- Email: [email protected]
Development of four-beam IH-RFQ linear accelerator
Nuclear Inst. and Methods in Physics Research B 462 (2020) 139–142
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of four-beam IH-RFQ linear accelerator a,⁎
b
c
Shota Ikeda , Masahiro Okamura , Noriyosu Hayashizaki
T
a
Riken, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan c Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Multi-beam acceleration IH-RFQ linac Heavy ion acceleration
It is difficult to accelerate a high intensity heavy ion beam because the upper limit of the beam current in an RFQ (Radio Frequency Quadrupole) linac depends on the applied voltage at the RFQ electrode and the strength of the space charge effect. A multi-beam RFQ linac, with multiple beam-channels accelerating several beams in parallel in one cavity, and utilizing multiple beams to decrease the space charge effect can be used to achieve a higher beam current. In order to demonstrate that a four-beam IH-RFQ (Interdigital H-type RFQ) linear accelerator is suitable for high-intensity, heavy ion beam acceleration, we have developed a four-beam prototype. A four-beam IH-RFQ cavity was fabricated according to design parameters obtained from our previous detailed simulations. Low-power RF property measurements of the fabricated four-beam IH-RFQ cavity were carried out with a network analyzer. After the low-power RF property measurements, we performed a highpower test and an acceleration test.
1. Introduction Radio frequency quadrupole (RFQ) linear accelerators are suitable for low energy, high-intensity ion beam acceleration and are typically used as injectors in hadron accelerator facilities and accelerator driven compact neutron sources. However, because of the space charge effect and an upper limit of the electric field strength, acceleration of high intensity heavy ion beam over 100 mA by using an RFQ linear accelerator is difficult. For higher intensity beam acceleration, the multi-beam acceleration method utilizes multiple beams to decrease the space charge effect, and the accelerated multiple beams are integrated using a beam funneling system. As a proof of principle, a four-rod IH-RFQ (Interdigital H-type RFQ) linear accelerator were demonstrated in reference [1], with possible extension to multi-beam acceleration. In particular, a two beam IH-RFQ linear accelerator with a two-beam LASER ion source with direct plasma injection scheme were developed at the Tokyo Institute of Technology, and the system accelerated 108 mA (2 × 54 mA/channel) carbon ions from 5 to 60 keV/u [2]. An IH-RFQ structure has some advantage for low energy heavy ion acceleration (high shunt impedance for low energy beam acceleration, small cavity diameter at low frequency, strong RF coupling). However, since the relationship between the RF properties and beam channel layout are not clear, a four-beam type RFQ linear ⁎
accelerator has not been demonstrated yet. In this study, we have designed and developed a prototype of a four beam IH-RFQ linear accelerator to establish its design scheme and verify the multi-beam acceleration properties. In this paper, a prototype was manufactured based on the design results [3], and we performed the first measurement of RF properties and four-beam acceleration. The layout of a four beam IH-RFQ linac, which consists of stem electrodes and 4 sets of RFQ electrodes installed alternately on upper and lower ridge electrodes, is shown in Fig. 1. The RF electromagnetic field is excited in the TE111 mode, and quadrupole fields are generated at each RFQ electrode. As a part of the development of the four-beam IH-RFQ linear accelerator, we have evaluated the relationship between the beam channel layout and the quadrupole distributions by using the threedimensional electromagnetic simulation software CST MICROWAVE STUDIO (CST MWS) [4] and beam tracking simulation software General Particle Tracer (GPT) [5]. Based on the simulation result, we developed the stem electrode design, in which the quadrupole distribution is the most symmetric and the difference between the maximum and minimum quadrupole strength is only 5.4%. As the simulation result, we have obtained the design of the prototype four-beam IH-RFQ linear accelerator which produced a total output beam current of 159.2 mA for an injection of 60 mA/channel with an inner cavity length of 650 mm and total power loss of 92.5 kW
Corresponding author. E-mail address: [email protected] (S. Ikeda).
