Septum magnet design for compact storage ring

Abstract

Compact storage rings have been recently constructed or are under construction for industrial or medical applications. Such compact rings operate at tens to hundreds MeV and their footprint is of the order of few meters. Since the size is a critical issue for many applications, the accelerator components installation is extremely dense. Beam injection into such a ring, therefore, takes place with a large bending angle. PAL-EUV is a 400 MeV synchrotron under construction at Pohang Accelerator Laboratory for semiconductor industries. The PAL-EUV accelerator consists of a 20 MeV injector linac, a booster ring and a 400 MeV storage ring, installed in a 15 m by 15 m area enclosed by a concrete wall. Three Lambertson septum magnets are used for the electron injection and extraction between the injector linac, the booster ring and the storage ring. The injection septum in the booster ring bends a 20 MeV electron beam transferred from the injector linac by 60°. The septum magnets for booster ring extraction and storage ring injection bend a 400 MeV beam by 30°. Due to the large bending angle of the septum magnets, the holes for the circulating beams extend to the outer side of the return yokes. In addition, the 2 mm septum thickness for the three septa is a concern for the leakage of the magnetic field which may generate higher order field multipoles in the field-free region of the circulating beam. In this article, we discuss the leakage fields in the septum magnets and how to minimize the impact on the circulating electron beams in the rings.