space-charge defocusing the velocity-dependent magnetic focusing forces are prohibitive for low-velocity ions. The ion velocity at injection is given by the maximum dc extraction and dc-column voltage which are inversely proportional to the ion current [6]. The application of electrical quadrupole focusing can give stronger focusing, but there were technical problems with feed&roughs, sparking and fringe fields. The application of electrical rf-focusing solves some of the technical problems by generating the necessary high voltages only close to the electrodes. Kapchinskij now introduced the mechanical modulation of the electrodes, adding an accelerating field component, and by this got a structure which accelerates and focuses with the same rf fields. Fig. 1 shows an example of modulated electrodes. 0168-583X/89/$03.50 0 Elsevier Science publishers B.V. (North-Holland Physics publishing Division)
Electrical focusing forces are velocity independent and if rf fields are applied, higher voltages than in a dc quadrupole system can be reached, giving a stronger focusing channel. Because the focusing structure is homogeneous the accelerating and focusing cells can be very short, which means that the beam aperture can be even larger than the cell length or that the operating frequency can be high. Short cells made it possible to apply the concept of adiabatic bunching, also proposed by Kapchinskij, where stable phases and accelerating fields are changed very slowly according to the increasing particle velocity, for instance to keep the phase space bucket constant. By this a dc beam from an ion source can be transformed into a bunched beam with minimum emittance growth. Particle losses are small, for instance over 90% of the dc beam can be accelerated.
m
m.a
y
-4%
m.a a
CL
L
L
m.a a
@
Fig. 1. Scheme of RFQ electrodes. VII. ACCELERATOR
TECHNOLOGY
Y
A. Schempp / RFQ ion accelerators with variable energy
938 2. VE-RFQ design
The advantages of the RFQ structure are clear: low-energy acceleration of ions from as low as 1 keV/amu up to approximately 1 MeV/amu with strong rf focusing and the possibility of high beam currents. The design of the RFQ has to be made for the heaviest particle to be accelerated and for a fixed output energy per nucleon or velocity. Lighter particles can be accelerated to the same velocity by reducing the electrode voltage accordingly to the higher specific charge of the ion. The design of the RFQ is to be done for a fixed velocity profile given by the resonance condition of all Wideroe-type structures [7]: L, = up/f (up is the particle velocity, f the frequency and L, the accelerating cell length). A change of the output energy is only possible by altering the electrode system, which means, in case of the RFQ, exchanging (parts) of the cavity, or by varying the resonance frequency of the cavity using the same electrode system. In the latter case the velocity ur, is proportional to the frequency f. For a fixed output velocity (energy per nucleon) the electrode voltage to be applied is inversely proportional to the specific charge of the ion species Uo a amu/q. Keeping the electrode voltage Uo constant, heavier particles can be accelerated in the same RFQ electrodes to the same final energy 7 at a lower frequency: r/a UQ (amu/q)f *. Fig. 2 shows the energy gain AT per atomic mass unit amu as a function of the ion mass M. Both quantities are normalized to their maximum values AT, and me at the highest accelerator frequency f,. For a fixed frequency all possible values are on one straight line and the electrode voltage has to be adjusted for the resonance condition. This is a well-known problem for rf ion accelerators, especially if they are used as injectors for cyclotrons or if they should have variable energy for example. Variable energy at a fixed frequency is achieved at the UNILAC of the GSI [8] by choosing one part of the
accelerator with a fixed velocity profile and a second part with independently phased cavities. The same principle is used for postaccelerators built for tandems [9,10] and a similar system is proposed for ion implantation [ll]. Postaccelerator cavities could be used to vary the energy of a fixed frequency RFQ. The system would consist of an RFQ and one or a few independently phased rf cavities, e.g. spiral or split-ring loaded resonators [12]. Depending on the chosen parameters the energy could be varied between 10% and 50%. Such a system would require additional focusing elements and separate transmitters for the energy variation cavities. The RILAC [13] is one example of a variablefrequency Wideroe-type structure. It serves as injector into the RIKEN SSC cyclotron. Here the frequency of the accelerator can be changed by a factor of 3 by using large moving short tuners in the six separated drifttube-loaded X/Cresonators. Focusing is done in each second drift tube, which is connected to the outer conductor of the coaxial resonator, similar to the GSI Wideroe. Thus a ~T/T focusing periodicity is chosen with a limited acceptance and no high beam current possibility. Looking for possibilities to change for frequency of an RFQ structure the only chance can be seen using a 4-rod-type RFQ structure, where the rod electrode system and the driving inductivities are practically separated. The tuning range can be large in this case. With a 4-vane structure the volume of the cavity, and by this the inductivity, has to be changed drastically to achieve a change in frequency. This is very impractical and leads to a lot of mechanical and electrical problems. In the case of 4-rod-type structures only the length of the driving conductor has to be changed, which is a much simpler solution. In the early stages of te 4-rod RFQ development [14] tuners were used, which could change the frequency of the structure by a factor of up to 3. They were used to tune to the frequency of the transmitter. New applications like ion implantation and cluster acceleration have led to new ideas in the development of frequency-variable RFQs, which are not aiming at highest beam currents and highest brilliance but on flexibility and energy variation.