https://doi.org/10.1016/j.nimb.2019.11.008 Received 9 June 2019; Received in revised form 6 November 2019; Accepted 6 November 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 462 (2020) 139–142
S. Ikeda, et al.
Fig. 1. Layout of the four-beam IH-RFQ linac. Table 1 Design parameters of the four-beam IH-RFQ linac. Charge to mass ratio (q/A) Input beam current (mA/channel) Input beam energy (keV/u) Single beam current (mA) Total output beam current (mA) Output beam current in Ch1 (mA) Output beam current in Ch2 (mA) Output beam current in Ch3 (mA) Output beam current in Ch4 (mA) Output beam energy (keV/u) Output beam power (kW) Synchronous phase (°) Focusing strength Operation frequency (MHz) Unloaded quality factor (Q0) Inter rod voltage (kV) Kilpatrick factor Wall loss (kW, normalized by interrod voltage) Radius of side shell (mm) Cavity inner length (mm) Rod length (mm) Average beam aperture radius (mm) Minimum beam aperture radius (mm) Maximum modulation index Stem number
≧1/6 60 3.6 40.2 159.2 38.9 40.6 40.5 39.2 41.6 36.7 −90 to −30 11.385 48 4254 92.2 1.8 55.8 150 650 600 6.43 3.04 3.04 8
(a) Stem electrodes, RFQ electrodes and the center plate
at 48 MHz [5]. The design parameters of the four-beam IH-RFQ linear accelerator are shown in Table 1, where the single beam current (mA) means the result of the PARMTEQM [6].
(b) Side shell with copper plating
2. Fabrication and RF measurement of a four-beam IH-RFQ cavity
Fig. 2. Fabricated four-beam IH-RFQ cavity.
2.1. Fabrication of a cavity
Before the copper plating, the resonance frequency was tuned to 47.87 MHz by decreasing the flame thickness of the side shells in the fabrication (Fig. 3). Then, the four-beam IH-RFQ cavity with copper plating and RF contacts exhibited a resonance frequency of 47.95 MHz, which is 0.08 MHz higher than that before the copper plating, and an unloaded Q value of 3687, which is about 88% of the design value. The RFQ electric field was measured using the perturbation method. A Teflon perturbator (12 mm × 12 mm × 15 mm) was set in a gap between the RFQ electrodes of each channel along the beam axis, and the frequency shift from the resonance frequency of the four-beam IH-RFQ cavity without the perturbation was measured. The square of the frequency shift is proportional to the electric field strength at the position of the perturbation. Fig. 4 presents the measured RFQ electric field strength distribution of each channel. The horizontal axis is the beam axis, and the vertical axis is the electric field strength, which is normalized with the maximum value of the electric field intensity in each beam channel as
We fabricated a four-beam IH-RFQ cavity based on the designed cavity parameters (Fig. 2). The side shell (aluminum 5052) and the center-plate (AISI 304) were plated with 50 µm copper. RF contacts were attached between the center-plate and the side shells. The left side shell had a grid vacuum port and two pickup ports, and the right one had a coupler port and two pickup ports. Copper brocks (OFC) were machined to the shape of the RFQ electrodes and stem electrodes by using a computer numerical control (CNC) milling machine. After the milling of the RFQ electrodes and the stem electrodes, they were aligned using spacers made of aluminum and fixed on the center-plate using bolts. 2.2. Low power RF test We have carried out low-power RF property measurements of the fabricated four-beam IH-RFQ cavity with a network-analyzer (Rohde & Schwarz FSG Spectrum Analyzer). 140
Nuclear Inst. and Methods in Physics Research B 462 (2020) 139–142
S. Ikeda, et al.
Fig. 5. Layout of the accelerator system.
Fig. 3. Flame thickness (Dotted area: the flame of the side shells).
100%. The tendency of the electric field distribution of each channel was similar to the simulation result. In addition, the maximum electric field intensity difference between the beam channels was 7.1%. 3. Four-beam acceleration test 3.1. Acceleration test set-up The layout of a test bench for the beam acceleration test of the fourbeam RFQ linac is shown in Fig. 5, which is constructed by combining the cavity with a four-beam laser ion-source and an RF amplifier. The RF amplifier consist of triode type amplifiers (Preamplifier: 3CX10000U7) and (Main amplifier: 4CW1000000E). The four-beam laser ion source with direct plasma injection scheme was developed and the time of flight spectra for each ion charge-state was measured using the test bench [7]. From the experimental result, we obtained a current of 36.1 mA/channel of C2+ ions with a focus lens
Fig. 6. Pulse structure of carbon beam at 330 mm from a target.
having a target distance of 400 mm and a pinhole diameter of 11 mm, using a laser beam with an energy of 175 mJ/channel (Fig. 6). In order to inject the RF power from an amplifier to the four beam IH-RFQ cavity, we used a coaxial waveguide (WX-203D) and a loopcoupler, which was connected to the cavity through magnetic coupling. Pulse waveforms in the high-power RF test are shown in Fig. 7 with
Fig. 4. Distribution of the electric field strength. 141
Nuclear Inst. and Methods in Physics Research B 462 (2020) 139–142
S. Ikeda, et al.
Fig. 9. Output signal of each beam channel (Input RF power: 84 kW, Extraction voltage: 21.6 kV).
