A Temperature-Dependent Calibration of Hall Probes for CPMU

A cryogenic permanent-magnet undulator (CPMU) with period 18 mm and magnetic length 2 m is being constructed for Taiwan Photon Source. The CPMU (gap 5.5 mm) can generate effective magnetic field 1.18 T at magnetic arrays temperature of 170K. When the field is measured under long time, the temperature of a Hall probe decreases at a cryogenic temperature; a calibration of the field strength and a thermostatic control system for the temperature is hence necessary. The calibration range of the field strength of a dipole electromagnet is from ±0.00055 to ±1.5T. A two-axis compact Hall probe (SENIS) is mounted on a copper plate in a vacuum chamber. The system to control temperature consists of a cryocooler, a sensor (PT100) and a heater to control temperature of the Hall probe. A higher-order polynomial surface fit is applied to analyze the data measured from the calibration system. The maximum error of field strength is less than 0.02% at the fitted surface. Details of the temperature-dependent calibration system are presented in this paper.