New damped acclerating cavities

In order to improve the beam stability at high stored currents, newly-developed damped acclerating cavities replaced the old rf cavities for the PF ring. The new cavities were designed to greatly reduce higher-order-mode (HOM) impedances which are the principal source of coupled-bunch beam instabilities.

500-MHz damped acclerating cavities which were installed in the PF storage ring.
(Note that a part of the waveguide is removed in this photo).

Inner shape of the damped cavity.
The cavity has relatively large beam ports. Power of higher-order-modes is extracted through the beam ports, and is absorbed by two cylinders of SiC (Silicon-Carbide) which were installed in the beam pipe. A few higher-order-modes, which remain in the cavity, are adjusted in their frequencies so that the coupled-bunch instabilities can be avoided during the storage-ring operations.

Principal parameters of the damped cavity

RF frequency	500.1 MHz
Shunt impedance	6.8 Mohms
Unloaded-Q	39,000
Coupling coefficient	2.3
Maximum dissipated power	140 kW

(This cavity was developed by a collabolation between the KEK Photon Factory and the ISSP of the University of Tokyo).