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RFQ accelerators for ion implantation
Nuclear Instruments North-Holland
and Methods
RFQ accelerators
in Physics Research
Nuclear Instruments & Methods in Physics Research SectmB
B62 (1992) 425-430
for ion implantation
A. Schempp Institut fiir Angewandte Physik, J. W’SGoethe-Uniuersitiit, Postfach 111932, D-6000 Frankfurt/M,
Germany
Radiofrequency quadrupoles (RFQI are rf structures which can efficiently accelerate high current ion beams to energies up to 1 MeV/u. Features are a strong electrical rf focusing which allows ion beam currents up to 10 mA and a compact and reliable system which is well suited for applications like high energy ion implantation. Examples of RFQs with fixed and variable energy for implantation and the status of work on these implanters will be discussed.
1. Introduction
2. Radiofrequency
Due to the increasing interest in ion implantation into semiconductors, ceramics and metals, high current ion beams with energies in the MeV range are required [1,2]. Compared to the well established techniques with beams of several hundred keV deeper and thicker regions of the material can be modified, resulting in novel structures. While research work could be done with standard low current equipment practical applications need high current beams for an appreciable throughput. For surface hardening of load carrying metal samples by deep implantation with doses of some 10” ions/cm*, e.g. N+ beams of up to 10 mA have to be accelerated which cannot be done with dc acceleration techniques at higher energies. While at low energies (100-400 keV) currents up to 10 mA are quite standard, the system at FOM [3] (1 MeV/l mA N+) represents the upper edge of the development. For rf accelerators the low energy part is the bottleneck (injection and bunching) while a high energy extension poses no principal problems. It is clear that rf acceleration even having these advantageous properties cannot replace dc accelerators concerning low emittance, energy spread, and good spatial resolution as needed in the field of ion beam analysis and diagnostic for materials characterisation. The typical energy spread of several percent poses no problem for high dose implantation [4]. So rf acceleration techniques are extending the range of applications to new parameters [S] and are even more valuable in combination with dc machines as has been proposed for the new facility for material research in the Institut fiir Kernphysik (IKE) in Frankfurt, in which a high current high energy implanter, an ECR ion source for highly charged heavy ions [6] in combination with a frequency variable RFQ with final energies between 100 and 200 keV/u will be used together with a 7 MV Van de Graaff accelerator [7-lo].
In low energy rf accelerators a wave is generated in a system of electrodes which moves along in synchronism with the ions; therefore the electrodes (drift tubes) are arranged with a special velocity profile. Because only the crest of the wave is used for acceleration the beam is bunched and occupies only approximately 60 ’ out of 360 ’ of the periodic field. Stable oscillations of the ions in the bunch result in an energy spread which is typically several percent. For compensating the defocusing action of the accelerating field and space charge repulsion an additional external focusing is necessary. Low energy beams, which for rf accelerators means energies up to several MeV/u, with significant space charge defocusing can be transported efficiently with the help of drift tube quadrupoles. Examples are the Wideroe-type structures as used for nuclear physics machines, some of which are divided in short structures which can be phased individually to adjust for a variable velocity profile 111,121. In radiofrequency quadrupole (RFQ) accelerators [13,14] the focusing and accelerating fields are generated by the same electrodes. The mechanical modulation of the quadrupole electrodes, which is shown schematically in fig. 1, adds an accelerating field component to the homogeneous quadrupole channel. Because electrical focusing forces are independent of ion velocity, the RFQ structure is especially attractive for the low energy part of ion linacs. Though the improvement of injectors for high energy machines has been the main goal of the early development, numerous proposals for other applications were made since. Ion implantation is an example for which RFQs have been proposed as beam sources for ion beams with an energy in the MeV range and a current of several milliamperes [4,15], At present several rf accelerators designed for the use as implanter are operating or under development.
