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 existi ng system was designed, replacing a passive section of the mechanical tuner with a magnetostrictive tuning element consisting of a nickel ro d and an industrially supplied 0.25 T superconducting solenoid, This a ssembly was tested with several magnetic shield configurations designe d to keep the stray flux at the niobium cavity below 1 mu T when the c avity was normal, to maintain cavity Q. Results of the tests, includin g change in cavity performance when the cavity was locally quenched ne ar the end of the solenoid, showed that a multilayer shield of 6mm of steel, with sheets of mu metal, niobium and mu metal spaced appropriat ely outside the thick steel, was effective in containing the flux, bot h remanent and current-driven, preventing any change in cavity Q upon cool down or quench with an external heater near the end of the soleno id, Hysteresis attributed to the nickel magnetostrictive element was o bserved.