Magnetic characterization for cryogenic permanent-magnet undulators: a first result.

The cryogenic permanent-magnet undulator (CPMU) is a novel insertion device recently proposed at SPring-8, in which permanent magnets (PMs) are cooled to cryogenic temperatures to improve the magnetic performances, such as remanence and coercivity. A new measurement system for carrying out high-precision magnetic field mapping using a Hall probe has been developed in order to characterize the magnetic field generated by PM arrays at cryogenic temperatures. In this system, alignment of the Hall probe was dynamically performed by means of detecting the variation in its transverse position using optical laser beams introduced into the vacuum chamber. Magnetic measurements of a CPMU prototype were made at different temperatures in order to investigate variations of the magnetic performances owing to temperature. The maximum remanence deduced from the average peak value of the field profile was found to be close to that obtained from a former measurement with a single piece of the same PM material. In addition, the error components in the field profile were found to be insensitive to temperature in terms of the electron trajectory and phase error. This result suggests that the field correction of CPMUs can be performed based on the field profile measured at room temperature, which considerably reduces the task and time necessary for construction of CPMUs.