0168-583X/92/$05.00
0 1992 - Elsevier
Science
Publishers
B.V. All rights reserved
accelerators
426
A. Schempp / RFQ accelerators for ion implantation
There is the Eaton variable phase linac [16] with a Wideroe-type structure with individually driven drift tubes for energy variation. The MEQALAC system at FOM [17,18] has four parallel beams, both accelerator concepts using electrostatic quadrupole focusing. The implanter designed by Shimadsu [19] is a 4-vane RFQ operating at 70 MHz, while the other RFQs are based on the 4-rod RFQ structure [20], for which now also systems with variable frequency and energy have been built [21].
3. RFQ design The RFQ structure is converting the dc beam extracted from an ion source into a bunched beam. The injection energy can be very low and the beam currents can be very high due to the homogeneous focusing structure with high rf field strengths. The design can be divided into two parts, namely the shaping of the electrodes with a proper “modulation program” and the design of a suitable rf structure for the transformation of rf power into electrode fields and beam power
La. The electrode shape is characterized by the aperture radii II,, the modulations mj and the cell lengths Lj along the RFQ. For a given ion energy and electrode voltage Uo the focusing gradients G (G = Xr l), the length of the corresponding X%/a*, quadrupole, which is determined by the operating frequency, and the beam channel cross section determine the limiting ion currents and the acceptance of the RFQ. The accelerating field E, (E, =AC&/L,, A < 1) is proportional to the modulation m of the electrode (ratio of minimum to maximum aperture) while the
focusing efficiency X goes down for increasing modulation. Another strong limitation is the maximum applicable field. The electrical breakdown fields and the electrode voltage for which sparks will prevent useful operation can be described by:
&,ax[kVl= 20( 1+
g[ mm] + 1.5 x 10-3f[MHz])
(g is the minimum electrode gap). The RFQ electrode design parameters have to be matched with ion source parameters and the requirements of the experiments. The resonator for driving the electrodes has to be efficient, compact, and reliable. While power efficiency (beam versus ac power) falls short compared to dc generators, size and handling are favourable. The rf power which is necessary for operation is proportional to the length L, and to the operating frequency f of the RFQ, and to the square of the electrode voltage Uo: N - U*L, f. The electrode voltage has to be adjusted proportional to the specific charge A/q to get the same beam dynamics in the accelerator. Therefore the dynamics has to be designed for the heaviest particle or the,lowest charge to mass ratio respectively. Typical rf accelerators are pulsed with a high pulse current. This is matched to the operation as injector but is also very efficient with respect to the rf power consumption. Cw RFQs are exceptional [23] but several resonators have been built and operated. Normally the power levels are lower than in pulsed operation, for which the problems of thermal and mechanical stability can be solved more easily. With respect to heavy ion acceleration the 4-rod structure is advantageous especially for high current implantation, where mostly singly charged ions are
Fig. 1. Scheme of the RFQ electrodes.
427
A. Schempp / RFQ accelerators for ion implantation
Table 1 Parameters of the N-RFQ
used at rather low frequencies. RFQs for protons and heavy ions with low and high currents and low and high duty cycles have been built and operated [24]. The operational experience is very promising. Two structures which demonstrate the flexibility and versatility are being built in Frankfurt at present. They will be described in the following sections.
4. High current implanter
Injection energy Final energy Charge to mass ratio Frequency Electrode voltage Duty cycle Aperture Modulation Tank diameter Tank length Radial acceptance (norm.) Input emittance Output emittance Longitudinal emittance Spiral-resonator voltage Rf power RFQ Rf power spiral
RFQ
For experiments in the field of high energy, high dose implantation a RFQ accelerator system is under construction, which will deliver N+ beams with energies well above 1 MeV and beam currents of up to 10 mA. The RFQ is operating at 108.5 MHz for which a transmitter from an earlier postaccelerator project [25] is available. The ion source is a CHORDIS developed at GSI by Keller [26], which has a very good emittance EN = 0.1~ mm mrad) and reliability. The injection energy has been chosen to be 50 keV (3.5 keV/u) and a beam current of 10 mA is envisaged. The design of the RFQ resulted in a structure with a final energy of 1.5 MeV (0.107 MeV/u), a length of 2 m and an rf power consumption of 80 kW as basic parameters. Table 1 summarizes characteristic parameters of this structure. Fig. 2 gives a typical variation of electrode parameters along the nitrogen implanter RFQ showing the adiabatic variation of the electrode parameters and the typical situation that a large fraction of the accelerator is used for the bunching of the beam with very little acceleration. The RFQ resonator is a 4-rod structure for which the mechanical design was adopted from the RFQ, which has been built for the new UNILAC injector at GSI [21].
