The China Spallation Neutron Source (CSNS) accelerator consists of an 80 MeV H^{-} Linac and a 1.6-GeV rapid cycling synchrotron (RCS). The painting scheme is one of the most important factors for the control of low beam loss at high beam power. In the painting injection of the CSNS/RCS, position scanning is used in both horizontal and vertical planes, and the anticorrelated painting is the design scheme. By using the anticorrelated painting, the beam power has successfully reached 50 kW. However, some problems have been found in the higher power beam commissioning, for instance, too large beam size after painting, nonuniform beam distribution, large transverse coupling effect, and so on, resulting in additional beam loss and making it difficult to satisfy the requirements of a high-power user mode. In order to solve these problems, flexibility in the CSNS design has been exploited to implement the correlated painting by using the rising current curve of the pulse power supply. The effectiveness of the new method has been verified in the simulation and beam commissioning, which can well control the full beam emittance and improve the beam distribution. By using the new method, the beam power on the target has successfully risen from 50 kW to the design value of 100 kW. Under the mechanical structure of the anticorrelated painting scheme where the position scanning is used in both horizontal and vertical planes, the correlated painting has been successfully achieved by using the rising current curve.

In the Japan Proton Accelerator Research Complex, the purpose of the 3 GeV rapid cycling synchrotron (RCS) is to accelerate a 1 MW, high-intensity proton beam. To achieve beam operation at a repetition rate of 25 Hz at high intensities, the RCS was elaborately designed. After starting the RCS operation, we carefully verified the validity of its design and made certain improvements to establish a reliable operation at higher power as possible. Consequently, we demonstrated beam operation at a high power, namely, 1 MW. We then summarized the design, actual performance, and improvements of the RCS to achieve a 1 MW beam.

We have established and also implemented a nondestructive online monitoring system for measuring the stripper foil degradation, such as foil thinning and pinhole outbreak, for the first time during beam operation in a high-intensity proton accelerator. We aimed to achieve a realistic and longer lifetime of a stripper foil by ensuring proper uses and determining its end of usefulness without any failure. A stripper foil is used for negative hydrogen (H-) stripping to proton (p) for multiturn charge-exchange injection in high-intensity proton accelerators. A longer foil lifetime is expected, while foil failure during operation should be avoided, as it reduces the accelerator availability and also has serious issues for regular accelerator maintenance. A proper use of the foil should also be ensured to minimize the replacement of the foil magazine, as it involves unhealthy radiation exposure to the workers. We have measured the partially stripped H-0 and unstripped H- charge fractions of the injection H- beam out of the stripper foil to understand details of foil degradation, such as foil thinning and pinhole formation due to high-intensity beam irradiation, which are believed to be foil breaking signals. We used two independent beam monitoring devices and precisely measured both H-0 and unstripped H- charge fractions by each monitor. As a result, we obtained a detail of foil degradation during operation to determine a realistic end of its usefulness by successfully achieving a record of nearly 2 yr of operation with a single foil without any failures. The detailed measurement result of the foil degradation also gives strong feedback for producing stronger and durable stripper foils. The present research was done by using simple and ordinary beam diagnostic devices including a nondestructive one, which can be easily applied to overcome the stripper foil issues in any similar existing and next-generation further higher-intensity accelerators.

The present four-terminal kicker at the Rapid Cycling Synchrotron (RCS) at the Japan Proton Accelerator Research Complex (J-PARC) has the power-saving benefit due to the doubled excitation currents by shorting two-terminals. On the other hand, beam instabilities are excited by the kicker impedances. In this report, we describe a scheme to reduce the kicker impedances using diodes (nonlinear devices) with resistors, while retaining the benefit of the doubled kicker excitation currents.

The 3-GeV rapid-cycling synchrotron at the Japan Proton Accelerator Research Complex has achieved the high-power beam operation equivalent to 1MW. As a next step, a study of an upgrade is in progress to mitigate the dose exposure of the maintenanceworkers in a high residual dose environment and realize the further high intensity beam power. Regarding the upgrade plan's provisions for radiation protection, a new injection scheme has been proposed to make space available for radiation shielding. The total length of the two shift bump magnets of the four magnets is reduced for this purpose, while the other two magnets in the injection straight section remain unchanged. As a result, the two types of pulse magnets are connected separately in series and are excited by two independent power supplies, which are made possible by splitting the presently installed power supply. A structural analysis of the new shift bump magnet is in progress, including simulations of the effects of eddy currents and the coil temperatures by OPERA-3D. This paper describes preliminary results of this analysis and the outlines the modified power supply design.

