Field Measurements of a Cryogenic Permanent Magnet Undulator at the TPS

A PrFeB-based cryogenic permanent-magnet undulator (CPMU) with a period length of 15 mm (CU15) is being constructed to provide high brilliant X-rays for the Taiwan Photon Source (TPS) nano-probe beamline. Field measurements of this undulator will be performed with a Hall probe in a vacuum environment. The in-situ Hall probe measurement system (Yang et al., 2014) was improved (Yang et al., 2016) to allow measurement and tuning of the CU15 magnetic field. A field strength and temperature dependent calibration of the Hall probe was done to obtain realistic field distributions. By tuning the differential adjusters and spring modules, we can correct magnetic field errors caused by assembly tolerances and temperature gradients along the magnetic arrays, thus lowering phase errors to reach our design goals. This paper describes details of improvements in the measurement system, field tuning methods, and measurement results at cryogenic temperatures.