A Tunable Reflecting Load for Multipactor Processing of the RF Power Coupler of a Superconducting Cavity

In recent years, superconducting RF cavities have been increasingly adopted for use in modern accelerator and light-source facilities. The microwave power is coupled into the accelerating cavity typically through a waveguide or a coaxial power coupler. The multipactor discharge in the RF power coupler constitutes one of the most serious problems to the reliable operation of superconducting cavities. Due to the lack of multipacting-free RF structures, RF processing of the RF power coupler becomes a required and routine procedure to ensure multipacting-free operation of a superconducting RF system. However, for a superconducting cavity with negligible wall losses, variations of the beam loading lead to significant magnitude and phase variations of the wave reflected from the cavity. This greatly complicates the multipacting behavior in the RF power coupler. Hence RF processing under fixed conditions, as is conventionally done, is inadequate for multipactor processing of RF power couplers used in superconducting cavities. Here we report the concept, design, and demonstration of a novel tunable reflecting load for multipactor processing of the RF power coupler. The load realistically models the RF cavity through a broad tuning range of the reflection coefficient, both in magnitude and in phase. It thus allows, for the first time, off-line multipactor processing of the RF power coupler under machine operational conditions before it is attached to the superconducting cavity. A high power prototype was assembled in-house using standard WR1800 rectangular waveguide components. Results of cold and high-power test up to 50 kW, CW were found to be in good agreement with theoretical predictions.