The fundamental power coupler prototype for the Spallation Neutron Source (SNS) superconducting cavities

The SNS linac contains superconducting cavities with /spl beta/.61 and .81 for the acceleration of H/sup -/ ions. Each of the 6-cell cavities, resonant at 805 MHz at 2.1 K, is powered by a 550-kW pulsed klystron via a coaxial coupler. The specifications for the fundamental power coupler (FPC) require that it withstand the full klystron power in full reflection for the duration of the RF pulse (1.3+0.15 [decay] ms) at a repetition rate of 60 pps with an average power of 48 kW. The FPC must provide a Q/sub ext/ of 7.3/spl times/10/sup 5/ and 7.0/spl times/10/sup 5/ for the medium-/spl beta/ and high-/spl 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 prototypes of the FPCs. The 50-/spl Omega/ coaxial line is scaled directly from 508 MHz to the SNS frequency of 805 MHz.