Beam Stability Experiments on DARHT-II

When completed, the DARHT-II linear induction accelerator (LIA) will produce a 2-kA, 18-MeV electron beam with more than 1500-ns current/energy "flat-top." In initial tests DARHT-II has already accelerated beams with current pulse lengths from 500-ns to 1200-ns full- width at half maximum (FWHM) with more than 1.2- kA, 12.5-MeV peak current and energy. New experiments are planned with a ~1600-ns pulse length but with reduced current and energy. These pulse lengths are all significantly longer than any other multi-MeV LIA, and they define a novel regime for high-current beam dynamics, especially with regard to beam stability. Although the initial tests demonstrated absence of BBU, the pulse length was too short to determine whether ion- hose instability would be present toward the end of a pulse longer than 1500-ns. The 1600-ns pulse experiments are designed to resolve these and other beam-dynamics issues with a long-pulse beam.