A Coupled-Bunch Ion Instability in the SLC Electron Damping Ring *

Under abnormally poor vacuum conditions in the electron damping ring, described in a companion paper at this conference, an emittance blowup was observed related to a transverse instability. The instability was only observed with two bunches in the ring, and could not be seen with single bunch operation, even at the same total beam current as for the two bunches. Measurements of the transverse Beam Transfer Function indicate strong coupling between the two bunches when ions ate present. This is described in terms of a simple model where the oscillating ion cloud provides the coupling between the two bunches. The frequency shift due to coupling can also cause the ion cloud to become unstable, and disappear, when the betatron tune of the electron bunch is just below the l/2 integer. Con~buted to the 1997 Particle Accelerator Conference, Vancouver, B.C., Canada, May 12-16,1997 ’ Worksupported by Department of Energy contract DE-AC03-76SF005 15. A Coupled-Bunch Ion Instability in the SLC Electron Damping Ring* P. Krejcik, Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309 USA