The new high field septum magnet for upgrading of fast extraction in main ring of J-PARC

Abstract

The upgrade of the beam power of J-PARC Main Ring to 750 kW by reducing the cycle from 2.48 s to 1.3 s is underway, and the upgrade of the four high-field (HF) septa will be completed by summer of 2023. The operation test of one HF septum, SM31, was conducted in 2020. First trial was 1 Hz operation test, and we confirmed the joule heating at the magnetic coil was lower than limit. Second was magnetic field measurement. We obtained a good linearity between the applied current and the gap field without saturation. The field integral of the gap field was measured to estimate the optimal current for beam extraction, and we found it is 3,400 A. We compared the gap field between in the neutrino line and that in the beam abort line, then we found the longitudinal distributions have a small discrepancy but it was acceptable. The large leakage field was observed around the end-fringes, therefore, in order to reduce the leakage field, we produced an additional duct shield for mounting in the circulating duct in 2022. We verified that the additional duct shield reduced the leakage field to ≈2% of unshielded leakage field. The new SM31 and other two HF septa have been installed in MR by end of May 2022. The beam operation with the new HF septa will be started in June 2022.