Optimization of the Pole Shape for Corrector Quadrupole Magnet in the J-PARC 3-GeV RCS

The corrector quadrupole magnet of J-PARC 3-GeV rapid cycling synchrotron (RCS) was developed as a pulse magnet with two functions. One is to correct the effect of magnetic field caused by the edge focus of the injection bump magnet, and the other is to control the tune at high speed during the beginning acceleration process of the beam (16 ms). At the beginning, the magnet is used for the first function. As RCS operates at 1 MW, beam loss needs to be as small as possible. However, beam loss is seen at the injection straight section of RCS caused by the edge focus of the bump magnet. In order to lessen this effect, the first 1 ms of beam injection has to be corrected using a quadrupole magnet. Field measurement was implemented in order to confirm performance of the fabricated magnets. However, the magnet did not show enough performance. We investigated the cause and found out that there was improper input of boundary condition during the magnetic field calculation. Then, a study to restore the magnetic field performance was carried out, and as a result, a pole piece was installed at the minimum gap of the magnetic pole, and the pole shape was optimized. Satisfactory performance of the correction quadrupole magnet with the pole piece was confirmed. In this paper, the authors will report optimization of a pole shape of the correction quadrupole magnets with pole pieces and a result of the field measurement carried out afterward.