Variable permanent magnet sextupole lens for focusing of pulsed cold neutrons

A Variable Permanent Magnet Sextupole (VPMS) lens that can focus pulsed slow neutrons is under development. It is based on the extended Halbach configuration to generate stronger magnetic field. In order to adjust the strength, the magnet is divided into two nested rings, where the inner ring is fixed and the outer ring can be rotated. This device will focus cold neutrons with wavelength of a few to several Angstroms at about 10 m focal length. We fabricated a half-scale model to measure the strength and the torque as functions of the rotor angle. Because the torque is measured as high as more than 30 N m, a mechanism to rotate the rotor is carefully designed. After installation of such a mechanism, a motor will drive the rotor and the temperature rise caused from eddy current loss will be measured. Detailed development of the system will be presented.