• Y. Yamamoto, H. Hayano, E. Kako, T. Matsumoto, S. Michizono, T. Miura, S. Noguchi, M. Satoh, T. Shishido, K. Watanabe
  KEK, Ibaraki

The Cryomodule test was finished at KEK-STF (Superconducting rf Test Facility) on December/2008. The four 9-cell cavities (MHI#1-#4) were installed into it and measured around 2K for totally a few months. MHI#2 cavity achieved around 32 MV/m (29 MV/m at vertical test) with the feed-back and the others around 20 MV/m. During the high power test with a klystron, the Lorenz detuning was observed and measured for these cavities. Generally, the Lorentz detuning is almost compensated by setting the offset of the cavity frequency in advance (pre-detuning) and driving the Piezo actuator with an optimum condition. The optimum driving condition for Piezo actuator was obtained, which controlled the detuning frequency of the cavity within ±30Hz. MHI#2 cavity was stably operated around 30 MV/m with Piezo compensation for several hours. During this operation, the r.m.s. of the detuning frequency was about 5Hz and the peak-to-peak of the gradient at the flat-top was below 0.1%. The “Two Modes Model” was devised to offer the physics explanation for the observational results of the Lorentz detuning. It was found that this model is valid, since it reproduces the real data.