The reduction of the leakage field of the injection septum magnet in main ring of J-PARC

Abstract

A new injection septum magnet 1 (Inj-SM1) was installed in the Main Ring (MR) of J-PARC in 2016 as an upgrade. We confirmed that the leakage field upstream from the circulating duct of Inj-SM1 was sufficiently smaller than that for the previous Inj-SM1. Nevertheless, we attempted to further reduce the leakage field by using a new magnetic shield to achieve a higher proton beam intensity. In 2017, the first version of the new magnetic shield was produced and installed. The strength of the leakage field with the shield was ≈30% of that without the shield; however, the magnetic field of a quadrupole magnet (QM) upstream of Inj-SM1 was reduced to below the permissible limit, ≈0.1% of the total field integral of the QM, from the point of view of beam optics. Consequently, the first version failed. A second shield, for which the defects of the first version were fixed, was produced in 2018. The strength of the leakage field of Inj-SM1 with the second shield was reduced to ≈30% of that without the shield, while that of the QM was maintained. Thus, we decided to use the second shield for beam operation starting in November 2019. We confirmed that the impact of the leakage field on the circulating beam was 1/20 of that of the previous Inj-SM1.