The SNS linac contains superconducting cavities with phase velocities .61 and .81 for the acceleration of H^- ions. Each of the 6-cell cavities, resonant at 805 MHz at 2 degrees K, is powered by a 550 kW pulsed klystron via a coaxial coupler. T he specifications for the coupler require that it withstand the full klystron power in full reflection for the duration of the RF pulse (1.3+1.5(decay) ms) at a repetition rate of 60 pps, with an average power of 48 kW. The coupler must provide a Q_{e xt} of 7.3 x 10⁵ for the medium beta and high beta cavities, respectively. A design derived from proven experience at other laboratories (primarily at KEK, for Tristan and KEK-B cavities at 508 MHz) has been adopted for the production of the 50 omega prototype couplers. The couplers include a planar coaxial window with matching compensation elements, cooled by conduction via a water system from the air side of the inner conductor. The coupler’s outer conductor between the window and the cavity is actively cooled by a helium circuit with input at 5 degrees K and output at about 165 degrees K. Vacuum, arcing, infrared, and electron current monitoring and interlocking is done through several ports located near the window. The coaxial cou pler is matched to a WR975 waveguide through a door-knob transition, also derived from the KEK design. Numerical calculations have been performed on the electromagnetic properties of the coupler and on the multipacting behavior of the coaxial line and win dow. A capacitor for DC bias will be used to control multipacting. The first coupler prototypes have been produced by industry and the results of the RF testing on them at room temperature will be presented.

This work was supported by U.S. DOE under contract DE-AC05-00OR22725.

Authors: I.E. Campisi, E. Daly, P. Kneisel, W. Schneider, M. Stirbet, K.M. Wilson

Abstract submitted to PAC 2001, Chicago, IL, June 18-24, 2001

Updated March 28, 2001

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