Self-trapping of a rotating ion ring

A fluid model is presented that describes the linear evolution of a thin la yer of beam ions in a conducting cylindrical cavity with a narrow rotating beam layer, and a dispersion relation is computed for the low-frequency ele ctromagnetic response of the system. For a broad range of ion ring paramete rs this configuration is found to be unstable to the generation of Alfvenic fluctuations. Simple estimates are provided of the time needed for these f luctuations to self-trap the ion beam. (C) 2001 American Institute of Physi cs.