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A variable energy RFQ for the acceleration of heavy clusters
NOMB
Nuclear Instruments and Methods in Physics Research B 88 (1994) 16-20 North-Holland
A variable energy RFQ for the acceleration
Beam Interactions with Materials &Atoms
of heavy clusters *
A. Schempp Institut fiir Angewandte Physik, J. W. Goethe Universitiit Frankfurt, Postfach 111932, D-50054 Frankfurt am Main, Germany
An accelerator for heavy clusters up to mass m = 1000 u is being designed and built for the Departement de Physique des MatCriaux of the Universit6 de Lyon. The design is based on the first VE-RFQ which is successfully used for research with light hydrogen clusters at the IPNL Lyon. For the heavy clusters, the operating frequency was lowered to 5-7 MHz, which required a new resonator design. Results of particle dynamics calculations and structure development for the first RFQ for such “particles” are presented, and the status of the project is discussed.
1. Introduction
Radio frequency quadrupoles (RFQs) are low energy rf accelerator structures which can efficiently transport and accelerate particle beams with low specific charge. RFQs have a fixed velocity profile and therefore fixed input and output energies per nucleon. The ion energy can be varied by changing the resonator frequency. This has been done for a 4-rod RFQ, i.e. the type of RFQ resonator developed in Frankfurt, which is well suited for such an application. In a RFQ structure [l-3], acceleration is achieved by a geometrical modulation of quadrupole electrodes leading to axial components of the field. These electrodes are part of a rf resonator, excited in a TE,,,,-like mode to generate the necessary field distribution. As indicated in Fig. 1, the modulation of the diameter adds an accelerating field component to the focusing channel, resulting in a structure which accelerates and focuses with the same rf fields. Electrical focusing forces are independent of the ion velocity up and if rf fields are applied, higher voltages than in a dc quadrupole system can be reached, giving a very strong focusing with a large radial acceptance. Because the focusing structure is homogeneous the accelerating and focusing cells can be very short, which makes it possible to apply the concept of adiabatic bunching. The phases ‘pS between the particles and the rf fields and the amplitude of the accelerating fields E, are changed very slowly with increasing ion velocity up, transforming a dc-beam from the ion source into a bunched beam. The beam bunches with rf repetition rate contain up to 90% of the particles of the ion source pulse (pulse time S+ rf period) with a minimum of emittance growth. * Supported by the BMFT. Elsevier Science B.V. 0168-583X(93)E1158-I
XSDI
The mechanical shape of the electrodes is characterized by the aperture radii ai, the modulations mi, and the modulation periods L,, which are varying along the structure as indicated in Fig. 1. Together with the electrode voltage Uo, this determines the acceleration and focusing fields. Therefore a voltage as high as possible (close to the breakdown voltage) and an aperture as small as possible will be chosen for a high focusing strength G - Ua/a2. For the same focusing strength the voltage Uo which has to be applied to the quadrupole electrodes is proportional to the ratio of ion mass A to charge q: U-A/q. The rf power N required to provide the design quadrupole voltage Uo on the electrodes is proportional to the length L, of the structure: N = L, U$R' . The characteristic structure impedance R’ depends on the type and parameters of the structure and the operating frequency f. The various applications of RFQ accelerators can be distinguished by the ion beam species, the ion energy and current, the emittance, the duty factor, and the specific charge of the ions, which lead to the different techniques of beam dynamics and rf-structure design, for which the choice of the frequency f and the electrode voltage U, are starting parameters. The electrode and cavity design is based on the development of 4-rod-RFQs, in which a resonator consists of coupled h/2 oscillators in a linear arrangement. The resonator is very stable with respect to rf operation because neighboring modes are clearly separated and all major current conducting parts can be cooled, which is important especially for high duty cycle operation. Normally, the velocity profile is fixed and can be changed by varying the frequency keeping the electrode structure unchanged: up = 2Li f. Keeping the
A. Schempp/ Nucl. In&. and Meth. in Phys.Rex B 88 (1994) 16-20
17
To change beam energy, the frequency of the 4-rod RFQ can be tuned by changing the effective length of the driving conductor with movable shorts as indicated in Fig. 2.
