The RCS at J-PARC is a kicker-impedance-dominant machine, which exceeds the impedance budget from a classical viewpoint. However, we have achieved a 1-MW beam without any transverse feedback by fully utilizing the indirect space charge effect to suppress beam instabilities. Although the indirect space charge effect is beneficial, the beam instability can still occur in a high-intensity beam with a smaller transverse emittance. To address this, we installed diode stacks and resistors at the ends of the four kicker power cables and have successfully conducted routine operations. This approach theoretically opens the door to achieving high-quality, higher-intensity beams, including a 2-MW beam, as no transverse feedback is required.

In the J-PARC 3-GeV rapid cycling synchrotron, second-order random resonances are excited by shifting the operating point to a higher tune side. To mitigate resonance-induced beam loss and further enhance tunability of the operating point, we studied the compensation of second-order resonances. By controlling the timing when the beam approaches resonance through adjusting the momentum offset and chromaticity-induced tune shift, the optimal resonance compensation is explored, considering the time dependence of lattice imperfections that drive the resonances. The analysis of resonance driving terms and numerical simulation was employed to deepen the understanding of the lattice imperfections. In this article, details of the method and experimental results will be reported.

At the J-PARC 3 GeV Rapid Cycling Synchrotron (RCS), a 400 MeV negative hydrogen (H-) beam from the linac is converted to proton (H+) using a charge-exchange foil and then accelerated to 3 GeV. As the beam power has increased toward the design value of 1 MW, foil de-formation and breakage of the SiC support fibers caused by beam irradiation have become major issues, leading to beam dump temperature rises that obstruct stable opera-tion. To improve the reliability of the charge-exchange foil under high power conditions, a stepwise develop-ment has been carried out. First, a pure carbon foil was developed to suppress irradiation induced deformation. Beam operation showed reduced deformation compared with previously used hybrid boron-mixed carbon (HBC) foil and graphene thin film (GTF), and beam dump tem-perature rises were not observed. Next, the arc deposition parameters were optimized by increasing the anode di-ameter, resulting in further reduction of deformation in beam operation. Finally, carbon nanotube (CNT) wires were adopted as support fibers instead of SiC fibers. The CNT-supported foil was successfully operated up to 940 kW-equivalent beam power for over 1 month without support failure and beam dump temperature rises. These results demonstrate improved durability of the charge-exchange foil and support stable RCS operation toward 1 MW beam power.

Stripper foils in high-intensity proton accelerators experience significant thermal loads during beam irradiation, causing deformation, degradation, and reduced lifetime. In the J-PARC Rapid Cycling Synchrotron (RCS), a carbon stripper foil is irradiated by a 400-MeV injecting H⁻ beam and the circulating proton beam, and simulations predict temperatures of up to about 1000 °C. Direct installation of an infrared (IR) camera near the foil is not feasible due to high radiation levels, so remote temperature monitoring is required. The RCS currently uses a long-distance optical system with sapphire lenses to monitor foil motion. Sapphire lenses transmit poorly in the infrared, making conventional IR thermography impractical. To overcome this, we propose a near-infrared (NIR) camera compatible with the existing optical path. To test feasibility, a test stand using an electron-beam irradiation device is being built to simulate localized foil heating. The preliminary design, expected performance, and current status of the test stand are presented. This study provides a foundation for in situ thermal diagnostics of stripper foils in the J-PARC RCS.