If!
3.5 keV/u 107 keV/u i 1 14108.5 MHz 80 kV 100% 3.0 mm l-l.65 35 cm 2.0 m 0.8~ mm mrad 0.5n mm mrad 0.6~ mm mrad 0.05~ MeV ns 0.5 MV 80 kW 60 kW
The resonator has rod electrodes with direct cooling, which are supported by cylindrical stems. The tank is designed with a top flange like a coffin, which gives easy access for installation, alignment and maintenance by the open top flange along the resonator and has diagnostic elements incorporated in the end plates. Fig. 3 shows a scheme of the input part of the resonator. This RFQ is a fixed energy structure. Energy variation is achieved with an additional spiral “postaccelerator” 112,271, which can accelerate or decelerate up to 0.5 MeV. The spiral loaded cavity is a two gap drift tube structure which requires 60 kW for the design field. The combination with the RFQ with its fixed output energy gives energy variation with unique beam
m, T
T\
[MeVl
.
0
Ip
.’
/ A,
.L.
._._._._.-.-*-.--.-
RFQ
CELL
NUMBER
/- i/T 200
Fig. 2. Beam parameters for the VE-RFQ implanter. VI. EQUIPMENT
A. Schempp / RFQ accelerators for ion implantation
428
BEAM AX
SUPPORT
STEMS
Fig. 3. Scheme of the 4-rod RFQ resonator.
properties because the energy can be modulated very fast distributing the beam over a large depth range.
5. ECR / VE-RFQ
combination
Normally RFQ accelerators have a fixed velocity profile, given by the electrode modulation. The Widerde resonance condition (Li= /3, h/2 = u,/2 f > connects the cell length L with the path length in half the rf period and also sets a lower limit to the accelerating fields or the electrode voltage in case of the RFQ, because the energy (velocity) gain must be sufficient to stay in synchronism. In variable energy RFQs (VE-RFQ) the frequency f of the resonator is varied (0, -f ). The electrode voltage Uo has to be adjusted to the specific charge and energy: Uo - (A/q)f ‘, T, - (A/q)f *. To change the frequency of the 4-rod RFQ the resonator can be tuned capacitively or inductively. The first VE-RFQ is the cluster accelerator at the IPNL in Lyon [29]. The design was made for S-10 keV/u injection energy and 50-100 keV/u final energies (energy gain up to 5 MeV for m = 50 u). For energy variation of the resonator the effective length of the driving conductor was changed with movable shorts as indicated in fig. 4 which resulted in a tuning range of 88 to 120 MHz. For these frequencies transmitters are easily available. This tuning range corresponds to an energy change by a factor of about 2. Compared to other RFQ designs the shaper-buncher part has been omitted in the cluster RFQ for very effective acceleration. The average field strength is as high as 2.5 MV/m, which is at the cost of a relatively small minimum transmission of 25%. The RFQ has been successfully used in first tests. This VE-RFQ is the model for the second implanter RFQ, planned for the acceleration of highly charged heavy ions from an
Fig. 4. Scheme
Table 2 Parameters
of a VE-RFQ
of the VE-RFQ
Injection energy Final energy Charge
module.
to mass ratio
Frequency Electrode voltage Duty cycle Aperture Modulation Tank diameter Tank length Radial acceptance (norm.) Input emittance Output emittance Longitudinal emittance (90%)
2-4 keV/u 100-200 keV/u
1 5-l
80-l 15 MHz 60 kV < 25% 3.0 mm l-2.2 50 cm 1.5 m 0.65~ mm mrad 0.377 mm mrad 0.47r mm mrad 3Orr keV ns
A. Schempp / RFQ accelerators for ion implantation
429
ECR
Fig. 5. Layout of the RFQ implanters in the IKF.