Fig. 7. RF signal from an amplifier (Input RF power: 100 kW).
consider the reasons of the total output beam current less than the simulation value are the input beam current of C2+ less than 60 mA/Ch and the low beam matching ratio between the ion source and the fourbeam IH-RFQ cavity. The pulse shapes in each channel are shown in Fig. 9. Because of the difference of ion spectra in each of the beam pulses, the beam pulse shapes of each channel were different. In particular, at beam channel 1, the pulse peak arrives at the Faraday cup faster than in the other channels. This could be due to higher charge spectra in the injected ion beam than in the other channels. 4. Summary and future plan A prototype of the four-beam IH-RFQ linac with a Q value of 3687 at 47.95 MHz was fabricated. The electric field strength measurements indicated that the difference between the maximum and minimum electric field strengths was 7.1%. In terms of beam acceleration, the four-beam IH-RFQ linac accelerated a carbon ion beam with a current of 32 mA (8 mA/channel × 4). In a future study, the output beam properties, such as ion spectra, and beam energy of each channel, will be measured in more detail.
Fig. 8. Peak output current on each channel as a function of the RF power.
Pick-up: the pick-up voltage signal from a monitoring antenna, Pf: the voltage signal of the incident wave to the cavity, Pr: the voltage signal of the reflection wave from the cavity, and PG: the output signal from a pulse generator, respectively. The test conditions were a RF pulse repetition frequency of 0.2 Hz and a pulse width of 200 µs. Ablation plasma drifted from a graphite target to the RFQ electrode, and ion beam was injected with an extraction voltage of 21.6 kV. A Faraday cup, which was made of copper and consisted of a cylindrical cup with a diameter of 26 mm and a deep groove to suppress the secondary electron generation as much as possible, was located 95 mm downstream from the output side of the RFQ electrode. The Faraday cup picked up the RF excited in the four-beam IH-RFQ cavity as noise. In order to suppress the noise to several mV, we set metallic meshes between the Faraday cup and the output side at the end face of the four-beam IHRFQ cavity.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Funding: This work was supported by JSPS KAKENHI Grant Number JP26246042.
3.2. Beam commissioning
References
The peak output current of each channel as a function of the RF power is shown in Fig. 8. The peak current of each beam channel does not increase linearly with increasing input RF power. We speculate that it is because the charge of the dominant carbon ion in the accelerating beam changes according to the high frequency input power. In order to verify the relation between the RF power and the dominant carbon ion, we need to carry out ion spectra measurement with a bending magnet and a faraday cup. At an input RF power of 84 kW, the four beam IH RFQ linear accelerator accelerated about 32 mA (4 × 8 mA/channel), and the variation of the peak current between the channels was about 0.4 mA. We
[1] U. Ratzinger, et al., The GSI 36 MHz high-current IH-type RFQ and HIIF-relevant extensions, Nucl. Instrum. Methods Phys. Res. Sect. A 415 (1998) 281–286. [2] T. Ishibashi, N. Hayashizaki, T. Hattori, Two-beam interdigital-H-type radio frequency quadrupole linac with direct plasma injection for high intensity heavy ion acceleration, Phys. Rev. Spec. Top.-Accelerators Beams 14 (2011) 060101. [3] S. Ikeda, A. Murata, N. Hayashizaki, Design of four-beam IH-RFQ linear accelerator, Nucl. Instrum. Methods Phys. Res. Sect. B 406 (2017) 239–243. [4] < http://www.cst.com > . [5] < http://pulsar.nl/gpt > . [6] K.R. Crandall, T.P. Wangler, Workshop on linear accelerator and beam optics codes, La Jolla, CA, USA, 1988, AIP Conference Proceedings No. 177, (1988). [7] S. Ikeda, T. Hosokai, N. Hayashizaki, Development of a laser ion source for a fourbeam interdigital-H type radio frequency quadrupole linac, AIP Conference Proceedings, AIP Publishing, 2018, p. 030006.
