Fundamental power coupler development for low-beta superconducting cavities

The construction of a re-accelerator for secondary ion beams is currently underway at the National Superconducting Cyclotron Laboratory (NSCL). This re-accelerating linear accelerator (linac) will use superconducting quarter-wave resonators operating at 80.5 MHz with beta = 0.041 and beta = 0.085, where beta is the velocity of a particle relative to the speed of light. Input coupling will be done using a coaxial probe-type radio frequency (RF) fundamental power coupler (FPC) for both quarter-wave resonator (QWR) cavities. Custom-designed power couplers are expensive, complicated, and can have long lead times. The FPCs presented in this thesis will be operated with fixed coupling, using continuous wave (CW) input power operating at a maximum of 1 kW. The key component of the coupler is a commercially-available RF feedthrough to reduce the cost and production time. This feedthrough allows the cavity to maintain a clean vacuum with pressure less than 1 x 10-8 torr. Another concern is the heat leak to the cryomodule's cold mass at 4.5 K. The design goal for each coupler is to produce less than 1 W of static heat load to the cold mass.;Four prototype FPCs were fabricated. Two of the FPCs were assembled back-to-back, pumped out, and conditioned with both traveling wave and standing wave power using a 1 kW RF amplifier. Experience with the prototype FPCs provided the basis for the design of production FPCs for the re-accelerator.