In the intensity-frontier proton synchrotron J-PARC Main Ring (MR), a power upgrade plan is underway to achieve 1.3 MW beam power for neutrino experiments. The upgrade has generally progressed as planned, with the beam power for the user operation reaching 830 kW as of May 2025. A major obstacle to upgrading the beam power of the MR is the radioactivation of accelerator components caused by beam loss. One significant contributor to beam loss is the betatron resonances, which are sensitive to the symmetry of optics. Precise optics corrections during beam tuning have therefore been playing a critical role in suppressing these resonances and reducing beam losses. This paper presents the strategies and recent results of optics measurements and corrections for the MR.

At the J-PARC Main Ring, a project is in progress to upgrade the beam power to 1.3 MW for the neutrino oscillation experiment. Control of the optics and beam orbit relative to the magnetic center of the accelerator components is crucial to suppress beam losses in intense beams. We need to reduce the determination error of the beta function from the current value of 2% to less than 1%. Furthermore, the closed-orbit offset from the magnetic center must be set within 10μm to weaken the effect of the resonances, while its precision is 30μm up to now. To satisfy such targets, ongoing upgrades of the data acquisition system for the BPMs will improve the accuracy to one-third than the current value. As a first step to requested fine measurements of optics parameters, we calculated how small an offset with respect to the magnetic center we can obtain via beam-based alignment with the new BPM circuits by simulation. We discussed the upgrade of the method for beam-based alignment to satisfy the requirements of the orbit correction to reduce the effect of the resonance in our system.

In the J-PARC Main Ring (MR), an upgrade project involving a total of 24 Trim-S units is in progress to further suppress beam loss caused by off-momentum particles. This effort targets key resonances, including the third-order resonance 3νx=64 and νx+2νy=64, and a fourth-order resonance 4νx=85, which are produced by the combination of vx=21 and 3vx=64. Four additional Trim-S units have been installed in the D3 power supply building, supplementing the original four units located in the D2 power supply building, resulting in a total of eight operational Trim-S units in J-PARC MR. To evaluate the feasibility of the full 24-unit upgrade, step-by-step experimental studies were carried out using first the individual sets of four Trim-S units in D2 and D3, and subsequently all eight units combined, for resonance correction. This paper reports the experimental procedures, analyzes the results, and outlines the plans for the next step based on the experimental findings.

Beam loss reduction is essential to achieve higher beam intensity in the main ring synchrotron (MR) of the Japan Proton Accelerator Research Complex (J-PARC). We have observed the effect of the third order non-structure resonances of 3νx=64 and νx+2νy=64. The resonance driving terms were measured with the current settings of the trim coils of the sextupole magnets. Both resonances have been corrected with four sets of the trim coil systems. Consequently, the beam loss was significantly reduced. Further correction of the resonances is planned to include the effect of off-momentum particles for the beam intensity upgrade. For that purpose, we have searched for sextupole error fields as causes of the resonances. An effective procedure for the search was devised using measurements of the nonlinear dispersion function. Because the sextupole field contributes to the first order nonlinear dispersion function, the distribution of the sextupole fields can be derived from the distribution of the nonlinear dispersion function measured using the beam position monitors.