ECR source, which will also be installed at the IKF in Frankfurt for atomic physics and materials research [8,9]. Basic parameters of this accelerator are summarized in table 2. This RFQ is close to completion. The tank has been built and copper plated, the electrodes and supporting stems have been manufactured and we are working on the tuning elements at present. The transmitter is now being converted to be tuned by remote control over the frequency range from 80 to 110 MHz. The delivery should be in July (1991). Rf tests and test beam operation will be possible this year.
6. Discussion We are presently building two RFQs, which will be used for ion implantation. Fig. 5 shows the layout of the implanters. The high current RFQ which will be used by the group of Thomae [7] is a fixed energy RFQ being designed for a 10 mA, 1.5 MeV Nt beam.
Energy variation can be done with a separately driven spiral loaded cavity. The second (low current) RFQ, which will be used by the group of Schmidt-B&king [8], is designed for a maximum output energy of 200 keV/u for ions from an ECR source with a charge to mass ratio of better than l/5 corresponding to a maximum “voltage gain” of 1 MV. Energy variation down to 100 keV/u is achieved by changing the frequency of the resonator. Fig. 6 shows the energy-mass plot for the two accelerators, normalized to the values of the “design particle” (T,, m,,, f,,). The VE-RFQ can be tuned to a typical triangle of T-m values between the lines of highest and lowest frequency and maximum energy. The RFQ-spiral-resonator system is characterized by two parallel lines to the (45 o 1 fixed frequency f0 line. For lighter ions the energy modulation can be up to lOO%, only limited by the acceptance of the beam line. While the high current implanter requires two transmitters with a total rf power of 140 kW, the (low beam current) VE-RFQ is driven by a single 80 kW transmitter with a duty cycle of up to 25%.
Acknowledgements The author wants to thank his friends and colleagues for valuable discussions and is especially grateful to H. Deitinghoff and R. Thomae for their help. Additionally he wants to mention those who knew everything beforehand anyhow, but waited long enough not to interfere.
References 1
Fig. 6. Energy-mass
1.5
m/m, plot for the RFQ implanters.
2
111 J.F. Ziegler, Nucl. Instr. and Meth. B6 (1985) 270. [2] J.G. Bannenberg and F. Saris, Nucl. Instr. and Meth. B37/38 (1989) 398. VI. EQUIPMEN-I
430
A. Schempp / RFQ accelerators for ion implantation
[3] A. Polmann et al., Nucl. Instr. and Meth. B37/38 (1989) 935. [4] R.W. Thornae, Nucl. Instr. and Meth. B50 (1990) 444. [5] R.W. Hamm, Linac90, LANL Rep. 12004 (1991) 558. [6] R. Geller et al., Linac88, CEBAF Rep. 89-001 (1989) 455. [7] R.W. Thomae et al., Nucl. Instr. and Meth. B37/38 (1989) 235. (81 D. Hofmann et al., Nucl. Instr. and Meth. B.50 (1990) 478. 191 A. Schempp, Nucl. Instr. and Meth. B50 (1990) 460. [lo] K. Bethge, EPAC I (World Scientific, 1989) p. 158. [ll] R. Wideroe, Archiv Elektrotechnik 21 (1928) 387. [12] E. Jaeschke, Linac84, GSI Rep. 84-11 (1984) 24. [13] I.M. Kapchinskij and V.A. Teplyakov, Prib. Tekh. Eksp. 119 (210970) 17, 19. [14] K.R. Crandall, R.H. Stokes and T.P. Wangler, BNL-51143 (1980) 20.
