Design study of a thin-septum magnet for slow extraction system of J-PARC 50 GeV ring

A large-aperture thin-septum magnet is employed in the slow-extraction system of J-PARC 50GeV ring. This magnet is excited with currents as high as 5000A to produce a field of 0.114T across the high-aperture gap (55mm). Due to the high-current density, the power dissipation in the septum conductor creates a serious thermal problem. Since the septum is edge cooled by 2 brazed-cooling tubes, a high temperature rise in the middle of septum with respect to the edge part is unavoidable. Consequently, the current density distribution is nonuniform, which inevitably destroys the gap-field uniformity and increases the leakage field. In this paper, the leakage fields affected by temperature distribution are studied in several cases.