Design of beam cleanup zone for DARHT-2

The DARHT-2 linear induction accelerator is designed to produce a 2 kA, 20 MV, 2 μs flat-top electron beam. The injector is driven directly by a Marx bank, and may have a risetime as large as one microsecond. As a result, there will be a considerable amount of beam charge which is mismatched to the magnetic transport channel. Depositing some of this charge near accelerating gaps could potentially cause insulator flashover. Deposition near the thermionic cathode could lead to cathode poisoning by desorbed neutrals and ions. The approach that has been adopted is to transport the beam-head without loss through the first eight cells, and then eliminate most of the beam-head in a "beam cleanup zone" (BCUZ). This consists of a 3 m section of beam pipe, two apertures, and three static solenoids. We describe two different beam tunes through this region: an aggressive "head cleanup" tune which removes the low-energy beam-head at the expense of a large local envelope modulation in the beam-body, and a "smooth" tune, which transports the beam-body with minimal envelope modulation. Calculations of emittance growth, electron energy deposition, secondary electron emission, and surface neutral and ion desorption in the BCUZ for each tune will be presented. We will also show the effect on transport through the rest of the accelerator. The calculations make use of an envelope code (LAMDA), beam-slice code (SPROP), and particle-in-cell code (LSP)