2. The IPNL post-accelerator
Fig. 1. Scheme of the 4-rod RFQ structure.
electrode voltage Uo constant, heavier particles with mass m = Au and charge state q can be accelerated at a lower frequency with the same RFQ electrodes to the same final energy T,: Tf N (A/q)f’.
for light clusters
A variable energy RFQ (VE-RFQ) has been developed in Frankfurt 141for a first application as a cluster post-accelerator at the 0.5 MV Cockroft-Walton facility at the IPNL in Lyon (France>, where several groups working on cluster physics were interested in an energy upgrade [5]. The upgrading of the Cluster Facility has been performed in a collaboration between IPN Lyon, KfK Karlsruhe and IAP Frankfurt [6,7]. The VE-RFQ post-accelerator for clusters was designed for 10 keV/u injection energy and up to 100 keV/u (3 MeV for m = 30 u) final energy. The resonator can be tuned within the FM range from about 80 to 110 MHz, corresponding to an energy change by a factor of approximately 2. The particle dynamics design is done for the heaviest particle resp. the minimum charge to mass ratio q/A, which is necessary for acceleration. Taking the “design particle” as reference, the obtainment of higher q/A ratios, or acceleration to lower energies, requires only a lower voltage: Uo - (A/q)f2. Compared to other RFQs the buncher part has been skipped for a very effective acceleration, e.g. an average field strength of up to 1.5 MV/m, which is at the cost of a relatively small transmission of approx. 25% during “rf-on” time 7, because for technical reasons the transmitter has to be pulsed with values of 7 = 0.5-5 ms (up to 50 Hz). Typical values of parameters of operation are 7 = 1.0 ms, df = 20%, N = 40 kW IS]. First results of the successful performance have been published [9,10]. The design of the Lyon VE-RFQ was not meant to obtain high beam currents but was optimized to get the energy factor as high as Tf/Ti = 10 for light clusters, taking into account the tight funding conditions. The success of this new VE-RFQ concept is also indicated by two new very similar systems presently under construction: two ECR-RFQ combinations at the IKF at the University of Frankfurt and at HMI (Berlin) as new injector for the VICKSI cyclotron [11,12]. Interesting new experimental possibilities will be available for atomic physics as well as for ion implantation and materials characterization.
3. The heavy cluster RFQ
Fig. 2. Scheme of the variable energy RFQ resonator insert.
A development towards particle beams with much lower charge-to-mass ratio has been made for a project
18
A. Schempp/Nucl.
Instr. and Meth. in Phys. Rex B 88 (1994) 16-20
Fig. 3. Cross section of the VE heavy-cluster RFQ tank.
for the Departement de Physique des Materiaux of the Universite de Lyon. The group of Perez has developed a “laser source” for heavy clusters [13], for which a post-accelerator is being built with a final energy of up to 1.0 MeV. Additional boundary conditions are the pulsed structure of the laser source and the low injection energy of only 20 eV/u. Particle dynamics requires the low frequency range of 5-7 MHz for this RFQ resonator. The solution was found in a new RFQ with a folded twin line resonator. It can be deduced from the light cluster resonator in Fig. 2 by skipping all but two electrode “legs”, increasing their length to
approx. 3.5 m and folding it, to let it fit into a rectangular vacuum tank. Fig. 3 shows a cross section of this low frequency resonator. Fig. 4 shows the range of masses and energies which will be covered by this machine. The basic parameters of this VE-RFQ are: input energy lo-20 keV, output energy Tf = 500-1000 keV, an operation frequency 5-7 MHz, electrode voltage 45 kV, length 2 m, number of cells 215, rf-pulse power 20 kw.
kd
f
/T 0
Fig. 4. Energy-mass plot for the VE heavy-cluster RFQ.
50
100
I 150
100 200
0
Fig. 5. Electrode design parameters ai, mi, pi, and energy Ti for the heavy-cluster RFQ as a function of the cell number.
A. Schempp / NucE. Instr. and Meth. in Phys. Res. B 88 (1994) 16-20
IA? CELL
RFQ
‘fE.?Y
215
.,
HEAVY
I =
Ci_USTER
0.00
MA
. i-1 .N -- i -29.5
-fd.5
t :... oh
tl lDb.5
7.d.4
Yj
Fig. 6. Results of PARMTEQ simulations for a 1 MeV, m =
19
20
A. Schempp/Nucl.
Instr. and Mefh. in Plays. Res. B 88 (1994) 16-20
Fig. 5 shows the design parameters ai, mi, rpi, and the particle energy Ti along the electrodes for the new cluster machine. The slow increase of the ion energy T as function of the RFQ cell number is demonstrating the fact that a significant part of the RFQ structure is required for bunching. PARMTEQ calculations done by Deitinghoff [14] show a good transmission well over 90% and a small radial emittance growth. Fig. 6 shows the output phase space distributions (m = 1000 u, T = 1 MeV) for a simulation with 20 f 0.5 keV input energy.