[15] N. Angert and R.W. Miiller, Vacuum 36 (1986) 969. 1161 H.F. Glavish, Nucl. Instr. and Meth. B24/25 (1987) 771. [17] J.G. Bannenberg et al., Linac90, LANL Rep. 12004 (1991) 250. [183 R. Wojke et al., Nucl. Instr. and Meth. A278 (1989) 318. [19] A. Hirakimoto et al., Nucl. Instr. and Meth. B37/38 (1989) 248. [20] A. Schempp et al., Nucl. Instr. and Meth. BlO/ll (19851 831. [21] A. Schempp, Linac90, LANL Rep. 12004 (1991) 535. [22] A. Schempp, EPAC I (World Scientific, 1989) p. 464. [23] G. McMichael, Linac90, LANL-120004 (1991). [24] A. Schempp, Linac88, CEBAF 89-0001 (1989) 560. [25] A. Schempp et al., Linac79, BNL Rep. 51134 (1979) 159. [26] R. Keller et al., Vacuum 36 (1986) 836. [27] J. Hauser et al., EPAC I (World Scientific, 19891 p. 1140.
and Methods
RFQ accelerators
in Physics Research
Nuclear Instruments & Methods in Physics Research SectmB
B62 (1992) 425-430
for ion implantation
A. Schempp Institut fiir Angewandte Physik, J. W’SGoethe-Uniuersitiit, Postfach 111932, D-6000 Frankfurt/M,
Germany
Radiofrequency quadrupoles (RFQI are rf structures which can efficiently accelerate high current ion beams to energies up to 1 MeV/u. Features are a strong electrical rf focusing which allows ion beam currents up to 10 mA and a compact and reliable system which is well suited for applications like high energy ion implantation. Examples of RFQs with fixed and variable energy for implantation and the status of work on these implanters will be discussed.
1. Introduction
2. Radiofrequency
Due to the increasing interest in ion implantation into semiconductors, ceramics and metals, high current ion beams with energies in the MeV range are required [1,2]. Compared to the well established techniques with beams of several hundred keV deeper and thicker regions of the material can be modified, resulting in novel structures. While research work could be done with standard low current equipment practical applications need high current beams for an appreciable throughput. For surface hardening of load carrying metal samples by deep implantation with doses of some 10” ions/cm*, e.g. N+ beams of up to 10 mA have to be accelerated which cannot be done with dc acceleration techniques at higher energies. While at low energies (100-400 keV) currents up to 10 mA are quite standard, the system at FOM [3] (1 MeV/l mA N+) represents the upper edge of the development. For rf accelerators the low energy part is the bottleneck (injection and bunching) while a high energy extension poses no principal problems. It is clear that rf acceleration even having these advantageous properties cannot replace dc accelerators concerning low emittance, energy spread, and good spatial resolution as needed in the field of ion beam analysis and diagnostic for materials characterisation. The typical energy spread of several percent poses no problem for high dose implantation [4]. So rf acceleration techniques are extending the range of applications to new parameters [S] and are even more valuable in combination with dc machines as has been proposed for the new facility for material research in the Institut fiir Kernphysik (IKE) in Frankfurt, in which a high current high energy implanter, an ECR ion source for highly charged heavy ions [6] in combination with a frequency variable RFQ with final energies between 100 and 200 keV/u will be used together with a 7 MV Van de Graaff accelerator [7-lo].
