Present status of the C-band activities at KEK

We have already developed three conventional and one periodic permanent (PPM) type 50-MW class klystrons, a smart modulator, and the first HOM-free accelerator structure (Choke-mode type, full-scale high power model). A very stable ceramic high voltage monitor was successfully tested up to 367-kV with a 4.5-μsec pulse. A new C-band SiC type high power rf-load, advancing the power handling capability up to 50-MW is now being designed. It should have excellent mass production characteristics as it uses circularly symmetric TM011 chained cavities. The first high power prototype of an rf compressor cavity made of a low thermal expansion material (super Invar) was designed to provide stable operation even with a very high Q of 200-k, it was successfully tested the output rf power up to 135-MW, 0.5-μsec pulse width and 50-pps repetition rate. The C-band linac rf-system will be used for the SASE-FEL (SCSS) production project at SPring-8, but SCSS will also serve to verify the design and components, which can eventually be deployed for the main linac rf system in a future linear collider. (author)