I. Introduction
Permanent magnet (PM) multipoles are advantageous over electromagnets for some applications [1]. A PM can generate steady magnetic field without any external power supply, which can be stronger than even a superconducting electromagnet in some cases. The maximum field appears in the coil for a superconducting magnet, which is beyond the bore radius. Since the magnetic field increases rapidly with radius for a higher multipole magnet, the peak magnetic field strength may exceed a critical field. On the other hand, the maximum field appears at the bore radius for permanent magnet multipoles. Besides higher strength, its variability is usually requested for real applications. Adjustability of PMQ is achieved by Gluckstern's 5-ring singlet structure where 5 PMQ rings with the same magnetic field gradient are carefully designed in lengths [2]–[4]. It has adjustable focusing strength without noticeable x-y coupling.