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.

Two new eddy-current-type septum magnets were installed at the fast extraction section of J-PARC main ring in April 2022. Eddy septum magnets (EDDYs) are energized pulse currents; thus, it is necessary to consider the possibility of misfires. Based on simulation results, if one of the EDDY misfires, the extraction beam will be irradiated to the ducts of the superconducting magnets (SCs) on the beamline to the neutrino facility. If this occurs, then the SC can be quenched owing to heating. Therefore, we built a safety device to prevent beam failure by monitoring the output pulse currents of the EDDYs. If a pulse-current anomaly is detected, the beam is kicked to the abort line using kicker magnets. This process must be completed within 390 µs, known as the pulse rise time period. The parameters used to determine the pulse-current waveform anomalies were determined from the beam optics simulation. This pulse-monitoring device has an added functionality to stop and protect the power supplies of the EDDYs when the pulse current waveform is abnormal. We consider that by 2023, it will be remotely controlled by the accelerator control system.