Development of movable mask with reduced-HOM design for KEKB

At the high energy ring of the KEK B-factory (KEKB), it was found that some bellows near the movable masks were overheated due to the higher order mode (HOM) as increasing the beam current over 900 mA. To cope with this problem, a new mask was designed where the length of ramps beside the mask head was expanded from 30 mm to 400 mm. MAFIA T3 simulations showed that the loss factor for the new long mask was about a half of that for the previous short one. The power of the TE mode, on the other hand, which can easily couple with the bellows through the finger-type RF-shield, was expected to reduce to about 6 percent of that for the short one. During the summer shutdown in 2002, two long masks were installed as a test. In the following run the temperature rise of bellows near the long masks was about 20 percent of those near the short ones and the new design was found to be effective to reduce the HOM. The decrease in the temperature rise was larger than the reduction of the HOM power estimated from the loss factor. This result indicates that the overheating of the bellows is mainly due to the TE mode like HOM rather than the total HOM.