Design of Thick Septa Magnets Based on 3D Field Calculation for the 3 GeV Rapid Cycling Synchrotron of J-PARC

Septum magnets are used in the injection and extraction section of the 3 GeV rapid cycling synchrotron (RCS) of J-PARC (Japan Proton Accelerator Research Complex). The RCS needs seven septum magnets, two for the injection line, two for the beam dump line and three for the extraction line, respectively. These septa require a large aperture for low-loss operation with high intensity beams, and high durability for minimizing maintenance after high activation. For insuring mechanical stability the septum magnets have been designed to work DC. The septa magnets work outside a vacuum, thus eliminating any trouble due to the leakage of cooling water into the vacuum system. We have been designing them by using the three dimensional magnet static field calculation code, TOSCA. This paper describes the flatness of the magnetic field in the gap and reduction of the leakage field of the septum magnets in the RCS