142
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of four-beam IH-RFQ linear accelerator a,⁎
b
c
Shota Ikeda , Masahiro Okamura , Noriyosu Hayashizaki
T
a
Riken, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan Tokyo Tech World Research Hub Initiative (WRHI), Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan c Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Multi-beam acceleration IH-RFQ linac Heavy ion acceleration
It is difficult to accelerate a high intensity heavy ion beam because the upper limit of the beam current in an RFQ (Radio Frequency Quadrupole) linac depends on the applied voltage at the RFQ electrode and the strength of the space charge effect. A multi-beam RFQ linac, with multiple beam-channels accelerating several beams in parallel in one cavity, and utilizing multiple beams to decrease the space charge effect can be used to achieve a higher beam current. In order to demonstrate that a four-beam IH-RFQ (Interdigital H-type RFQ) linear accelerator is suitable for high-intensity, heavy ion beam acceleration, we have developed a four-beam prototype. A four-beam IH-RFQ cavity was fabricated according to design parameters obtained from our previous detailed simulations. Low-power RF property measurements of the fabricated four-beam IH-RFQ cavity were carried out with a network analyzer. After the low-power RF property measurements, we performed a highpower test and an acceleration test.
1. Introduction Radio frequency quadrupole (RFQ) linear accelerators are suitable for low energy, high-intensity ion beam acceleration and are typically used as injectors in hadron accelerator facilities and accelerator driven compact neutron sources. However, because of the space charge effect and an upper limit of the electric field strength, acceleration of high intensity heavy ion beam over 100 mA by using an RFQ linear accelerator is difficult. For higher intensity beam acceleration, the multi-beam acceleration method utilizes multiple beams to decrease the space charge effect, and the accelerated multiple beams are integrated using a beam funneling system. As a proof of principle, a four-rod IH-RFQ (Interdigital H-type RFQ) linear accelerator were demonstrated in reference [1], with possible extension to multi-beam acceleration. In particular, a two beam IH-RFQ linear accelerator with a two-beam LASER ion source with direct plasma injection scheme were developed at the Tokyo Institute of Technology, and the system accelerated 108 mA (2 × 54 mA/channel) carbon ions from 5 to 60 keV/u [2]. An IH-RFQ structure has some advantage for low energy heavy ion acceleration (high shunt impedance for low energy beam acceleration, small cavity diameter at low frequency, strong RF coupling). However, since the relationship between the RF properties and beam channel layout are not clear, a four-beam type RFQ linear ⁎
accelerator has not been demonstrated yet. In this study, we have designed and developed a prototype of a four beam IH-RFQ linear accelerator to establish its design scheme and verify the multi-beam acceleration properties. In this paper, a prototype was manufactured based on the design results [3], and we performed the first measurement of RF properties and four-beam acceleration. The layout of a four beam IH-RFQ linac, which consists of stem electrodes and 4 sets of RFQ electrodes installed alternately on upper and lower ridge electrodes, is shown in Fig. 1. The RF electromagnetic field is excited in the TE111 mode, and quadrupole fields are generated at each RFQ electrode. As a part of the development of the four-beam IH-RFQ linear accelerator, we have evaluated the relationship between the beam channel layout and the quadrupole distributions by using the threedimensional electromagnetic simulation software CST MICROWAVE STUDIO (CST MWS) [4] and beam tracking simulation software General Particle Tracer (GPT) [5]. Based on the simulation result, we developed the stem electrode design, in which the quadrupole distribution is the most symmetric and the difference between the maximum and minimum quadrupole strength is only 5.4%. As the simulation result, we have obtained the design of the prototype four-beam IH-RFQ linear accelerator which produced a total output beam current of 159.2 mA for an injection of 60 mA/channel with an inner cavity length of 650 mm and total power loss of 92.5 kW
Corresponding author. E-mail address: [email protected] (S. Ikeda).
