Coarse and fine tuners for the CERN PS 40 MHz buncher cavity

A 40 MHz cavity has been constructed at CERN as part of the PS upgrade program for injection into LHC. To compensate for the variable capacitive power coupling and mechanical tolerance during fabrication, a coarse tuner has been envisaged to correct the resonant frequency of the PS 40 MHz cavity. This consists of a shorted loop inside the cavity and by rotating the loop, the magnetic coupling to the cavity can be varied and hence a coarse tuning is obtained. The coarse tuner can not be changed when the cavity is in vacuum. The fine tuner is to compensate for slow variation of resonant frequency of the cavity due to temperature and pressure variation. The tuner employs a variable capacitor as the tuning element. It is connected to a loop inside the cavity via a ceramic window and a rigid transmission line of suitable length. The desired tuning range of the coarse tuner is 1% and the fine tuner is 0.5%. The coarse tuner has been designed and tested at TRIUMF on a full scale wooden model of the CERN cavity. The fine tuner was designed using PSPICE with the same loop as the coarse tuner. This tuner has been constructed at CERN and also installed in the cavity. Excellent agreement was found between the predicted and the measured tuning ranges of both the tuners on the 40 MHz CERN cavity.