3. AccSys VE - RFQ
Fig. 2. Energy range for VE-RFQ.
Beam dynamics requires frequencies in the range of 10 to 100 MHz for the acceleration of heavy ions with lower charge states. This is the design basis for the variable-frequency RFQ, built by AccSys [15,16]. The design is made for an ion implanter with variable energy at modest beam currents of a few mA. The RFQ resonator consists of a multiple-helix-driven 4-rod structure. The electrodes have the shape of slim vanes, which
A. Schempp / RFQ ion accelerators with variable energy
939
Table 1 Parameters of ion implantation VE-RFQ Initial energy [keV/amu] Final energy per u [keV/amu] Output kinetic energy [MeV], singly charged ions Ion mass [amuX mJ Length of structure [m] Clear aperture between electrodes [mm] Maximum modulation of electrodes Frequency [MHz] Peak voltage [kV]
Power [kW] Number of tuning coils
Fig. 3. AccSys VE-RFQ resonator module.
allow application of the Cvane vane-tip milling procedures and the use of very small beam apertures and high values for the electrode modulation. The inductivity is chosen to be a helix, respectively a system of helices, which oscillate in opposite phase in such a way that two helices form an interlacing current loop, a technique well known from tunable rf transmitters. It is a relatively simple and practical way of changing the inductivity by using a helix and a sliding contact between adjacent windings, which gives a large frequency range. Chains of such helical drive elements are connected to the RFQ electrodes. INJECTOR VOLTACE [kVl 20
40
60
80
II0
120
140
I
I
I 30
I 600
900
1200 1500
FINAL ENERGY
1800
I 2100
IkeVl
Fig. 4. Implanter RFQ output energy vs frequency and injection energy.
34 40 10
Fig. 3 shows the scheme of such a module of this accelerator with two pairs of helices, shortening pieces which are movable to change the frequency. Helices have been chosen because for the lowest frequency of 17 MHz the length of the conductor has to be as long as 6 m. By application of the helix the physical size of the resonator can be very small. This advantage has been used for the work on helix-postaccelerators [17]. The advantage of the combination used by AccSys is twofold: firstly, beam dynamics designs can be done using the experience and the results of 4-vane cavities. Secondly, the experience in transmission line cavities with relatively simple manufacture, stable operation and simplicity in cooling can be used. Prototypes of this heavy-ion accelerator with varying frequency for use as an ion implanter have been built. Fig. 4 shows the output energy as function of the frequency for singly charged ions. Table 1 gives some parameters of such an implanter VE-RFQ.
4. VERFQ
0
0.21-9.0 12.4-136 1.5 11-121 2.4 1.9 2.08 17-100
cluster accelerator
The 4-rod structure was developed in Frankfurt at first to have some simple rf drive for the RFQ electrodes that were studied [18]. So the first resonators with 4-rod electrodes can be described as chains of spiral A/Coscillators connected by the electrodes in an axial O-mode forming basic h/2 units. They are excited in a radial n-mode producing the quadrupole fields as known from coupled postaccelerator structures [lo]. The frequency tuning of these devices was done by cutting the length of the spiral or helix. In first order the cavity size did not determine the frequency. Several significant changes of the 4-rod h/2-resonator have been made since: first, straight stems were introduced; later, a linear arrangement of these stems, transforming a TM- to a TE-like mode [19], was applied. In the early versions of the A/2-mode 4-rod structure the tuning bars, which are able to shift the frequency VII. ACCELERATOR
TECHNOLOGY
940
A. Schempp / RFQ ion accelerators with variable energy
Fig. 5. Scheme of the 4-rod RFQ.
