Development of the movable mask free from trapped mode for the KEKB

The movable mask is a device that cuts off spent electrons/positrons near the beam orbit and reduces background of the detector. For the KEKB, sixteen movable masks had been installed for each ring. The originally designed masks, however, had severe troubles of heating, arcing and vacuum leaks. Several kinds of improved masks had been designed partially employing RF technologies in dealing with HOM and were installed to the ring step by step. The latest masks are working almost as expected now. Here the problems of movable masks so far are summarized and the structure of the latest movable masks and their status are presented.