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Yamada, Ippei; Kojima, Kunihiro; Chimura, Motoki
Nuclear Instruments and Methods in Physics Research A, 1084, p.171261_1 - 171261_12, 2026/04
Times Cited Count：0 Percentile：0.00(Instruments & Instrumentation)no abstracts in English
Kojima, Kunihiro; Harada, Hiroyuki; Saha, P. K.
Proceedings of 22nd Annual Meeting of Particle Accelerator Society of Japan (Internet), p.289 - 293, 2026/03
Uncontrolled beam loss in the 3 GeV rapid-cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC) was investigated. A significant amount of uncontrolled beam loss was observed, especially at each arc section, despite the RCS being equipped with a collimation system designed to collect halo particles with one scatterer and five absorbers. Based on the time-dependence of the beam loss measured in the collimation system and arc sections, the particles lost at the arc sections are found to be likely scattered by the absorber rather than the scatterer, contrary to the design of the collimation system. By changing the configuration of the collimation system, it was determined that the absorber through which the majority of the lost particles passed was the second absorber. In addition, the trajectory of such a particle was identified by using numerical simulation assuming the lattice structure of the RCS.
Kojima, Kunihiro; Harada, Hiroyuki; Chimura, Motoki; Nagayama, Shota*; Saha, P. K.
Proceedings of 22nd Annual Meeting of Particle Accelerator Society of Japan (Internet), p.104 - 108, 2026/03
A detailed study on the random resonance was conducted to ensure sufficient tunability in the 3-GeV rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex (J-PARC). The progressive improvement in beam intensity, aiming for an output beam power of 1.5 MW, inevitably excites the effect of the structure resonance and requires the operating point to be shifted higher. Employing a low-intensity beam, the excitation of the sum linear resonance locating just above the current operating point is revealed. The linear sum resonance was confirmed to be compensated by making local bumps in the arc section by using the steering magnets. Based on the resonance driving term, the main source of the imperfection field is identified as the leakage field from extraction magnets.
Nagayama, Shota; Kinsho, Michikazu; Harada, Hiroyuki; Yamada, Ippei; Chimura, Motoki; Kojima, Kunihiro; Yamamoto, Kazami; Shimogawa, Tetsushi*; Sato, Atsushi*
Proceedings of 16th International Particle Accelerator Conference (IPAC25) (Internet), p.1025 - 1028, 2025/11
Slow extraction method of synchrotrons is used in nuclear and particle physics experiments, radiation therapy, and many other applications. In the slow extraction process, beam loss at the septum electrode, which separates the extracted beam from the circumferential side, induces activation and damage to the device. To solve this problem, we have devised an electric field measurement device named "Beam separation test device" to evaluate the two-dimensional electric field distribution of the nondestructive electrostatic septum that we are developing. The device consists of a prototype septum, horizontal and vertical wire scanners, and an electron gun installed on a movable stage fixed to a drive unit. This device measures the electric field by injecting an electron beam into the electric field and measuring the bending angle of the beam orbit. In developing this device, an additional optics system was designed to make a narrow electron beam to improve the measurement accuracy of the electric field distribution. In addition, we designed a beam dump that reduces secondary electron emission by utilizing energy loss when particles collide with materials. According to calculations of secondary electron emission, this beam dump can reduce secondary-emission into the chamber by up to 98%.
Kojima, Kunihiro; Harada, Hiroyuki; Okabe, Kota; Chimura, Motoki; Saha, P. K.
Proceedings of 16th International Particle Accelerator Conference (IPAC25) (Internet), p.2244 - 2247, 2025/11
In the 3-GeV rapid cycling synchrotron of the Japan Proton Accelerator Research Complex, the beam power ramp-up aiming to surpass the design of 1 MW enhances the space charge effect and pushes the beam toward the structure resonance. To mitigate the beam loss, the operating point is required to be apart from the structure resonance corresponding to the beam power ramp-up, although any changes in the operating point lead to an increase in the beam loss due to the approach to other resonances. We recently compensated the horizontal half-integer resonance located just above the current operating point to address this issue, and we are now ready to explore a higher tune region than that. We confirmed in a tune survey that the vertical half-integer and linear sum resonances are substantially excited. In addition, the linear sum resonance is found to be well compensated by making a vertical local bump in the arc section.
