Tests of a prototype magnetostrictive tuner for superconducting cavities

The Continuous Electron Beam Accelerator (CEBA) uses mechanical tuners at 2 K driven by room temperature stepping motors in a feedback loop to maintain cavity frequency at 1497 MHz, A modification of the existing system was designed, replacing a passive section of the mechanical tuner with a magnetostrictive tuning element consisting of a nickel rod and an industrially supplied 0.25 T superconducting solenoid, This assembly was tested with several magnetic shield configurations designed to keep the stray flux at the niobium cavity below 1 mu T when the cavity was normal, to maintain cavity Q. Results of the tests, including change in cavity performance when the cavity was locally quenched near the end of the solenoid, showed that a multilayer shield of 6mm of steel, with sheets of mu metal, niobium and mu metal spaced appropriately outside the thick steel, was effective in containing the flux, both remanent and current-driven, preventing any change in cavity Q upon cool down or quench with an external heater near the end of the solenoid, Hysteresis attributed to the nickel magnetostrictive element was observed.