https://doi.org/10.1016/j.nimb.2019.11.008 Received 9 June 2019; Received in revised form 6 November 2019; Accepted 6 November 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 462 (2020) 139–142
S. Ikeda, et al.
Fig. 1. Layout of the four-beam IH-RFQ linac. Table 1 Design parameters of the four-beam IH-RFQ linac. Charge to mass ratio (q/A) Input beam current (mA/channel) Input beam energy (keV/u) Single beam current (mA) Total output beam current (mA) Output beam current in Ch1 (mA) Output beam current in Ch2 (mA) Output beam current in Ch3 (mA) Output beam current in Ch4 (mA) Output beam energy (keV/u) Output beam power (kW) Synchronous phase (°) Focusing strength Operation frequency (MHz) Unloaded quality factor (Q0) Inter rod voltage (kV) Kilpatrick factor Wall loss (kW, normalized by interrod voltage) Radius of side shell (mm) Cavity inner length (mm) Rod length (mm) Average beam aperture radius (mm) Minimum beam aperture radius (mm) Maximum modulation index Stem number
≧1/6 60 3.6 40.2 159.2 38.9 40.6 40.5 39.2 41.6 36.7 −90 to −30 11.385 48 4254 92.2 1.8 55.8 150 650 600 6.43 3.04 3.04 8
(a) Stem electrodes, RFQ electrodes and the center plate
at 48 MHz [5]. The design parameters of the four-beam IH-RFQ linear accelerator are shown in Table 1, where the single beam current (mA) means the result of the PARMTEQM [6].
(b) Side shell with copper plating
2. Fabrication and RF measurement of a four-beam IH-RFQ cavity
Fig. 2. Fabricated four-beam IH-RFQ cavity.
2.1. Fabrication of a cavity
Before the copper plating, the resonance frequency was tuned to 47.87 MHz by decreasing the flame thickness of the side shells in the fabrication (Fig. 3). Then, the four-beam IH-RFQ cavity with copper plating and RF contacts exhibited a resonance frequency of 47.95 MHz, which is 0.08 MHz higher than that before the copper plating, and an unloaded Q value of 3687, which is about 88% of the design value. The RFQ electric field was measured using the perturbation method. A Teflon perturbator (12 mm × 12 mm × 15 mm) was set in a gap between the RFQ electrodes of each channel along the beam axis, and the frequency shift from the resonance frequency of the four-beam IH-RFQ cavity without the perturbation was measured. The square of the frequency shift is proportional to the electric field strength at the position of the perturbation. Fig. 4 presents the measured RFQ electric field strength distribution of each channel. The horizontal axis is the beam axis, and the vertical axis is the electric field strength, which is normalized with the maximum value of the electric field intensity in each beam channel as
We fabricated a four-beam IH-RFQ cavity based on the designed cavity parameters (Fig. 2). The side shell (aluminum 5052) and the center-plate (AISI 304) were plated with 50 µm copper. RF contacts were attached between the center-plate and the side shells. The left side shell had a grid vacuum port and two pickup ports, and the right one had a coupler port and two pickup ports. Copper brocks (OFC) were machined to the shape of the RFQ electrodes and stem electrodes by using a computer numerical control (CNC) milling machine. After the milling of the RFQ electrodes and the stem electrodes, they were aligned using spacers made of aluminum and fixed on the center-plate using bolts. 2.2. Low power RF test We have carried out low-power RF property measurements of the fabricated four-beam IH-RFQ cavity with a network-analyzer (Rohde & Schwarz FSG Spectrum Analyzer). 140
Nuclear Inst. and Methods in Physics Research B 462 (2020) 139–142
S. Ikeda, et al.
Fig. 5. Layout of the accelerator system.
Fig. 3. Flame thickness (Dotted area: the flame of the side shells).
100%. The tendency of the electric field distribution of each channel was similar to the simulation result. In addition, the maximum electric field intensity difference between the beam channels was 7.1%. 3. Four-beam acceleration test 3.1. Acceleration test set-up The layout of a test bench for the beam acceleration test of the fourbeam RFQ linac is shown in Fig. 5, which is constructed by combining the cavity with a four-beam laser ion-source and an RF amplifier. The RF amplifier consist of triode type amplifiers (Preamplifier: 3CX10000U7) and (Main amplifier: 4CW1000000E). The four-beam laser ion source with direct plasma injection scheme was developed and the time of flight spectra for each ion charge-state was measured using the test bench [7]. From the experimental result, we obtained a current of 36.1 mA/channel of C2+ ions with a focus lens
Fig. 6. Pulse structure of carbon beam at 330 mm from a target.
having a target distance of 400 mm and a pinhole diameter of 11 mm, using a laser beam with an energy of 175 mJ/channel (Fig. 6). In order to inject the RF power from an amplifier to the four beam IH-RFQ cavity, we used a coaxial waveguide (WX-203D) and a loopcoupler, which was connected to the cavity through magnetic coupling. Pulse waveforms in the high-power RF test are shown in Fig. 7 with
Fig. 4. Distribution of the electric field strength. 141
Nuclear Inst. and Methods in Physics Research B 462 (2020) 139–142
S. Ikeda, et al.
Fig. 9. Output signal of each beam channel (Input RF power: 84 kW, Extraction voltage: 21.6 kV).