In low energy rf accelerators a wave is generated in a system of electrodes which moves along in synchronism with the ions; therefore the electrodes (drift tubes) are arranged with a special velocity profile. Because only the crest of the wave is used for acceleration the beam is bunched and occupies only approximately 60 ’ out of 360 ’ of the periodic field. Stable oscillations of the ions in the bunch result in an energy spread which is typically several percent. For compensating the defocusing action of the accelerating field and space charge repulsion an additional external focusing is necessary. Low energy beams, which for rf accelerators means energies up to several MeV/u, with significant space charge defocusing can be transported efficiently with the help of drift tube quadrupoles. Examples are the Wideroe-type structures as used for nuclear physics machines, some of which are divided in short structures which can be phased individually to adjust for a variable velocity profile 111,121. In radiofrequency quadrupole (RFQ) accelerators [13,14] the focusing and accelerating fields are generated by the same electrodes. The mechanical modulation of the quadrupole electrodes, which is shown schematically in fig. 1, adds an accelerating field component to the homogeneous quadrupole channel. Because electrical focusing forces are independent of ion velocity, the RFQ structure is especially attractive for the low energy part of ion linacs. Though the improvement of injectors for high energy machines has been the main goal of the early development, numerous proposals for other applications were made since. Ion implantation is an example for which RFQs have been proposed as beam sources for ion beams with an energy in the MeV range and a current of several milliamperes [4,15], At present several rf accelerators designed for the use as implanter are operating or under development.
0168-583X/92/$05.00
0 1992 - Elsevier
Science
Publishers
B.V. All rights reserved
accelerators
426
A. Schempp / RFQ accelerators for ion implantation
There is the Eaton variable phase linac [16] with a Wideroe-type structure with individually driven drift tubes for energy variation. The MEQALAC system at FOM [17,18] has four parallel beams, both accelerator concepts using electrostatic quadrupole focusing. The implanter designed by Shimadsu [19] is a 4-vane RFQ operating at 70 MHz, while the other RFQs are based on the 4-rod RFQ structure [20], for which now also systems with variable frequency and energy have been built [21].
3. RFQ design The RFQ structure is converting the dc beam extracted from an ion source into a bunched beam. The injection energy can be very low and the beam currents can be very high due to the homogeneous focusing structure with high rf field strengths. The design can be divided into two parts, namely the shaping of the electrodes with a proper “modulation program” and the design of a suitable rf structure for the transformation of rf power into electrode fields and beam power
La. The electrode shape is characterized by the aperture radii II,, the modulations mj and the cell lengths Lj along the RFQ. For a given ion energy and electrode voltage Uo the focusing gradients G (G = Xr l), the length of the corresponding X%/a*, quadrupole, which is determined by the operating frequency, and the beam channel cross section determine the limiting ion currents and the acceptance of the RFQ. The accelerating field E, (E, =AC&/L,, A < 1) is proportional to the modulation m of the electrode (ratio of minimum to maximum aperture) while the
focusing efficiency X goes down for increasing modulation. Another strong limitation is the maximum applicable field. The electrical breakdown fields and the electrode voltage for which sparks will prevent useful operation can be described by:
&,ax[kVl= 20( 1+
g[ mm] + 1.5 x 10-3f[MHz])
(g is the minimum electrode gap). The RFQ electrode design parameters have to be matched with ion source parameters and the requirements of the experiments. The resonator for driving the electrodes has to be efficient, compact, and reliable. While power efficiency (beam versus ac power) falls short compared to dc generators, size and handling are favourable. The rf power which is necessary for operation is proportional to the length L, and to the operating frequency f of the RFQ, and to the square of the electrode voltage Uo: N - U*L, f. The electrode voltage has to be adjusted proportional to the specific charge A/q to get the same beam dynamics in the accelerator. Therefore the dynamics has to be designed for the heaviest particle or the,lowest charge to mass ratio respectively. Typical rf accelerators are pulsed with a high pulse current. This is matched to the operation as injector but is also very efficient with respect to the rf power consumption. Cw RFQs are exceptional [23] but several resonators have been built and operated. Normally the power levels are lower than in pulsed operation, for which the problems of thermal and mechanical stability can be solved more easily. With respect to heavy ion acceleration the 4-rod structure is advantageous especially for high current implantation, where mostly singly charged ions are
Fig. 1. Scheme of the RFQ electrodes.
