When completed, the DARHT-II linear induction accelerator (LIA) will produce a 2-kA, 17-MeV electron beam in a 1600-ns flat-top pulse. In initial tests, DARHT-II accelerated beams with current pulse lengths from 500 to 1200 ns full-width at half-maximum (FWHM) with more than 1.2-kA, 12.5-MeV peak current and energy. Experiments have now been done with a 1600-ns pulse length. These pulse lengths are all significantly longer than any other multimegaelectronvolt LIA, and they define a novel regime for high-current beam dynamics, especially with regard to beam stability. Although the initial tests demonstrated insignificant beam-breakup instability (BBU), the pulse length was too short to determine whether ion-hose instability would be present toward the end of a long, 1600-ns pulse. The 1600-ns pulse experiments reported here resolved these issues for the long-pulse DARHT-II LIA.
The ion-hose instability sets limits on the allowable vacuum in the DARHT-2 linear induction accelerator (2kA, 18.6MeV, 2渭s). Lamda is a transport code which advances the beam centroid and envelope in a linear induction accelerator from the injector to the final focus region. The code computes the effect of magnet misalignment, beam breakup instability, imagedisplacement instability, and gap voltage fluctuation. To support the experiments, we have implemented the SM model of ion-hose instability into Lamda. Unlike the ordinary SM ion-hose code which assumes the uniform axial magnetic field, Lamda ion-hose calculation includes varying axial magnetic field, accelerating beam, gas pressure file, varying beam radius, and elliptical beam. This paper describes the Lamda ion-hose instability code, the benchmarks against a semi-analytical SM code, and the particle-in-cell code Lsp. A prediction of ion-hose instability for a 2.5MeV-1.4kA beam in the DARHT-2 is also presented.
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