Characterization of longitudinal impedances in storage rings via multibunch effects

Abstract

In this paper we present two new techniques for frequency-resolved characterization of longitudinal impedances in storage rings. The first method is based on transient measurements of the growth rates and tune shifts of unstable coupled-bunch modes. In the second approach, estimates of the impedances are obtained from analysis of the steady-state synchronous phases of the bunches for uneven fill patterns. These techniques are applicable to measurements of both fundamental and higher-order mode (HOM) impedances and allow characterization of shunt impedances and quality factors of the HOMs. Methods presented here are complementary to laboratory bench measurements of rf cavities, in that the beam-based measurements directly sense the physical impedance in the installed configuration. Experimental results from the Advanced Light Source and BESSY-II are presented showing the use of these techniques to measure complex impedances.