4. Status
A prototype of the RFQ (without electrode modulation) has been built and operated at high rf power [15], which showed good results, concerning the shunt impedance and the operational stability. At present the RFQ, the transmitter and the control system are being built and assembled. Most parts will be delivered to Lyon in October, where final installation and commissioning should be completed in early 1994. As usual, we are somewhat late but optimistic.
Acknowledgements I have to thank A. Perez for his confidence despite warnings and good cooperation in this difficult project as well as my colleagues in IPNL, Frankfurt and Darm-
stadt for continuous help and support in this work, which led, amongst others, to a remarkable poyster session.
References [l] I.M. Kapchinskiy and V. Teplyakov, Prib. Tekh. Eksp.
119 (1970) 17 and 19. [2] R.H. Stokes and T.P. Wangler, Ann. Rev. Nucl. Part. Sci. 38 (1988) 97. [3] A. Schempp, Habilitationsschrift Univ. Frankfurt (1990). [41 A. Schempp, Nucl. Instr. and Meth. B 40/41 (1989) 937. [5] B. Mazuy, A. Belkacem, M. Chevallier, M.J. Gaillard, J.C. Poizat and J. Remillieux, Nucl. Instr. and Meth. B 33 (1988) 105. 161A. Schempp and H.O. Moser, J. Phys. (Paris) 50 C2 (1989) 205. 171A. Schempp et al., Proc. EPAC90, (Ed. Front&es, 1990) p. 40. [8] R. Genre, private communication. [9] M.J. Gaillard et al., Proc. ISSPIC6, Chicago, USA, 1992, Z. Phys. D 26 (1993) 347. [lo] B. Farizon et al., these Proceedings (Conf. on Polyatomic Ion Impact on Solids and Related Phenomena, St. Malo, France, 1993) Nucl. Instr. and Meth. B 88 (1994) 86. [ll] D. Hofmann et al., Nucl. Instr. and Meth. B 50 (1990) 478. [12] A. Schempp, Proc. Linac 92, AECL-10728 (1992) p. 545. [13] A. Perez et al., these Proceedings (Conf. on Polyatomic Ion Impact on Solids and Related Phenomena, St. Malo, France, 1993) Nucl. Instr. and Meth. B 88 (1994) 25. [14] H. Deitinghoff, Univ. Frankfurt, IAP-Int. Rep. 93-6 (1993). [15] H. Vormann, Diplom Thesis, Univ. Frankfurt (1993).
Nuclear Instruments and Methods in Physics Research B 88 (1994) 16-20 North-Holland
A variable energy RFQ for the acceleration
Beam Interactions with Materials &Atoms
of heavy clusters *
A. Schempp Institut fiir Angewandte Physik, J. W. Goethe Universitiit Frankfurt, Postfach 111932, D-50054 Frankfurt am Main, Germany
An accelerator for heavy clusters up to mass m = 1000 u is being designed and built for the Departement de Physique des MatCriaux of the Universit6 de Lyon. The design is based on the first VE-RFQ which is successfully used for research with light hydrogen clusters at the IPNL Lyon. For the heavy clusters, the operating frequency was lowered to 5-7 MHz, which required a new resonator design. Results of particle dynamics calculations and structure development for the first RFQ for such “particles” are presented, and the status of the project is discussed.
1. Introduction
Radio frequency quadrupoles (RFQs) are low energy rf accelerator structures which can efficiently transport and accelerate particle beams with low specific charge. RFQs have a fixed velocity profile and therefore fixed input and output energies per nucleon. The ion energy can be varied by changing the resonator frequency. This has been done for a 4-rod RFQ, i.e. the type of RFQ resonator developed in Frankfurt, which is well suited for such an application. In a RFQ structure [l-3], acceleration is achieved by a geometrical modulation of quadrupole electrodes leading to axial components of the field. These electrodes are part of a rf resonator, excited in a TE,,,,-like mode to generate the necessary field distribution. As indicated in Fig. 1, the modulation of the diameter adds an accelerating field component to the focusing channel, resulting in a structure which accelerates and focuses with the same rf fields. Electrical focusing forces are independent of the ion velocity up and if rf fields are applied, higher voltages than in a dc quadrupole system can be reached, giving a very strong focusing with a large radial acceptance. Because the focusing structure is homogeneous the accelerating and focusing cells can be very short, which makes it possible to apply the concept of adiabatic bunching. The phases ‘pS between the particles and the rf fields and the amplitude of the accelerating fields E, are changed very slowly with increasing ion velocity up, transforming a dc-beam from the ion source into a bunched beam. The beam bunches with rf repetition rate contain up to 90% of the particles of the ion source pulse (pulse time S+ rf period) with a minimum of emittance growth. * Supported by the BMFT. Elsevier Science B.V. 0168-583X(93)E1158-I
