Design of Hom Power Absorbers for the Kek B-factory

The Higher-Order-Mode (HOM) power deposition is one of most crucial problems in the KEK B-factory to achieve a total beam current of 2.6A at the Low Energy Ring (LER). The estimate shows that the HOM power of about 20 kW can pass at any point in the arc of the ring. A particular worry is the Interaction Point (IP) where the maximum tolerable heat deposition of the IP chamber is only 200 W. Two and one types of HOM power absorbers have been studied to protect the beamline components at the arc and the IP chamber, respectively. One structure for the arc consists of a radial line connected a coaxial pipe where an absorbing material is housed. Another one has an absorbing material directly attached to the inner surface of the copper chamber. The absorbing material is segmented to a dozen pieces of pencil shape, each being titled by 30-45 degrees for good couplings with both TM and TE modes. Calculations with MAFIA and HFSS show that they have absorbing efficiencies more than 10% and the loss factors of 0.1-0.2 V/pC. Our choice of absorber for the Interaction Region (IR) is a conventional cylindrical SiC attached to the IR chamber. It has an absorption rate of 70% in the wide range of frequency.