We have suffered from multipactor problem of some SDTL after the Great East Japan Earthquake. As a designed operating rf power of the SDTL is in the multipactor region, we had to operate at higher power of the designed one. From the result of the simulation and the observation of the SDTL cavity, it became clear that the multipactor occurred on the inner surface of the cavity. We think that one of the cause of the maultipactor is the contamination on the inner surface of the cavity, we

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... the cleaning of the inner surface of the cavity by using acetone. The cleaning was very effective and the multipactor region was reduced dramatically or disappeared. The multipactor problem has not occurred since then.