Fig. 7. RF signal from an amplifier (Input RF power: 100 kW).
consider the reasons of the total output beam current less than the simulation value are the input beam current of C2+ less than 60 mA/Ch and the low beam matching ratio between the ion source and the fourbeam IH-RFQ cavity. The pulse shapes in each channel are shown in Fig. 9. Because of the difference of ion spectra in each of the beam pulses, the beam pulse shapes of each channel were different. In particular, at beam channel 1, the pulse peak arrives at the Faraday cup faster than in the other channels. This could be due to higher charge spectra in the injected ion beam than in the other channels. 4. Summary and future plan A prototype of the four-beam IH-RFQ linac with a Q value of 3687 at 47.95 MHz was fabricated. The electric field strength measurements indicated that the difference between the maximum and minimum electric field strengths was 7.1%. In terms of beam acceleration, the four-beam IH-RFQ linac accelerated a carbon ion beam with a current of 32 mA (8 mA/channel × 4). In a future study, the output beam properties, such as ion spectra, and beam energy of each channel, will be measured in more detail.
Fig. 8. Peak output current on each channel as a function of the RF power.
Pick-up: the pick-up voltage signal from a monitoring antenna, Pf: the voltage signal of the incident wave to the cavity, Pr: the voltage signal of the reflection wave from the cavity, and PG: the output signal from a pulse generator, respectively. The test conditions were a RF pulse repetition frequency of 0.2 Hz and a pulse width of 200 µs. Ablation plasma drifted from a graphite target to the RFQ electrode, and ion beam was injected with an extraction voltage of 21.6 kV. A Faraday cup, which was made of copper and consisted of a cylindrical cup with a diameter of 26 mm and a deep groove to suppress the secondary electron generation as much as possible, was located 95 mm downstream from the output side of the RFQ electrode. The Faraday cup picked up the RF excited in the four-beam IH-RFQ cavity as noise. In order to suppress the noise to several mV, we set metallic meshes between the Faraday cup and the output side at the end face of the four-beam IHRFQ cavity.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Funding: This work was supported by JSPS KAKENHI Grant Number JP26246042.
3.2. Beam commissioning
References
The peak output current of each channel as a function of the RF power is shown in Fig. 8. The peak current of each beam channel does not increase linearly with increasing input RF power. We speculate that it is because the charge of the dominant carbon ion in the accelerating beam changes according to the high frequency input power. In order to verify the relation between the RF power and the dominant carbon ion, we need to carry out ion spectra measurement with a bending magnet and a faraday cup. At an input RF power of 84 kW, the four beam IH RFQ linear accelerator accelerated about 32 mA (4 × 8 mA/channel), and the variation of the peak current between the channels was about 0.4 mA. We
[1] U. Ratzinger, et al., The GSI 36 MHz high-current IH-type RFQ and HIIF-relevant extensions, Nucl. Instrum. Methods Phys. Res. Sect. A 415 (1998) 281–286. [2] T. Ishibashi, N. Hayashizaki, T. Hattori, Two-beam interdigital-H-type radio frequency quadrupole linac with direct plasma injection for high intensity heavy ion acceleration, Phys. Rev. Spec. Top.-Accelerators Beams 14 (2011) 060101. [3] S. Ikeda, A. Murata, N. Hayashizaki, Design of four-beam IH-RFQ linear accelerator, Nucl. Instrum. Methods Phys. Res. Sect. B 406 (2017) 239–243. [4] < http://www.cst.com > . [5] < http://pulsar.nl/gpt > . [6] K.R. Crandall, T.P. Wangler, Workshop on linear accelerator and beam optics codes, La Jolla, CA, USA, 1988, AIP Conference Proceedings No. 177, (1988). [7] S. Ikeda, T. Hosokai, N. Hayashizaki, Development of a laser ion source for a fourbeam interdigital-H type radio frequency quadrupole linac, AIP Conference Proceedings, AIP Publishing, 2018, p. 030006.
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