between 65 and 180 MHz, were only being used to tune the resonator to the frequency of 108.5 MHz and then soldered in place. Frequency variation during operation was not planned originally but the potential of this structure was seen. The linear structure as shown schematically in fig. 5 was developed since 1983. It had end tuners and for the HERA RFQ [lo] these end tuners could vary the frequency up to 20 MHz. Here they were used to flatten the field in addition. A similar system has been adapted at Texas A&M for studies as a frequency-tunable RFQ [21] but the frequency range was still limited. The restart of some work on energyvariable RFQs was initiated for the application as a
cluster postaccelerator at the IPNL in Lyon, France. Here an energy variation of the postaccelerator for the 0.5 MV cluster CW facility was planned. The upgrading of the cluster facility is being performed in a collaboration between IPN Lyon, KIK Karlsruhe and IAP Frankfurt [22,23]. The RFQ for postacceleration of clusters is designed for 500 keV (10 keV/u) injection energy and 5 MeV (100 keV/u) final energy for singly charged clusters with a mass of 50 amu. Fig. 6 shows a scheme of the cluster accelerator system, which is designed to increase the cluster energy given by the injector by a factor of up to 10. Such a combination can use the typical advantages of an RFQ. Firstly, the ion source, in this case the rather huge cluster preaccelerator (height 8 m), can be on ground potential. Higher energy by means of dc acceleration is hardly possible. Rf acceleration offers in principle unlimited energy gain at rather low peak potentials. The second advantage of the RFQ is the use of strong radiofrequency electrical quadrupole focusing, which solves the focusing problems. Alternatively, a very low frequency and a very bulky postacceleration with a classical structure, e.g. of the Wideroe type, would be required. Because the specific energy per
High
Voltage
luster
Ion
ccelerator
Sour Gap
Deflector
Fig. 6. Scheme of the cluster accelerator at IPN Lyon.
'ce
A. Schempp / RFQ ion accelerators with variable energy
941
The capacitive tuners would have no current on contacts but they would increase the losses by capacitive action. Fig. 7 shows schemes of the three kinds of tuners which work on each cell along the accelerator structure. The largest frequency swing could be obtained with the trombone tuner. We plan to install magnetic and moving short tuners for the first structure to be built. Table 2 gives parameters of the Lyon cluster postaccelerator.
5. Conclusions
Fig. 7. Frequency tuner layout.
nucleon is very low, the limited focussing strength is the main problem of rf postacceleration at these energies (per mass unit amu). The third feature of the RFQ is of special advantage for cluster acceleration: due to the limited energy acceptance ( - k 5%) the RFQ postaccelerator will select a narrow band out of the broad energy range and mass spectrum of the incoming cluster beam. This will open new experimental possibilities. The possibility to vary the final energy by means of changing the frequency of the RFQ has been included. In the Lyon project the frequency of the 4-rod RFQ structure will be appreciably tuned to scan the FM range from about 80 to 110 MHz. Here commercial transmitters with sufficient duty cycle (25lOO%), developed to operate on high Q loads, are available [24]. The 4-rod structure can be tuned either with perturbation magnetic tuners or trombone-like tuners with a change of the length of the resonators, as well as capacitively by elongating the drift tube holders [25]. All the possibilities were tested and preference was given to the magnetic tuners, which have no sliding contacts.
Table 2 Cluster RFQ parameters Initial kinetic energy per mass [keV/an 14
Final kinetic energy per u [keV/amu] Maximum total kinetic energy [MeV] Maximum ion mass [amu] Length of structure [m] Clear aperture between electrodes [mm] Number of cells Maximum modulation of electrodes Diameter of vacuum chamber [m] Frequency [MHz] Peak voltage [kV] Power [kW] Maximum field strength [MV/m] Duty factor
10 100 5 50 2.5 2.2 214 2.0 0.5 SO-110 100 80 16.5 25%
The work on RFQs with variable energy will be used at first for an ion implanter and a cluster postaccelerator. The applied schemes show how the advantages of the RFQ can be used together with additional flexibility by frequency variation. First results of experiments and operational experience should be available in 1989.
