New waveguide-type HOM damper for ALS storage ring cavities.

The ALS storage ring 500 MHz RF system uses two re-entrant accelerating cavities powered by a single 320kW PHILLIPS YK1305 klystron. During several years of initial operation, the RF cavities were not equipped with effective passive HOM damper systems, however, longitudinal beam stability was achieved with careful cavity temperature control and by implementing an active longitudinal feedback system (LFB), which was often operating at the edge of its capabilities. As a result, longitudinal beam stability was a significant operations issue at the ALS. During three consecutive shutdown periods (April2002, 2003 and 2004) we installed E-type HOM dampers on the main and third harmonic cavities. These devices dramatically decreased the Q-values of the longitudinal anti-symmetric HOM modes. The next step is to damp the remaining longitudinal HOM modes in the fundamental RF cavities below the synchrotron radiation damping level. This will hopefully eliminate the need for the active LFB system and set the stage for a possible increase in beam current. A “waveguide” type of HOM damper is the only option that does not significantly compromise the vacuum performance of the rf cavity. In this report we further discuss the design process and the results of the low level measurements of the waveguide type damper.