Fundamental and Higher Order Mode Couplers on Superconducting RF Cavities for Electron Storage Rings

The design of a coaxial high power loop coupler for use on a 500 MHz superconducting (sc) accelerating cavity of "spherical" shape is described. Nonsuperconducting construction materials (copper and copper plated stainless steel) are used. Cooling is obtained by cold helium gas in counter flow. The coupler is located at the cavity equator and a choke provides an uncritical RF joint at a minimum current flow position. For the coupling required (Qext = 105-106) the loop does not protrude into the cavity. Test experiments up to 70 kW incident RF power with a matched copper cavity and up to 17.5 kW with a superconducting cavity (corresponding to an acceleration field Eacc = 3.6 MV/m) are described. Design and measurements with two types of compact higher order mode (hom) couplers, coupling to the magnetic and electric field respectively are described. Both couplers are manufactured from Nb and are low Q resonant devices with their centrefrequency matched to the relevant part of the cavities' higher order mode spectrum. All couplers have been operated with a sc cavity up to the design field level.