Superconducting multipole wiggler with large magnetic gap for HEPS-TF

A 16-pole superconducting multipole wiggler with a large gap of 68 mm was designed and fabricated to serve as a multipole wiggler for HEPS-TF. The wiggler consists of 16 pairs of NbTi superconducting coils with a period length of 170 mm, and its maximum peak field is 2.6 Tesla. In magnet design, magnet poles were optimized. Furthermore, the Lorentz force on the coils and electromagnetic force between the upper and lower halves were computed and analyzed along with the stored energy and inductance at different currents. To enhance the critical current of the magnet coil, all the pole coils selected for the magnet exhibited excellent performance, and appropriate prestress derived from the coil force analysis was applied to the pole coils during magnet assembly. The entire magnet structure was immersed in 4.2-K liquid helium in the cryostat cooled solely by four two-stage cryocoolers, and the performance test of the superconducting wiggler was appropriately completed. Based on the measured results, the first and second field integrals on the axis of the superconducting wiggler were significantly improved at different field levels after the compensation of the corrector coils. Subsequently, the wiggler was successfully installed in the storage ring of BEPCII operation with beams.