A rotary tuner for UHF drift-tube linacs

A rotary tuner has been developed and tested in a 425-MHz ramped gradient drift-tube linac (DTL) and also in a 350-MHz DTL cold model tank. This type of tuner utilizes a rotary paddle to displace the magnetic field at the tank wall to vary the inductance and resonant frequency of the drift-tube linac. The tuning paddle effective area is about 7% of the cross-sectional area of the DTL tank and shifts the resonant frequency by about 350 kHz for 90° of rotation. Advantages of the tuner include low friction for fast and accurate response, no RF seals, low wall currents at the shaft feedthrough, and minimal cutout volume at the tank wall. The small cutout volume is important in applications in which tank wall cooling is critical. A smaller helium-cooled version of this device is being developed for use in 850-MHz, cryogenically cooled drift-tube linacs