XSDI
The mechanical shape of the electrodes is characterized by the aperture radii ai, the modulations mi, and the modulation periods L,, which are varying along the structure as indicated in Fig. 1. Together with the electrode voltage Uo, this determines the acceleration and focusing fields. Therefore a voltage as high as possible (close to the breakdown voltage) and an aperture as small as possible will be chosen for a high focusing strength G - Ua/a2. For the same focusing strength the voltage Uo which has to be applied to the quadrupole electrodes is proportional to the ratio of ion mass A to charge q: U-A/q. The rf power N required to provide the design quadrupole voltage Uo on the electrodes is proportional to the length L, of the structure: N = L, U$R' . The characteristic structure impedance R’ depends on the type and parameters of the structure and the operating frequency f. The various applications of RFQ accelerators can be distinguished by the ion beam species, the ion energy and current, the emittance, the duty factor, and the specific charge of the ions, which lead to the different techniques of beam dynamics and rf-structure design, for which the choice of the frequency f and the electrode voltage U, are starting parameters. The electrode and cavity design is based on the development of 4-rod-RFQs, in which a resonator consists of coupled h/2 oscillators in a linear arrangement. The resonator is very stable with respect to rf operation because neighboring modes are clearly separated and all major current conducting parts can be cooled, which is important especially for high duty cycle operation. Normally, the velocity profile is fixed and can be changed by varying the frequency keeping the electrode structure unchanged: up = 2Li f. Keeping the
A. Schempp/ Nucl. In&. and Meth. in Phys.Rex B 88 (1994) 16-20
17
To change beam energy, the frequency of the 4-rod RFQ can be tuned by changing the effective length of the driving conductor with movable shorts as indicated in Fig. 2.
2. The IPNL post-accelerator
Fig. 1. Scheme of the 4-rod RFQ structure.
electrode voltage Uo constant, heavier particles with mass m = Au and charge state q can be accelerated at a lower frequency with the same RFQ electrodes to the same final energy T,: Tf N (A/q)f’.
for light clusters
A variable energy RFQ (VE-RFQ) has been developed in Frankfurt 141for a first application as a cluster post-accelerator at the 0.5 MV Cockroft-Walton facility at the IPNL in Lyon (France>, where several groups working on cluster physics were interested in an energy upgrade [5]. The upgrading of the Cluster Facility has been performed in a collaboration between IPN Lyon, KfK Karlsruhe and IAP Frankfurt [6,7]. The VE-RFQ post-accelerator for clusters was designed for 10 keV/u injection energy and up to 100 keV/u (3 MeV for m = 30 u) final energy. The resonator can be tuned within the FM range from about 80 to 110 MHz, corresponding to an energy change by a factor of approximately 2. The particle dynamics design is done for the heaviest particle resp. the minimum charge to mass ratio q/A, which is necessary for acceleration. Taking the “design particle” as reference, the obtainment of higher q/A ratios, or acceleration to lower energies, requires only a lower voltage: Uo - (A/q)f2. Compared to other RFQs the buncher part has been skipped for a very effective acceleration, e.g. an average field strength of up to 1.5 MV/m, which is at the cost of a relatively small transmission of approx. 25% during “rf-on” time 7, because for technical reasons the transmitter has to be pulsed with values of 7 = 0.5-5 ms (up to 50 Hz). Typical values of parameters of operation are 7 = 1.0 ms, df = 20%, N = 40 kW IS]. First results of the successful performance have been published [9,10]. The design of the Lyon VE-RFQ was not meant to obtain high beam currents but was optimized to get the energy factor as high as Tf/Ti = 10 for light clusters, taking into account the tight funding conditions. The success of this new VE-RFQ concept is also indicated by two new very similar systems presently under construction: two ECR-RFQ combinations at the IKF at the University of Frankfurt and at HMI (Berlin) as new injector for the VICKSI cyclotron [11,12]. Interesting new experimental possibilities will be available for atomic physics as well as for ion implantation and materials characterization.
3. The heavy cluster RFQ
Fig. 2. Scheme of the variable energy RFQ resonator insert.
