Development of C-Band Accelerator Structure with Smooth Shape Couplers

A new C-band accelerator structure for the SuperKEKB injector linac is under development. This structure corresponds to an upstream half part in a tandem connection of two structures. A measure against RF breakdown is required because the upstream structure is exposed to higher RF field than that of the downstream structure. In order to suppress a RF breakdown around a coupler iris, a fat and smooth iris is adopted for the structure. The coupler dimensions such as an aperture of the iris and a diameter of the coupler cavity is first optimized by computer simulation and then decided by cold model measurements. Electric field symmetry is retained by shifting the coupler cavity from a beam centre. An amount of offset is also decided by simulations. The coupler body is made with a milling machine. A rather rough surface of the inner coupler is finished by an electro-polishing method. This structure will be installed in the beam line of the KEKB injector linac this summer.