Construction of the baseline SC cavity system for STF at KEK

The STF-Baseline superconducting cavity system, which includes four TESLA-type 9-cell cavities, input couplers and frequency tuners, has been developed for the future ILC project. A main improvement in the cavity system is a very stiff design in a He vessel and a cavity tuning system, which can relax a cavity deformation due to Lorentz force. Vertical tests of four 9-cell cavities have been carried out repeatedly, and the attained accelerating gradients have reached to 20 _~ 29 MV/m with Qo values higher than 1times10¹⁰. Peculiar phenomena, like increasing of Qo values with higher Eacc or very slow degradation of Qo values, were observed in the vertical tests. RF processing of input couplers was successfully carried out up to 1.0 MW with a 1.5 msec and 5 Hz operation .