RESULTS OF THE APT RF POWER COUPLER DEVELOPMENT FOR SUPERCONDUCTING LINACS*

For the new baseline APT (Accelerator Production of Tritium) linac design, the power couplers are required to transmit 420 kW of CW RF power to the superconducting cavities at 700 MHz. These couplers consist of an airside waveguide-to-coax transition, an air/vacuum break made by two planar, coaxial windows, and a vacuum-side coaxial antenna section. The coaxial antenna allows adjustability of the RF matching to the superconducting cavities. Design, fabrication, and testing of the power coupler/window occurred over the last four years, and room temperature testing of the prototype design is complete. Coupler/window assemblies have transmitted power to 1 MW, CW and have handled full reflected 850 kW, CW over a limited standing-wave phase range. Couplers were tested with a portion of the outer conductor cooled by liquid nitrogen to study the effects of condensed gases. Final results, conclusions, and lessons learned about the coupler design, fabrication, and testing will be discussed. 1 OVERVIEW AND REQUIREMENTS The power coupler was one of the challenges for the Accelerator Production of Tritium (APT) linac, which has a high-energy superconducting (SC) section spanning 210–1030 MeV [1]. Due to the 100 mA p+ current, a large amount of continuous wave (CW) radio frequency (RF) power must be transmitted into the SC cavities through the input power coupler. In 1997, the coupler requirement of 210 kW, CW (at 700 MHz) was chosen for the APT couplers as a reasonable extrapolation of the technology. Later, due to testing results, this was increased to 420 kW, CW requiring only one coupler per cavity. Multipacting and window failure concerns governed the design features of the coupler/window assembly. Figure 1 shows the APT coupler layout