High-accuracy stretched-wire measurement system for cryogenic permanent magnet undulator (CPMU) in High Energy Photon Source (HEPS)

A new stretched-wire system is built for a cryogenic permanent magnet undulator in High Energy Photon Source Testing Facility. The system has two functions: integral field measurement and magnet gap measurement. Integral field measurement and gap measurement are important for evaluation and optimization of the magnetic performance of the undulator in cryogenic-vacuum environment. Two high-precision, high-load motion stages are used for accurate positioning. A special fix structure of stretched wire is adopted for vacuum environment. To reduce the deflection of the 3-meter-long wire, constant tension is maintained along the wire. The measurement repeatability of field integral and magnetic gap is the key performance which depends on the stability of wire and suppression of the electric noise. Strategy of improving the measurement accuracy and stability is presented.