R&D of Short-Period NBTI and Nb3Sn Superconducting Undulators for the APS

Superconducting undulators (SCUs) with a period of 14.5 mm are under development for the Advanced Photon Source (APS). The undulators have been designed to achieve a peak field on the beam axis higher than 0.8 T with an 8 mm pole gap and current densities over 1 kA/mm²in the NbTi and Nb3Sn coils. Upper-half NbTi SCUs of short sections have been fabricated and were charged up to near the critical current density of 1.43 kA/mm²to achieve a peak field about 1 T. The stability margin of the SCU was measured by imposing steady-state heat fluxes on the pole/coil face of the SCU in a pool-boiling liquid He (LHe) dewar at 4.2 K. Near the critical current density, where the temperature stability margin is minimal, the heat flux density to quench the SCU was about 1.3 mW/mm², of which 60% was attributed to LHe at the interface of the SCU and the vacuum chamber. The peak fields of the SCU were mapped along the beam axis using a Hall probe in a vertical dewar. The first test of a Nb3Sn short-section SCU was charged to an average current density of 1.45 kA/mm², slightly higher than the critical current density for the NbTi SCU.