A mechanical tuner for the ISAC-II quarter wave superconducting cavities

TRIUMF is developing a new mechanical tuner system capable of both coarse (kHz) and fine (Hz) frequency adjustments for maintaining frequency lock on the superconducting quarter wave cavities of the ISAC-II heavy ion linac. A 1 mm thick Niobium plate at the high field end of the cavity is actuated by a vertically mounted permanent magnet linear servo motor, at the top of the cryostat, using a 'zero backlash' lever and push rod configuration through a bellows feed-through. The system resolution at the tuner plate center is similar to 0.055mum (0.3 Hz) with a dynamic range of 8 KHz and a manual coarse tuning range of 33 kHz. In cold tests with a prototype quarter wave cavity the tuning system's ability to compensate perturbations indicated a bandwidth up to 100 Hz. A large mechanical resonance at 20 Hz should be eliminated in the on-line device. The rf control is based on a self-excited loop with a locking circuit for amplitude and phase regulation. The tuner is fed a position signal integrated from the control loop phase error. Details of the mechanical device and results of open and closed loop cold tests will be given.