Thermal Analysis of a Helical Superconducting Undulator Cryostat

A novel cryocooler-cooled helical superconducting undulator (HSCU) was built and installed in the Advanced Photon Source (APS) storage ring and has been in operation since January 2018. The new cryostat for the HSCU is a major improvement of the cryogenic system used for the existing APS planar superconducting undulators. In both systems, superconducting magnets are cooled independently from the beam chamber by liquid helium (LHe) passing through a channel in the magnet/mold assembly. The original planar undulator cryostat consists of three thermal circuits: the outer shield circuit to cool both the warm part of the magnet current leads and the outer shield, the beam chamber cooling circuit to cool the beam chamber and the inner shield, and the magnet cooling circuit to cool the magnets and the LHe tank. On the other hand, the new system consists of only two thermal circuits: the thermal shield circuit to cool the beam chamber, the shield, and the warm part of the magnet leads; and the magnet cooling circuit to cool the LHe tank and the magnet/mold assembly. An finite element analysis-based thermal model of the new HSCU cryostat was developed while the cryogenic performance of the HSCU was measured in a series of standalone tests as well as after installation in the storage ring. This paper presents a thermal analysis of the new HSCU cryostat and, in particular, a comparison of calculated and measured temperatures as well as the excess cooling capacity of the new cryostat in static and dynamic (beam-induced heating) modes.