Design of Cryogen-Free Mini-Pole Superconducting Undulator

A cryogen-free cryostat for a superconducting undulator with a period of 15 mm (SU15) was developed at the National Synchrotron Radiation Research Center (NSRRC). A 0.44 mm-thin superconducting NbTi wire was wound on iron pole with a racetrack profile. The design field strength of SU15 was 1.4 T and the coil had 148 turns/pole at 230 A; the magnet gap was 5.6 mm. The liquid helium (LHe) recondensation cryostat was designed for SU15. The cryostat provides the advantage of uniform cooling and reduces the amount of LHe consumed. A0.3mm-thick stainless steel (SS) beam duct was formed to separate the vacuum in the storage ring from the magnet. Therefore, all of dirty material is produced out of storage ring vacuum. Most of the heat leaked from the LHe vessel leaks from the LHe-fill port heat leak and the 300 K-4.2 K beam duct or was associated with image current power dissipation (ICPD) and bending magnet power dispersion (BMPD). The leaking of heat from the main coil was prevented by the high-temperature superconducting current leads (HTSC). The beam duct was coated with a copper layer with a high-RRR ratio to reduce image current heating. The heat loads on the magnet were cooled using two 1.5 W Gifford-MacMahon (GM) type cryocoolers via LHe medium.