Implementation of spread mass model of ion hose instability in lamda

The ion-hose instability sets limits on the allowable vacuum in the DARHT-2 linear induction accelerator (2 kA, 18.6 MeV, 2 mus). 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, image- displacement 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 2.5 MeV-1.4 kA beam in the DARHT-2 is also presented.