Achieving high gradient performance of 9-cell cavities at KEK for the international linear collider

Since 2008, vertical tests of 1.3 GHz 9-cell cavities at the superconducting RF test facility (STF) in KEK have been carried out for the International Linear Collider (ILC). The tested cavities are mainly KEK-05 through KEK-22. Test results showed that KEK-12, KEK-13, KEK-17, and KEK-21 attained the unloaded Q value (Q0) of 0.8×1010 at the accelerating gradient (Eacc) of 35 MV/m (ILC specification) only by the electro-polishing (EP) process. These four cavities had no problematic defect on any electron beam welding seams at their equators; however, field emission was still observed in the vertical tests. This paper reports on the recent progress of the cavity performance at KEK-STF, along with a detailed analysis of problematic defects and the effect of the local mechanical grinding.