High Power Co-Axial SRF Coupler

A superconducting RF (SRF) power coupler capable of handling 500 kW CW RF power at 750 MHz is required for present and future storage rings and linacs. There are over 35 coupler designs for SRF cavities ranging in frequency from 325 to 1500 MHz. Coupler windows vary from cylinders to cones to disks and RF power couplers will always be limited by the ability of ceramic windows and their matching systems to withstand the stresses due to non-uniform heating from dielectric and wall losses, multipactor, and mechanical flexure. We propose a novel robust co-axial SRF coupler design which uses two windows manufactured with compression ring technology and no matching elements in the coax line. This technology will allow the use of high thermally conductive materials for cryogenic windows because the braze joint will be in compression. In Phase II a scaled down version of the 500kW CW double window assembly will be built and tested at high power in order to finalize the design for the full size version. A compressed co-axial window was designed and fabricated using standard ceramics. The mechanical design incorporated several critical process elements that made sure other materials could be used without concern for metalizing issues. Low power tests confirmed the broadband match characteristics of a double window design. Window materials will be used in the assembly processes ranging from soft boron nitride to lossy aluminum nitride and sapphire. A scaled down version of the ultimate double window design for 500kW CW at 750 MHz will be built and stress tested at cryogenic temperatures as well as room temperature with varying pressures and gases. As a result of these comprehensive tests, a design scaled up to full power CW can be reliably produced. Commercial Applications and other Benefits: Double-window, coaxial input couplers or coaxial pressure barriers have high power CW applications ranging from RF guns for injectors to high power magnetron sputtering systems. Single half-wavelength coaxial windows built with our compression technology will handle significantly more power than standard windows.