Characterization of Longitudinal Impedances in Storage Rings via Multibunch Effects∗

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 coupledbunch 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 measurement of both fundamental and higher-order mode impedances and allow characterization of shunt impedances and quality factors of the HOMs. Methods presented here are complementary to lab bench measurements of RF cavities, in that the beam based measurements directly sense the physical impedance in the installed configuration. Experimental results from ALS and BESSY-II are presented showing the use of these techniques to measure complex impedances. Submitted to Phys Rev. ST Accel. Beams ∗Work supported by Department of Energy contract DE–AC03–76SF00515. Characterization of Longitudinal Impedances in Storage Rings via Multibunch Effects D. Teytelman, J. Fox, S. Prabhakar Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309, U.S.A J. M. Byrd Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720 Department of Physics, University of California, Davis, Davis, California 95616 (January 7, 2002) In this paper we present two new techniques for frequencyresolved 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 measurement of both fundamental and higher-order mode impedances and allow characterization of shunt impedances and quality factors of the HOMs. Methods presented here are complementary to lab bench measurements of RF cavities, in that the beam based measurements directly sense the physical impedance in the installed configuration. Experimental results from ALS and BESSY-II are presented showing the use of these techniques to measure complex impedances.