427
A. Schempp / RFQ accelerators for ion implantation
Table 1 Parameters of the N-RFQ
used at rather low frequencies. RFQs for protons and heavy ions with low and high currents and low and high duty cycles have been built and operated [24]. The operational experience is very promising. Two structures which demonstrate the flexibility and versatility are being built in Frankfurt at present. They will be described in the following sections.
4. High current implanter
Injection energy Final energy Charge to mass ratio Frequency Electrode voltage Duty cycle Aperture Modulation Tank diameter Tank length Radial acceptance (norm.) Input emittance Output emittance Longitudinal emittance Spiral-resonator voltage Rf power RFQ Rf power spiral
RFQ
For experiments in the field of high energy, high dose implantation a RFQ accelerator system is under construction, which will deliver N+ beams with energies well above 1 MeV and beam currents of up to 10 mA. The RFQ is operating at 108.5 MHz for which a transmitter from an earlier postaccelerator project [25] is available. The ion source is a CHORDIS developed at GSI by Keller [26], which has a very good emittance EN = 0.1~ mm mrad) and reliability. The injection energy has been chosen to be 50 keV (3.5 keV/u) and a beam current of 10 mA is envisaged. The design of the RFQ resulted in a structure with a final energy of 1.5 MeV (0.107 MeV/u), a length of 2 m and an rf power consumption of 80 kW as basic parameters. Table 1 summarizes characteristic parameters of this structure. Fig. 2 gives a typical variation of electrode parameters along the nitrogen implanter RFQ showing the adiabatic variation of the electrode parameters and the typical situation that a large fraction of the accelerator is used for the bunching of the beam with very little acceleration. The RFQ resonator is a 4-rod structure for which the mechanical design was adopted from the RFQ, which has been built for the new UNILAC injector at GSI [21].
If!
3.5 keV/u 107 keV/u i 1 14108.5 MHz 80 kV 100% 3.0 mm l-l.65 35 cm 2.0 m 0.8~ mm mrad 0.5n mm mrad 0.6~ mm mrad 0.05~ MeV ns 0.5 MV 80 kW 60 kW
The resonator has rod electrodes with direct cooling, which are supported by cylindrical stems. The tank is designed with a top flange like a coffin, which gives easy access for installation, alignment and maintenance by the open top flange along the resonator and has diagnostic elements incorporated in the end plates. Fig. 3 shows a scheme of the input part of the resonator. This RFQ is a fixed energy structure. Energy variation is achieved with an additional spiral “postaccelerator” 112,271, which can accelerate or decelerate up to 0.5 MeV. The spiral loaded cavity is a two gap drift tube structure which requires 60 kW for the design field. The combination with the RFQ with its fixed output energy gives energy variation with unique beam
m, T
T\
[MeVl
.
0
Ip
.’
/ A,
.L.
._._._._.-.-*-.--.-
RFQ
CELL
NUMBER
/- i/T 200
Fig. 2. Beam parameters for the VE-RFQ implanter. VI. EQUIPMENT
A. Schempp / RFQ accelerators for ion implantation
428
BEAM AX
SUPPORT
STEMS
Fig. 3. Scheme of the 4-rod RFQ resonator.
properties because the energy can be modulated very fast distributing the beam over a large depth range.
