RF conditioning studies of input power couplers for superconducting cavities operating in pulsed mode

We have performed systematic studies of RF conditioning on a series of 30 power couplers (the so-called TTF-III coupler) to be used with the superconducting cavities of the FLASH VUV Free Electron Laser (FEL) and, eventually, the European X-ray FEL. We have investigated the reduction in conditioning time resulting from the effects of baking-out the couplers. We show that preservation of conditioning after lengthy storage of the couplers under vacuum is conditional on this bake-out. We confirm the beneficial effect of DC bias on this coupler to counteract multipactoring. By carrying out successive variations of the control algorithm for the conditioning procedure (vacuum thresholds and interlock levels, loop control speed, pulse repetition rate), we have demonstrated a factor of three reduction in the time required to fully condition the couplers at ambient temperature on a test stand. This result has been confirmed on tests of the couplers on cavities at high gradient in cryogenic conditions.