Methods for measuring transverse coupling impedances in circular accelerators

Charged particle beams in circular accelerators often become unstable and execute coherent transverse oscillations that grow in amplitude. These are driven by deflecting fields set up by currents induced in the vacuum chamber walls or other equipment by the oscillating beam itself. This unwanted interaction between the beam and its surroundings is characterised by a transverse coupling impedance Z/sub T/. The authors show that it is relatively easy to measure Z/sub T/ for accelerator components with large impedance such as kicker magnets, and that useful results can also be obtained for components with small impedance, such as vacuum chambers, although in this case one is close to the limits of accuracy with ordinary laboratory equipment. (15 refs)