The RCS at J-PARC is a kicker-impedance dominant machine, which violates the impedance budget from a classical viewpoint. Nevertheless, we have recently succeeded to accelerate a 1-MW equivalent beam by making maximum use of the space charge effect on the beam instabilities. In this report, we explain the manipulation to suppress the beam instability, at first. Then, we discuss some issues to suppress the beam instabilities for beams with much smaller transverse emittance, as well as the present status of our efforts to reduce the kicker impedance toward the realization of the higher beam power at the RCS.

... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Rapid Cycling Synchrotron (RCS), whose beam energy ranges from 400 MeV to 3 GeV and which is located in the Japan Proton Accelerator Research Complex, is a kicker-impedancedominated machine, which violates the impedance budget from a classical viewpoint. Contrary to conventional understanding, we have succeeded in accelerating a 1 MW equivalent beam. The machine has some interesting features: e.g., the beam tends to be unstable for the smaller transverse beam size and the beam is stabilized by increasing the peak current. Space charge effects play an important role in the beam instability at the RCS. In this study, a new theory has been developed to calculate the beam growth rate with the head-tail and coupled-bunch modes (m,μ) while taking space charge effects into account. The theory sufficiently explains the distinctive features of the beam instabilities at the RCS. ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

In the 3-GeV RCS (Rapid Cycling Synchrotron) of JPARC (Japan Proton Accelerator Research Complex), experimental studies are under preparation for POP (proofof-principle) demonstration of 400 MeV H− stripping to protons by using only lasers. The aim is to established an alternative H− stripping injection method without using any stripper foils for that purpose. This is because realistic issues involved with conventional H− stripping by using foil, such as short lifetime and extremely high residual radiation at the injection area due to the interactions of foil with the beam are already big limitations in all existing high intensity accelerators, and also serious concerns to aim for multi-MW beam power. The present method consists of 3 steps for H− stripping to protons. The H− is first neutralized to H0 by using a Nd:YAG laser of 1064 nm. The ground state (n=1) H0 is excited to two level higher states (n=3) producing H0∗ by using an Excimer laser of 193 nm in the 2nd step. The H0∗ is then stripped to proton in the 3rd step by using another Nd:YAG laser of 1064 nm. The characteristic feature of this method is that no magnetic field is used, which requires extremely high magnetic field at lower H− energy. The POP experimental studies will be conducted at the L-3BT (Linac to 3-GeV Beam Transport) of J-PARC. The present status and plan of POP experiment for 400 MeV H− stripping to protons are presented in this paper.

The untuned magnetic alloy (MA) loaded cavity will be adopted in the recent proposed projects based on compact proton and heavy-ion synchrotron in China. The MA loaded cavity has been operated successfully in many accelerators, but there is no construction and operation experience in China. The small-size MA cores with outer diameter >100 mm have been supplied by Chinese firms, and the properties of cores are tested and show good consistency with Hitachi material. Based on the MA cores testing results, the schematic design of a cavity is conducted, which could obtain 1 kV gap voltage with less than 1.5 kW power dissipation in the frequency range of 0.5~7 MHz. The analytic and simulation results were compared on the resonant frequency, shunt impedance and Q value.

The China Spallation Neutron Source/Rapid Cycling Synchrotron（CSNS/RCS） accelerates a highintensity proton beam from 80 Me V to 1.6 Ge V. Since the beam current and beam power is high, the beam loading is a severe problem for the stability of the circulating beam in the RCS. To study the beam loading effect in the CSNS/RCS theoretically, the RLC circuit model of the rf cavity, the method of Fast Fourier Transform and the method of Laplace transform have been employed to obtain the impedance of the rf system, the beam spectrum and the beam-induced voltage, respectively. Based on these physical models, the beam dynamics equations have been revised and a beam loading model has been constructed in the simulation code ORIENT. By using the code, the beam loading effect on the rf system of the CSNS/RCS has been investigated. Some simulation results have been obtained and conclusions have been drawn.

In the 3-GeV rapid cycling synchrotron of Japan Proton Accelerator Research Complex, we have clearly measured a continuous degradation of a stripper foil during more than 6 months operation with a beam power of 300 kW. A hybrid-type boron doped carbon stripper foil of 200 μg/cm2 was used for 181 MeV H− stripping injection. In order to know an absolute change of the foil thickness as well as information on the pinhole formation, we precisely measured an absolute change of the partially-stripped H0 and the un-stripped H− waste beams. Two absolutely independent monitor systems were used and the results were found to be very consistent with each other. The foil thickness at the injected beam spot was measured to be gradually thickening and it was more than 10 % thicker at the end compared to the initial thickness, where the measurement accuracy was obtained to be better than 2 %. The missing un-stripped H− were also measured to be gradually increasing due to foil deformation and curling but there was however no clear indication of foil thinning or pinhole formation.

