High power tests of KEK-ERL input coupler for main linac under liquid nitrogen condition

Sakai, Hiroshi*; Furuya, Takaaki*; Nakamura, Norio*; Umemori, Kensei*; Shinoe, Kenji*; Sawamura, Masaru; Cenni, E.*

We fabricated the prototype of an input coupler with two ceramic windows for ERL main linac and performed the high power test. In high power test, the one cold window was installed into the vacuum insulating chamber and cooled by liquid nitrogen. First, the multipacting limited the power level to 10 kW. The pulse processing method for 8 hours enabled the final power level to 25 kW with standing wave condition. We could also keep 20 kW power feeding into the coupler for 16 hours. The maximum measured temperature rises under feeding the 20 kW power to coupler were 120 K near the bellows parts and these are not so severe values to operate ERL main linac. The thermal cycle test of cold window of coupler was done. After 10-times thermal cycle tests between the room temperature and liquid nitrogen temperature, no leaks or cracks were observed. From these results of high power test, this prototype coupler satisfies our thermal and RF requirements.