Development of a movable collimator with low beam impedance

A movable collimator (mask) with low beam impedance is proposed for high-intensity colliders. The collimator head is supported by a dielectric rod, instead of a conventional metal block or rod. Owing to the dielectric support, the beams hardly see the head, and thus the beam impedance decreases. Based on the simulation study, the first test model was manufactured and installed in the KEK B-factory (KEKB) positron ring, and tested with the beams. The head and the support were made of alumina ceramic, and the head is coated by copper. However, excess heating of the head and the support was observed from an extremely lower beam current than expected. High frequency HOM seemed to cause the considerable dielectric loss not only inside the support but also the head, which formed an RF cavity due to the copper coating. The revised model is now under design based on the experience.