A development towards particle beams with much lower charge-to-mass ratio has been made for a project
18
A. Schempp/Nucl.
Instr. and Meth. in Phys. Rex B 88 (1994) 16-20
Fig. 3. Cross section of the VE heavy-cluster RFQ tank.
for the Departement de Physique des Materiaux of the Universite de Lyon. The group of Perez has developed a “laser source” for heavy clusters [13], for which a post-accelerator is being built with a final energy of up to 1.0 MeV. Additional boundary conditions are the pulsed structure of the laser source and the low injection energy of only 20 eV/u. Particle dynamics requires the low frequency range of 5-7 MHz for this RFQ resonator. The solution was found in a new RFQ with a folded twin line resonator. It can be deduced from the light cluster resonator in Fig. 2 by skipping all but two electrode “legs”, increasing their length to
approx. 3.5 m and folding it, to let it fit into a rectangular vacuum tank. Fig. 3 shows a cross section of this low frequency resonator. Fig. 4 shows the range of masses and energies which will be covered by this machine. The basic parameters of this VE-RFQ are: input energy lo-20 keV, output energy Tf = 500-1000 keV, an operation frequency 5-7 MHz, electrode voltage 45 kV, length 2 m, number of cells 215, rf-pulse power 20 kw.
kd
f
/T 0
Fig. 4. Energy-mass plot for the VE heavy-cluster RFQ.
50
100
I 150
100 200
0
Fig. 5. Electrode design parameters ai, mi, pi, and energy Ti for the heavy-cluster RFQ as a function of the cell number.
A. Schempp / NucE. Instr. and Meth. in Phys. Res. B 88 (1994) 16-20
IA? CELL
RFQ
‘fE.?Y
215
.,
HEAVY
I =
Ci_USTER
0.00
MA
. i-1 .N -- i -29.5
-fd.5
t :... oh
tl lDb.5
7.d.4
Yj
Fig. 6. Results of PARMTEQ simulations for a 1 MeV, m =
19
20
A. Schempp/Nucl.
Instr. and Mefh. in Plays. Res. B 88 (1994) 16-20
Fig. 5 shows the design parameters ai, mi, rpi, and the particle energy Ti along the electrodes for the new cluster machine. The slow increase of the ion energy T as function of the RFQ cell number is demonstrating the fact that a significant part of the RFQ structure is required for bunching. PARMTEQ calculations done by Deitinghoff [14] show a good transmission well over 90% and a small radial emittance growth. Fig. 6 shows the output phase space distributions (m = 1000 u, T = 1 MeV) for a simulation with 20 f 0.5 keV input energy.
4. Status
A prototype of the RFQ (without electrode modulation) has been built and operated at high rf power [15], which showed good results, concerning the shunt impedance and the operational stability. At present the RFQ, the transmitter and the control system are being built and assembled. Most parts will be delivered to Lyon in October, where final installation and commissioning should be completed in early 1994. As usual, we are somewhat late but optimistic.
Acknowledgements I have to thank A. Perez for his confidence despite warnings and good cooperation in this difficult project as well as my colleagues in IPNL, Frankfurt and Darm-
stadt for continuous help and support in this work, which led, amongst others, to a remarkable poyster session.
References [l] I.M. Kapchinskiy and V. Teplyakov, Prib. Tekh. Eksp.
119 (1970) 17 and 19. [2] R.H. Stokes and T.P. Wangler, Ann. Rev. Nucl. Part. Sci. 38 (1988) 97. [3] A. Schempp, Habilitationsschrift Univ. Frankfurt (1990). [41 A. Schempp, Nucl. Instr. and Meth. B 40/41 (1989) 937. [5] B. Mazuy, A. Belkacem, M. Chevallier, M.J. Gaillard, J.C. Poizat and J. Remillieux, Nucl. Instr. and Meth. B 33 (1988) 105. 161A. Schempp and H.O. Moser, J. Phys. (Paris) 50 C2 (1989) 205. 171A. Schempp et al., Proc. EPAC90, (Ed. Front&es, 1990) p. 40. [8] R. Genre, private communication. [9] M.J. Gaillard et al., Proc. ISSPIC6, Chicago, USA, 1992, Z. Phys. D 26 (1993) 347. [lo] B. Farizon et al., these Proceedings (Conf. on Polyatomic Ion Impact on Solids and Related Phenomena, St. Malo, France, 1993) Nucl. Instr. and Meth. B 88 (1994) 86. [ll] D. Hofmann et al., Nucl. Instr. and Meth. B 50 (1990) 478. [12] A. Schempp, Proc. Linac 92, AECL-10728 (1992) p. 545. [13] A. Perez et al., these Proceedings (Conf. on Polyatomic Ion Impact on Solids and Related Phenomena, St. Malo, France, 1993) Nucl. Instr. and Meth. B 88 (1994) 25. [14] H. Deitinghoff, Univ. Frankfurt, IAP-Int. Rep. 93-6 (1993). [15] H. Vormann, Diplom Thesis, Univ. Frankfurt (1993).