Saha, P. K.; Harada, Hiroyuki; Tamura, Fumihiko; Okabe, Kota; Yoshimoto, Masahiro; Shobuda, Yoshihiro; Okita, Hidefumi; Kojima, Kunihiro; Nakanoya, Takamitsu; Hatakeyama, Shuichiro; et al.
Physical Review Accelerators and Beams (Internet), 28(7), p.074201_1 - 074201_23, 2025/07
Times Cited Count：1 Percentile：59.26(Physics, Nuclear)Kojima, Kunihiro; Harada, Hiroyuki; Tamura, Fumihiko; Okita, Hidefumi; Chimura, Motoki; Saha, P. K.
Progress of Theoretical and Experimental Physics (Internet), 2025(1), p.013G01_1 - 013G01_19, 2025/01
Times Cited Count：0 Percentile：0.00(Physics, Multidisciplinary)A comprehensive study on the random resonances was conducted to mitigate beam losses and ensure sufficient tunability of the operating point for further beam power ramp-up in the 3-GeV rapid cycling synchrotron of the Japan Proton Accelerator Research Complex. Low-intensity beam studies revealed considerable excitation of the half-integer random resonance. This half-integer random resonance was successfully compensated using trim quadrupole magnets without exciting other higher-order resonances. By implementing a conventional theoretical procedure based on resonance driving terms, we identified the leakage field from extraction magnets as the primary source of the error field driving the random resonance. High-intensity beam studies confirmed that our resonance compensation approach substantially mitigated beam loss in higher-tune regions, making it highly effective in improving operating point tunability.
Kojima, Kunihiro; Harada, Hiroyuki; Chimura, Motoki; Saha, P. K.
Proceedings of 21st Annual Meeting of Particle Accelerator Society of Japan (Internet), p.118 - 122, 2024/10
Further beam power ramp-up going beyond the design output beam power of 1 MW is proceeding in the J-PARC RCS. In MW-class rings, the residual radioactivity accompanying the beam loss strictly limits the available beam power. While the current beam loss is mitigated to 0.1% level at the designed beam power, further beam loss mitigation prepared for the beam power ramp-up is required. The resonance crossing of the tune spread can cause unallowable beam loss. Since the beam power ramp-up definitely extends the space-charge-induced tune spread, the sufficiently wide stability region in the betatron tune map must be secured for low beam loss. We conducted low-intensity beam studies on the half-integer random resonances that likely have significant effects on the width of the stability region. Thanks to the simplification coming from the mitigation of the space-charge force, the successful compensation of the resonance is realized by using the trim quadrupole magnets. In the high-intensity beam studies, the resonance compensation was found to be beneficial in extending the stability region.
Nagayama, Shota; Harada, Hiroyuki; Shimogawa, Tetsushi*; Sato, Atsushi*; Yamada, Ippei; Chimura, Motoki; Kojima, Kunihiro; Yamamoto, Kazami; Kinsho, Michikazu
Proceedings of 20th Annual Meeting of Particle Accelerator Society of Japan (Internet), p.526 - 530, 2023/11
We have been developing "Non-destructive electrostatic septum" for a slow extraction. This septum has multiple electrodes placed around the region without the beam hitting and separate the beam by its electric field. To evaluate its electric field, we have built a prototype septum and a test machine, which consists of an electron gun and monitors. This test machine can measure the electric field indirectly by using a narrow electron beam. The experiment results of prototype septum is good agreement with the calculation one. However, this electric field distribution is not enough to separate the beam. A step function-like electric field distribution is ideal for the beam separation with minimal negative effect on the beam. We have studied to improve the electrode configuration to match the beam shape. In this paper, we present the result of the electric field measurements and the septum improvement. Additionally, we describe the future plan of this development.
Kojima, Kunihiro; Harada, Hiroyuki; Saha, P. K.