References 111 I.M. Kapchinskij and V.A. Tepliakov, Prib. Tekh. Eksp. 4 (1970) 17, 19. PI K.R. Crandah, R.H. Stokes and T.P. Wangler, BNL-51143, Brookhaven Nat. Lab. (1980) 20. 131 R.H. Stokes, T.P. Wangler and K.R. Crandall, PAC 81, IEEE Trans. Nucl Sci. NS-28 (3) (1981) 1999. 141 S.O. Schriber, PAC 85, IEEE Trans. Nucl. Sci. NS-32 (5) (1985) 3134. 151 H. Klein, PAC 83, IEEE Trans. Nucl. Sci. NS-30 (4) (1983) 3313. 161 A. Schempp, Space charge dominated accelerators, Proc. 1st Europ. Particle Accelerator Conf., Rome (June 1988). 171 R. Wideroe, Archiv fur Elektrotechnik 21 (1928) 387. PI N. Angert and C. Schmelzer, Kemtechnik 19 (1977) 57. 191 E. Jaeschke, Linac ‘84, GSI 84-11 (1984) 24. WI A. Schempp et al., Linac ‘79, BNL-51134 (1980) 159. [111 H. Glavish, Nucl. Instr. and Meth. B24/25 (1987) 771. WI R. Thomae et al., Proc. 1st Europ. Particle Accelerator Conf., Rome (June 1988). D31 M. Odera et al., Nucl. Instr. and Meth. 227 (1984) 187. 1141 A. Schempp et al., PAC ‘83, IEEE Trans. Nucl. Sci. NS-30 (4) (1983) 3536. 1151 R. Hamm, Linac ‘86, SLAC Rep. 303 (1986) 33. Ml R. Hamm et al., Proc. 1st Europ. Particle Accelerator Conf., Rome (June 1988). P71 A. Schempp and H. Klein, Nucl. Instr. and Meth. 136 (1976) 29. WI J. Mtiller and A. Schempp, Int. Rep. 79-l(1979). LA-TR 82-28 LANL, Los Alamos, NM, USA (1982). 1191 A. Schempp et al., Nucl. Instr. and Meth. BlO/ll (1985) 831. PO1 A. Schempp, M. Ferch and H. Klein, PAC ‘87, IEEE CH2387-9 (1987) 267. VII. ACCELERATOR
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A. Schempp / RFQ ion accelerators with variable energy
[21] D. DiBonito et al. Nucl. Instr. and Meth. B24/25 (1987) 783. [22] H.O. Moser and A. Schempp, Nucl. Instr. and Meth. B24/25, (1987) 759. [23] A. Schempp, H.O. Moser, 2nd Int. Workshop on MeV and keV Ion and Cluster Interactions with Surfaces and
Materials, Orsay, France, Sept. 1988, to be published in J. Phys. (Paris). [24] Herfurth GmbH, D-2000 Hamburg, FRG. (251 A. Schempp, Int. Rep. 86-11, Inst. f. Angewandte Physik, Frankfurt, FRG (1987).
(1989) 937-942
RFQ ION ACCELERATORS
ENERGY
WITH VARIABLE
937
A. SCHEMPP Institut ftir Angewandte
Physik, J. W. Goethe Universitiit, Postfach I 11932, D-6000 Frpzkfurt,
FRG
RFQs are low-energy rf accelerator structures which can efficiently transport and accelerate high-current ion beams. Whilst usually RFQs, like all rf accelerators, have a fixed velocity profile and output energy per amu, attempts have been made to vary the ion beam energy by changing the resonator frequency. The 4-rod X/2-RFQ resonator developed in Frankfurt for heavy-ion acceleration is well suited for such an application. Results of the work on variable-energy VE-RFQs are presented and the status of the variable-energy cluster accelerator project is discussed.
