Measurement and analysis of higher-order-mode (HOM) damping in B-factory R-F cavities

We have previously described a fully automated “bead” puller system which can be used to measure both longitudinal and transverse field profiles. Converting bead-puller data to impedances requires knowledge of the Q-values of the individual modes. A second system enables us to acquire cavity spectra and then simultaneously fit up to three resonances plus a uniform background. R/Q measurements on the “bare” cavity (i.e., with only the beam ports open) have been consistent with predictions by URMEL. Due to overlapping resonances, we have not been able to measure R/Q for many of the damped modes, but for those for which we have, there is generally good agreement with the undamped cases. Assuming this to be true in general, we use the measured Q-values for the damped modes in conjunction with the “best available” measured R/Q, and conclude that with a few possible exceptions (which if need be can be dealt with by additional dedicated dampers), the damping for the higher order modes is adequate for the operation of the machine