Vertical cold test of the crab cavity with a co-axial beam pipe

Abstract

A crab cavity was designed for the KEKB electron-positron collider-accelerator. The aim of this cavity is to deflect the beam bunch and realize the crab-crossing scheme. The cavity, operating in the TM110 mode, has a squashed cell with a co-axial beam pipe coupling scheme to extract the lowest order mode (TM010). Operating voltage should be high enough to deflect a beam bunch for a finite beam-crossing angle. For the R & D of this complicated structure, a prototype cavity and a simplified inner conductor for the co-axial beam pipe structure have been fabricated. The RF performance of the cavity was tested with the inner conductor in a vertical cryostat. During the tests, a serious Q-degradation was observed, which is so called Hydrogen Q-disease. A calorimetric RF loss measurement showed that the loss at the inner conductor is a cause of the Q-degradation. A quick cool-down procedure was applied to the inner conductor and a required deflecting voltage was achieved.