1. Introduction RFQs are favourable injector structures for high-current beams as well as for low-current beams, e.g. heavy ions. The concept of spatial homogeneous strong fucusing proposed by Kapchinskij and Tepliakov [l] has closed the low-velocity gap of high-frequency accelerators. The work triggered a large number of research activities starting with the work at Los Alamos [2-41. While the first aim was the improvement of injectors for high-energy accelerators, the possibility of an application for heavy ions and polarized beams was seen early too [5]. The capability of conventional accelerators is limited in respect to low-energy and high-current beams. For low-current accelerators - “low current” means a negligible influence of beam space-charge forces on beam dynamics - the choice of a lower operating frequency for which the rf-defocusing is weaker and longer drift tube magnets are possible can improve the acceptance. For beams with significant space-charge defocusing the velocity-dependent magnetic focusing forces are prohibitive for low-velocity ions. The ion velocity at injection is given by the maximum dc extraction and dc-column voltage which are inversely proportional to the ion current [6]. The application of electrical quadrupole focusing can give stronger focusing, but there were technical problems with feed&roughs, sparking and fringe fields. The application of electrical rf-focusing solves some of the technical problems by generating the necessary high voltages only close to the electrodes. Kapchinskij now introduced the mechanical modulation of the electrodes, adding an accelerating field component, and by this got a structure which accelerates and focuses with the same rf fields. Fig. 1 shows an example of modulated electrodes. 0168-583X/89/$03.50 0 Elsevier Science publishers B.V. (North-Holland Physics publishing Division)
Electrical focusing forces are velocity independent and if rf fields are applied, higher voltages than in a dc quadrupole system can be reached, giving a stronger focusing channel. Because the focusing structure is homogeneous the accelerating and focusing cells can be very short, which means that the beam aperture can be even larger than the cell length or that the operating frequency can be high. Short cells made it possible to apply the concept of adiabatic bunching, also proposed by Kapchinskij, where stable phases and accelerating fields are changed very slowly according to the increasing particle velocity, for instance to keep the phase space bucket constant. By this a dc beam from an ion source can be transformed into a bunched beam with minimum emittance growth. Particle losses are small, for instance over 90% of the dc beam can be accelerated.
m
m.a
y
-4%
m.a a
CL
L
L
m.a a
@
Fig. 1. Scheme of RFQ electrodes. VII. ACCELERATOR
TECHNOLOGY
Y
A. Schempp / RFQ ion accelerators with variable energy
938 2. VE-RFQ design
The advantages of the RFQ structure are clear: low-energy acceleration of ions from as low as 1 keV/amu up to approximately 1 MeV/amu with strong rf focusing and the possibility of high beam currents. The design of the RFQ has to be made for the heaviest particle to be accelerated and for a fixed output energy per nucleon or velocity. Lighter particles can be accelerated to the same velocity by reducing the electrode voltage accordingly to the higher specific charge of the ion. The design of the RFQ is to be done for a fixed velocity profile given by the resonance condition of all Wideroe-type structures [7]: L, = up/f (up is the particle velocity, f the frequency and L, the accelerating cell length). A change of the output energy is only possible by altering the electrode system, which means, in case of the RFQ, exchanging (parts) of the cavity, or by varying the resonance frequency of the cavity using the same electrode system. In the latter case the velocity ur, is proportional to the frequency f. For a fixed output velocity (energy per nucleon) the electrode voltage to be applied is inversely proportional to the specific charge of the ion species Uo a amu/q. Keeping the electrode voltage Uo constant, heavier particles can be accelerated in the same RFQ electrodes to the same final energy 7 at a lower frequency: r/a UQ (amu/q)f *. Fig. 2 shows the energy gain AT per atomic mass unit amu as a function of the ion mass M. Both quantities are normalized to their maximum values AT, and me at the highest accelerator frequency f,. For a fixed frequency all possible values are on one straight line and the electrode voltage has to be adjusted for the resonance condition. This is a well-known problem for rf ion accelerators, especially if they are used as injectors for cyclotrons or if they should have variable energy for example. Variable energy at a fixed frequency is achieved at the UNILAC of the GSI [8] by choosing one part of the
accelerator with a fixed velocity profile and a second part with independently phased cavities. The same principle is used for postaccelerators built for tandems [9,10] and a similar system is proposed for ion implantation [ll]. Postaccelerator cavities could be used to vary the energy of a fixed frequency RFQ. The system would consist of an RFQ and one or a few independently phased rf cavities, e.g. spiral or split-ring loaded resonators [12]. Depending on the chosen parameters the energy could be varied between 10% and 50%. Such a system would require additional focusing elements and separate transmitters for the energy variation cavities. The RILAC [13] is one example of a variablefrequency Wideroe-type structure. It serves as injector into the RIKEN SSC cyclotron. Here the frequency of the accelerator can be changed by a factor of 3 by using large moving short tuners in the six separated drifttube-loaded X/Cresonators. Focusing is done in each second drift tube, which is connected to the outer conductor of the coaxial resonator, similar to the GSI Wideroe. Thus a ~T/T focusing periodicity is chosen with a limited acceptance and no high beam current possibility. Looking for possibilities to change for frequency of an RFQ structure the only chance can be seen using a 4-rod-type RFQ structure, where the rod electrode system and the driving inductivities are practically separated. The tuning range can be large in this case. With a 4-vane structure the volume of the cavity, and by this the inductivity, has to be changed drastically to achieve a change in frequency. This is very impractical and leads to a lot of mechanical and electrical problems. In the case of 4-rod-type structures only the length of the driving conductor has to be changed, which is a much simpler solution. In the early stages of te 4-rod RFQ development [14] tuners were used, which could change the frequency of the structure by a factor of up to 3. They were used to tune to the frequency of the transmitter. New applications like ion implantation and cluster acceleration have led to new ideas in the development of frequency-variable RFQs, which are not aiming at highest beam currents and highest brilliance but on flexibility and energy variation.
