The main ring synchrotron (MR) of the Japan Proton Accelerator Research Complex (J-PARC) has provided high-intensity proton beams to the T2K long-baseline neutrino experiment, which requires high statistics to confirm the existence of CP violation. We plan to increase the beam power from 0.5 MW in 2021 operation to 1.3 MW by 2028 in the fast extraction mode of the MR. This upgrade supports higher statistics for T2K and the Hyper-Kamiokande long-baseline project, which will start from 2027. The scheme of the upgrade is to quicken the repetition period by a factor of two from 2.5 s in 2021 operation, and to increase the number of protons per pulse 30% more. This scheme requires hardware upgrades on the power supplies of the main magnets, high gradient RF system, collimator system, injection and fast extraction systems, and beam monitors. The upgrade of the MR is on schedule. The hardware upgrade for high-repetition operation was completed by 2022. The remaining upgrades will be accomplished in following several years to increase the number of protons per pulse. The improvement of the beam dynamics in the MR is also necessary to manage higher space charge effects due to increase of the beam intensity, and to localize beam losses at the collimator section in the MR more efficiently. This presentation reports the first results of the MR beam operation in the high repetition rate and the strategies to 1.3 MW operation based on beam study results.

In the J-PARC Main Ring, a project to upgrade the beam power to 1.3 MW is currently underway. The most important issues in realizing such a high-power beam operation are controlling and minimizing beam loss, which are essential for sustainable beam operation allowing hands-on maintenance. In this paper, we report on our recent efforts to understand the mechanism of beam loss and to reduce it.

As part of the goal of increasing the beam power of the Main Ring for Fast eXtraction (FX) in J-PARC to 750 kW, the two low-field septa and three high-field septa for FX were installed into MR in 2022. The most significant goals regarding the magnets are achieving an extremely low leakage field in the circulating line. To reduce the leakage field in the circulating line, the new pure iron duct-type magnetic shields were produced for all the septa in 2021, and mounted in the circulating line in 2022. We verified that the leakage field in the circulating line of a low-field septum and high-field septa were greatly reduced. We also confirmed that the impact of the leakage field of all of the septa for FX on the 3-GeV circulating beam was below 1/10 of that of the previous septa for FX in beam test in July 2022. We also measured the leakage field in the circulating line of the new high field septum magnets. We verified that the field integral was about 1/10 lower than previous septa. The quadrupole component was about 1/100 lower than previous septa. Consequently, the leakage field of high field septa could be reduced extremely.

J-PARC MR is a high intensity synchrotron that accelerates protons from 3 GeV to 30 GeV. In MR, beam study for 1.3 MW upgrade plan is now in progress. The upgrade is done by shortening the repetition period and increasing the number of protons, and it is crucial to understand their effects on beam motion. Especially, the betatron function is one of the most important parameters that determines the beam motion. In MR, the betatron function has been measured by using turn-by-turn signal of the beam position monitor. Betatron function has been adjusted to match with model within 3% accuracy in relative error in low energy period. However, in evaluating the effects of space charge forces and eddy currents on beam optics whose impact will be largen by the upgrade, the accuracy of betatron function measurement during the injection and acceleration period will be even more important. In this study, we have attempt to match betatron function to model within 1% accuracy in relative error both in injection and acceleration period which has never been achieved in MR, by performing beta function measurement using COD response from the steering magnets in MR.