Suppression of microwave instabilities

Abstract

The longitudinal resistive microwave instability in storage rings has been identified in previous workshops as a key problem in heavy ion fusion. Here we show by means of computer simulation that the predictions from the linearized theory of instability are over-pessimistic in the case of non-relativistic ions. This is due to an early nonlinear saturation of the instability caused by the formation of a thin stabilizing tail of the momentum distribution, which provides enough Landau damping to suppress further growth. The main condition for this non-destructive self-stabilization lies in the slow resistive growth rate, as compared with the large coherent frequency shift induced by space charge. The important conclusion from this is a significant extension of the stable storage ring filling time to values that are comfortable from a design point of view. For the HIBALL reactor driver accelerator a safe storage time of 5 to 10 msec is expected, instead of only 1 msec according to the linearized theory e-folding times.