3. AccSys VE - RFQ
Fig. 2. Energy range for VE-RFQ.
Beam dynamics requires frequencies in the range of 10 to 100 MHz for the acceleration of heavy ions with lower charge states. This is the design basis for the variable-frequency RFQ, built by AccSys [15,16]. The design is made for an ion implanter with variable energy at modest beam currents of a few mA. The RFQ resonator consists of a multiple-helix-driven 4-rod structure. The electrodes have the shape of slim vanes, which
A. Schempp / RFQ ion accelerators with variable energy
939
Table 1 Parameters of ion implantation VE-RFQ Initial energy [keV/amu] Final energy per u [keV/amu] Output kinetic energy [MeV], singly charged ions Ion mass [amuX mJ Length of structure [m] Clear aperture between electrodes [mm] Maximum modulation of electrodes Frequency [MHz] Peak voltage [kV]
Power [kW] Number of tuning coils
Fig. 3. AccSys VE-RFQ resonator module.
allow application of the Cvane vane-tip milling procedures and the use of very small beam apertures and high values for the electrode modulation. The inductivity is chosen to be a helix, respectively a system of helices, which oscillate in opposite phase in such a way that two helices form an interlacing current loop, a technique well known from tunable rf transmitters. It is a relatively simple and practical way of changing the inductivity by using a helix and a sliding contact between adjacent windings, which gives a large frequency range. Chains of such helical drive elements are connected to the RFQ electrodes. INJECTOR VOLTACE [kVl 20
40
60
80
II0
120
140
I
I
I 30
I 600
900
1200 1500
FINAL ENERGY
1800
I 2100
IkeVl
Fig. 4. Implanter RFQ output energy vs frequency and injection energy.
34 40 10
Fig. 3 shows the scheme of such a module of this accelerator with two pairs of helices, shortening pieces which are movable to change the frequency. Helices have been chosen because for the lowest frequency of 17 MHz the length of the conductor has to be as long as 6 m. By application of the helix the physical size of the resonator can be very small. This advantage has been used for the work on helix-postaccelerators [17]. The advantage of the combination used by AccSys is twofold: firstly, beam dynamics designs can be done using the experience and the results of 4-vane cavities. Secondly, the experience in transmission line cavities with relatively simple manufacture, stable operation and simplicity in cooling can be used. Prototypes of this heavy-ion accelerator with varying frequency for use as an ion implanter have been built. Fig. 4 shows the output energy as function of the frequency for singly charged ions. Table 1 gives some parameters of such an implanter VE-RFQ.
4. VERFQ
0
0.21-9.0 12.4-136 1.5 11-121 2.4 1.9 2.08 17-100
cluster accelerator
The 4-rod structure was developed in Frankfurt at first to have some simple rf drive for the RFQ electrodes that were studied [18]. So the first resonators with 4-rod electrodes can be described as chains of spiral A/Coscillators connected by the electrodes in an axial O-mode forming basic h/2 units. They are excited in a radial n-mode producing the quadrupole fields as known from coupled postaccelerator structures [lo]. The frequency tuning of these devices was done by cutting the length of the spiral or helix. In first order the cavity size did not determine the frequency. Several significant changes of the 4-rod h/2-resonator have been made since: first, straight stems were introduced; later, a linear arrangement of these stems, transforming a TM- to a TE-like mode [19], was applied. In the early versions of the A/2-mode 4-rod structure the tuning bars, which are able to shift the frequency VII. ACCELERATOR
TECHNOLOGY
940
A. Schempp / RFQ ion accelerators with variable energy
Fig. 5. Scheme of the 4-rod RFQ.