5. ECR / VE-RFQ
combination
Normally RFQ accelerators have a fixed velocity profile, given by the electrode modulation. The Widerde resonance condition (Li= /3, h/2 = u,/2 f > connects the cell length L with the path length in half the rf period and also sets a lower limit to the accelerating fields or the electrode voltage in case of the RFQ, because the energy (velocity) gain must be sufficient to stay in synchronism. In variable energy RFQs (VE-RFQ) the frequency f of the resonator is varied (0, -f ). The electrode voltage Uo has to be adjusted to the specific charge and energy: Uo - (A/q)f ‘, T, - (A/q)f *. To change the frequency of the 4-rod RFQ the resonator can be tuned capacitively or inductively. The first VE-RFQ is the cluster accelerator at the IPNL in Lyon [29]. The design was made for S-10 keV/u injection energy and 50-100 keV/u final energies (energy gain up to 5 MeV for m = 50 u). For energy variation of the resonator the effective length of the driving conductor was changed with movable shorts as indicated in fig. 4 which resulted in a tuning range of 88 to 120 MHz. For these frequencies transmitters are easily available. This tuning range corresponds to an energy change by a factor of about 2. Compared to other RFQ designs the shaper-buncher part has been omitted in the cluster RFQ for very effective acceleration. The average field strength is as high as 2.5 MV/m, which is at the cost of a relatively small minimum transmission of 25%. The RFQ has been successfully used in first tests. This VE-RFQ is the model for the second implanter RFQ, planned for the acceleration of highly charged heavy ions from an
Fig. 4. Scheme
Table 2 Parameters
of a VE-RFQ
of the VE-RFQ
Injection energy Final energy Charge
module.
to mass ratio
Frequency Electrode voltage Duty cycle Aperture Modulation Tank diameter Tank length Radial acceptance (norm.) Input emittance Output emittance Longitudinal emittance (90%)
2-4 keV/u 100-200 keV/u
1 5-l
80-l 15 MHz 60 kV < 25% 3.0 mm l-2.2 50 cm 1.5 m 0.65~ mm mrad 0.377 mm mrad 0.47r mm mrad 3Orr keV ns
A. Schempp / RFQ accelerators for ion implantation
429
ECR
Fig. 5. Layout of the RFQ implanters in the IKF.
ECR source, which will also be installed at the IKF in Frankfurt for atomic physics and materials research [8,9]. Basic parameters of this accelerator are summarized in table 2. This RFQ is close to completion. The tank has been built and copper plated, the electrodes and supporting stems have been manufactured and we are working on the tuning elements at present. The transmitter is now being converted to be tuned by remote control over the frequency range from 80 to 110 MHz. The delivery should be in July (1991). Rf tests and test beam operation will be possible this year.
6. Discussion We are presently building two RFQs, which will be used for ion implantation. Fig. 5 shows the layout of the implanters. The high current RFQ which will be used by the group of Thomae [7] is a fixed energy RFQ being designed for a 10 mA, 1.5 MeV Nt beam.
Energy variation can be done with a separately driven spiral loaded cavity. The second (low current) RFQ, which will be used by the group of Schmidt-B&king [8], is designed for a maximum output energy of 200 keV/u for ions from an ECR source with a charge to mass ratio of better than l/5 corresponding to a maximum “voltage gain” of 1 MV. Energy variation down to 100 keV/u is achieved by changing the frequency of the resonator. Fig. 6 shows the energy-mass plot for the two accelerators, normalized to the values of the “design particle” (T,, m,,, f,,). The VE-RFQ can be tuned to a typical triangle of T-m values between the lines of highest and lowest frequency and maximum energy. The RFQ-spiral-resonator system is characterized by two parallel lines to the (45 o 1 fixed frequency f0 line. For lighter ions the energy modulation can be up to lOO%, only limited by the acceptance of the beam line. While the high current implanter requires two transmitters with a total rf power of 140 kW, the (low beam current) VE-RFQ is driven by a single 80 kW transmitter with a duty cycle of up to 25%.
Acknowledgements The author wants to thank his friends and colleagues for valuable discussions and is especially grateful to H. Deitinghoff and R. Thomae for their help. Additionally he wants to mention those who knew everything beforehand anyhow, but waited long enough not to interfere.
References 1
Fig. 6. Energy-mass
1.5
m/m, plot for the RFQ implanters.
2
111 J.F. Ziegler, Nucl. Instr. and Meth. B6 (1985) 270. [2] J.G. Bannenberg and F. Saris, Nucl. Instr. and Meth. B37/38 (1989) 398. VI. EQUIPMEN-I
430
A. Schempp / RFQ accelerators for ion implantation
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