Superconducting Undulators: From NbTi-Based Magnets to Nb 3 Sn and 2G-HTS Devices

Developments of short-period superconducting undulators (SCUs) continue at the Advanced Photon Source (APS). In these developments three practical superconducting materials: NbTi, Nb3​Sn, and second-generation high-temperature superconductors (2G-HTSS) are used. For NbTi SCUs a new magnet design with enhanced coil-to-ground insulation was implemented and successfully tested. The magnet fabrication process included frequent high-voltage testing, and the quench protection system was enhanced by adding parallel dump resistors to each magnet. These modifications enabled reliable SCU quench training up to 500 A after about 15 quenches in 1.5-m-long NbTi magnets. This provides a comfortable margin for reliable SCU operation at the maximum current of about 450 A. By using mechanical shims, the SCU magnetic gap was adjusted along the device length. That procedure allowed us to achieve an RMS phase error less than 5° across the full range of the SCU operating current. Development of a 14-mm-period Nb3Sn SCU targeting K>2 at a 6 mm magnetic gap is in progress. These developments also include the design of a liquid helium-free (LHe-free) cryostat. The prototyping of LHe-free cryostat components is in progress. In parallel, force-balanced 2G-HTS SCU prototypes aiming for an undulator period of 10 mm are being designed and fabricated.