Beam clean-up zone calculations for 2.5MV, 1.4kA experiments on DARHT-2

The injector for the second axis of the Dual-Axis Radiographic Hydrotest Facility (DARHT) produces 2 us, 2kA, 3.2 MeV electron beam with a nominal rise time of 600 ns. The injector diode is powered through a ceramic column by a Marx generator. The off-energy electrons (beam head and tail) during the rise and fall time are mismatched to the magnetic transport channel. These electrons may hit the accelerating gaps, resulting in insulator flashover or a vacuum arc. The beam cleanup zone (BCUZ) has been designed to remove low energy beam electrons and to keep mismatched charge from reaching the downstream accelerator. Eliminating the beam head requires a strongly dispersive magnetic field tune in the BCUZ. In this paper, we will present simulation results on developing a beam clean-up tune and on the beam transport through BCUZ for 2.5MeV, 1.4kA experiments on DARHT-2 using the particle-in-cell code Lsp and the envelope code Lamda. The energy loss and wall temperature rise, electron stimulated desorption due to beam scrape-off in the BCUZ and the effects of slow approach to the flat top on transport of the beam body are also investigated. In addition, the experimentally measured voltage profiles for the injector diode and the accelerating cells were used in these simulations. The current transport from Lamda calculations is compared with Lsp results.