A ceramic chamber is utilized at the 3 GeV Rapid Cycling Synchrotron (RCS) in Japan Proton Accelerator Research Complex (J-PARC). To conduct the image currents on chambers, the outside of the chamber is surrounded by copper stripes equipped with capacitors. When magnetic fields are excited outside the chamber, the currents on the stripes generate rippling fields in the chamber. A three-dimensional theory is developed to cope with the field oscillations. In order to avoid adverse effects on the beam from the field oscillations, we found that the positions of the stripes with the capacitors should be as identical as possible among the different chambers.

The new horizontal shift bump (SB) power supply for beam injection system of the J-PARC (Japan Proton Accelerator Research Complex) 3-GeV RCS (Rapid-Cycling Synchrotron) has been developed and manufactured. The injection energy was increased from 181 MeV to 400 MeV, and the power capacity of the new power supply was doubled. The power supply newly adopted a capacitor commutation method to form the trapezoid waveform pattern (bump waveform) by the IGBT (Insulated Gate Bipolar Transistor) switch. This paper reports characteristic, the problems and the user operation status about the new shift bump power supply.

Both longitudinal and transverse coupling impedance for some critical components need to be measured for accelerator design. The twin wires method is widely used to measure longitudinal and transverse impedance on the bench. A mode error is induced when the twin wires method is used with a two-port network analyzer. Here, the mode error is analyzed theoretically and an example analysis is given. Moreover, the mode error in the measurement is a few percent when a hybrid with no less than 25 dB isolation and a splitter with no less than 20 dB magnitude error are used.

In the 3 GeV rapid cycling synchrotron (RCS) of the Japan proton accelerator research complex, we adopted the multi-turn charge exchange injection scheme using the stripper foils. After the fine beam tuning in the RCS, most of the beam losses occur at the foil only. However, the high residual doses were detected around the stripper foil, which cannot be explained by the direct scattering of the injected H− and circulating proton beams with the stripper foil. From the measurements and simulations, it is identified that secondary particles produced in the nuclear reactions with the foil had caused the high residual activity around there. The radio-activation of the foil itself and the high residual dose around the foil are intrinsic problems for all high power beam accelerators with stripping foil. A safe and efficient maintenance in the high radiation and narrow space in these accelerators is important. The foil maintenance under such an environment is required to keep staff radiation exposure as low as possible to reduce the risk of radioactive foil breakup or dispersion and to retrieve the foil without breaking for its analysis. We achieved a safe and efficient maintenance method to retrieve the radioactivated stripper foils.

It is considered that muon-electron conversion would be one of the most obvious evidence of the new physics beyond the standard model.An experimental search for muon-electron conversion in the nuclear field, DeeMe, is proposed at J-PARC Materials and Life Science Experimental Facility (MLF).DeeMe experiment will be carried out at a brand-new beamline (H-Line) which will be constructed at J-PARC MLF Muon Science Establishment (MUSE).The single event sensitivity achieved by the experiment with the current graphite production target of MUSE is estimated to be 1.2 × 10 -13 .This is smaller than the current upper limit but it is desirable to make the experiment more sensitive.In order to improve the sensitivity, it is planned to replace the graphite target with a silicon carbide (SiC) target.Thanks to the larger pion production rate and the larger muon capture rate of silicon nucleus, the number of muonic atoms formed in a SiC target is expected to be totally 6 times as large as in a graphite target and then the single event sensitivity for SiC target is estimated to be 2.1 × 10 -14 , nearly two orders of magnitudes below current upper limit.The current status of the preparation for the use of a SiC target is reported.

The stripping cross-sections of 181MeV H− (negative hydrogen) ions by the carbon stripper foil are measured with good accuracy. The present experiment was carried out at the 3-GeV RCS (Rapid Cycling Synchrotron) of J-PARC (Japan Proton Accelerator Research Complex). The stripping cross-sections for different charge states, also known as electron loss cross-sections of H− ion, are denoted as σ−11, σ−10 and σ01, for both electrons stripping (H−→H+), one-electron stripping (H−→H0) and the 2nd-electron stripping (H0→H+) proceeding σ−10, respectively. We have established very unique and precise techniques for such measurements so as also to determine a foil stripping efficiency very accurately. The cross-sections σ−11, σ−10 and σ01 are obtained to be (0.002±0.001)×10−18cm2, (1.580±0.034)×10−18cm2 and (0.648±0.014)×10−18cm2, respectively. The presently given cross-sections are newly available experimental results for an incident H− energy below 200MeV and they are also shown to be consistent with recently proposed energy (1/β2) scaled cross-sections calculated from the previously measured data at 200 and 800MeV. The present results have a great importance not only at J-PARC for the upgraded H− beam energy of 400MeV but also for many new and upgrading similar accelerators, where H− beam energies in most cases are considered to be lower than 200MeV.