between 65 and 180 MHz, were only being used to tune the resonator to the frequency of 108.5 MHz and then soldered in place. Frequency variation during operation was not planned originally but the potential of this structure was seen. The linear structure as shown schematically in fig. 5 was developed since 1983. It had end tuners and for the HERA RFQ [lo] these end tuners could vary the frequency up to 20 MHz. Here they were used to flatten the field in addition. A similar system has been adapted at Texas A&M for studies as a frequency-tunable RFQ [21] but the frequency range was still limited. The restart of some work on energyvariable RFQs was initiated for the application as a
cluster postaccelerator at the IPNL in Lyon, France. Here an energy variation of the postaccelerator for the 0.5 MV cluster CW facility was planned. The upgrading of the cluster facility is being performed in a collaboration between IPN Lyon, KIK Karlsruhe and IAP Frankfurt [22,23]. The RFQ for postacceleration of clusters is designed for 500 keV (10 keV/u) injection energy and 5 MeV (100 keV/u) final energy for singly charged clusters with a mass of 50 amu. Fig. 6 shows a scheme of the cluster accelerator system, which is designed to increase the cluster energy given by the injector by a factor of up to 10. Such a combination can use the typical advantages of an RFQ. Firstly, the ion source, in this case the rather huge cluster preaccelerator (height 8 m), can be on ground potential. Higher energy by means of dc acceleration is hardly possible. Rf acceleration offers in principle unlimited energy gain at rather low peak potentials. The second advantage of the RFQ is the use of strong radiofrequency electrical quadrupole focusing, which solves the focusing problems. Alternatively, a very low frequency and a very bulky postacceleration with a classical structure, e.g. of the Wideroe type, would be required. Because the specific energy per
High
Voltage
luster
Ion
ccelerator
Sour Gap
Deflector
Fig. 6. Scheme of the cluster accelerator at IPN Lyon.
'ce
A. Schempp / RFQ ion accelerators with variable energy
941
The capacitive tuners would have no current on contacts but they would increase the losses by capacitive action. Fig. 7 shows schemes of the three kinds of tuners which work on each cell along the accelerator structure. The largest frequency swing could be obtained with the trombone tuner. We plan to install magnetic and moving short tuners for the first structure to be built. Table 2 gives parameters of the Lyon cluster postaccelerator.
5. Conclusions
Fig. 7. Frequency tuner layout.
nucleon is very low, the limited focussing strength is the main problem of rf postacceleration at these energies (per mass unit amu). The third feature of the RFQ is of special advantage for cluster acceleration: due to the limited energy acceptance ( - k 5%) the RFQ postaccelerator will select a narrow band out of the broad energy range and mass spectrum of the incoming cluster beam. This will open new experimental possibilities. The possibility to vary the final energy by means of changing the frequency of the RFQ has been included. In the Lyon project the frequency of the 4-rod RFQ structure will be appreciably tuned to scan the FM range from about 80 to 110 MHz. Here commercial transmitters with sufficient duty cycle (25lOO%), developed to operate on high Q loads, are available [24]. The 4-rod structure can be tuned either with perturbation magnetic tuners or trombone-like tuners with a change of the length of the resonators, as well as capacitively by elongating the drift tube holders [25]. All the possibilities were tested and preference was given to the magnetic tuners, which have no sliding contacts.
Table 2 Cluster RFQ parameters Initial kinetic energy per mass [keV/an 14
Final kinetic energy per u [keV/amu] Maximum total kinetic energy [MeV] Maximum ion mass [amu] Length of structure [m] Clear aperture between electrodes [mm] Number of cells Maximum modulation of electrodes Diameter of vacuum chamber [m] Frequency [MHz] Peak voltage [kV] Power [kW] Maximum field strength [MV/m] Duty factor
10 100 5 50 2.5 2.2 214 2.0 0.5 SO-110 100 80 16.5 25%
The work on RFQs with variable energy will be used at first for an ion implanter and a cluster postaccelerator. The applied schemes show how the advantages of the RFQ can be used together with additional flexibility by frequency variation. First results of experiments and operational experience should be available in 1989.
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TECHNOLOGY
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A. Schempp / RFQ ion accelerators with variable energy
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Materials, Orsay, France, Sept. 1988, to be published in J. Phys. (Paris). [24] Herfurth GmbH, D-2000 Hamburg, FRG. (251 A. Schempp, Int. Rep. 86-11, Inst. f. Angewandte Physik, Frankfurt, FRG (1987).