Proceedings of 20th Annual Meeting of Particle Accelerator Society of Japan (Internet), p.127 - 131, 2023/11
no abstracts in English
Kojima, Kunihiro; Harada, Hiroyuki; Chimura, Motoki; Saha, P. K.
no journal, ,
While the current beam loss in the RCS is successfully mitigated to 0.1% level, further reduction in the beam loss prepared for the ongoing beam power ramp-up is essential. Especially, securing sufficient dynamic aperture for the off-momentum particle is important to reduce beam loss since the RCS features a large momentum acceptance. For this purpose, we constructed a new 3rd-order resonance compensation method using the octupole field together with the sextupole field. This method allows one to achieve the 3rd-order resonance compensation regardless of the momentum deviation of the particle. Our simulation shows a substantial improvement in the dynamic aperture for the off-momentum particles with the proper addition of the octupole field. The details of the newly constructed method and simulation results will be presented in this meeting.
Chimura, Motoki; Yamada, Ippei; Kojima, Kunihiro
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no abstracts in English
Yamada, Ippei; Kojima, Kunihiro; Chimura, Motoki
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no abstracts in English
Moriya, Katsuhiro; Kojima, Kunihiro*
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no abstracts in English
Kojima, Kunihiro*; Okamoto, Hiromi*; Moriya, Katsuhiro
no journal, ,
no abstracts in English
Kojima, Kunihiro*; Okamoto, Hiromi*; Moriya, Katsuhiro
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Nagayama, Shota; Harada, Hiroyuki; Shimogawa, Tetsushi*; Sato, Atsushi*; Yamada, Ippei; Chimura, Motoki; Kojima, Kunihiro; Yamamoto, Kazami; Kinsho, Michikazu
no journal, ,
no abstracts in English
Kojima, Kunihiro; Harada, Hiroyuki; Saha, P. K.
no journal, ,
We have studied the compensation of random resonance for further beam power ramp-up and beam loss mitigation in the 3 GeV rapid cycling synchrotron (RCS). The particle loss will produce high levels of radioactivity in high-intensity synchrotrons such as the RCS and can limit the available beam power. While the current beam loss has been successfully mitigated to 0.1 % at the designed beam power of 1 MW, further beam loss mitigation and expansion of the stability region are required to realize the beam power of 1.5 MW. For this purpose, we have conducted a tune survey with a low current beam to identify the beam loss source. We reveal the enhancement of the 2nd-order random resonance (
) and demonstrate the compensation of the resonance by using the trim quadrupole magnets (QDT). In addition, the resonance compensation using the QDT is confirmed to be effective in reducing beam loss for the high current beam provided to the material and life science facility. In this meeting, the results of the study of random resonance compensation will be presented.
Nagayama, Shota; Harada, Hiroyuki; Shimogawa, Tetsushi*; Sato, Atsushi*; Yamada, Ippei; Chimura, Motoki; Kojima, Kunihiro; Yamamoto, Kazami; Kinsho, Michikazu
no journal, ,
no abstracts in English
Kojima, Kunihiro; Harada, Hiroyuki; Saha, P. K.
no journal, ,
In the J-PARC 3 GeV rapid cycling synchrotron (RCS), the 3rd-order resonance has been compensated by using the sextupole magnet. The resonance compensation has been confirmed well reduce the beam loss due to the 3rd-order resonance. On the other hand, the resonance compensation for the off-momentum particle is likely to be not perfect, since the focusing structure of the off-momentum particle is more or less different from that of the on-momentum particle. To achieve better resonance compensation for the off-momentum particle and further reduction in the resonance-induced beam loss, we constructed a new resonance compensation method using the octupole magnet. Our numerical simulation assuming the lattice structure of the RCS verifies that the new method effectively enlarges the physical aperture of the off-momentum particle.
Kojima, Kunihiro; Harada, Hiroyuki; Chimura, Motoki; Saha, P. K.
no journal, ,
We have realized a design output beam power of 1 MW of the 3-GeV rapid cycling synchrotron (RCS) in the Japan Proton Accelerator Research Complex (J-PARC), and now we are proceeding further beam power ramp-up aiming at an output beam power beyond the design. In MW-class high-power proton machines such as the RCS, the beam loss will produce high levels of radioactivity and limit the available beam power. While the current beam loss is successfully reduced to the order of 0.1% at the designed beam power, further beam loss mitigation prepared for the beam power ramp-up is required. For this purpose, we conducted a series of low-intensity beam tests to investigate the lattice imperfection of the RCS. It was confirmed that the half-integer random resonance was significantly excited and was able to be compensated by the proper addition of the quadrupole field using the trim quadrupole magnets. In addition, the effect of the resonance compensation was verified in the high-intensity beam tests.
