3D calculations for a 4 kA, 3.5 MV, 2.5 microsecond injector

The DARHT-2 machine under construction at Los Alamos National Laboratory requires a long-pulse (2.5 μsec) 2-4 kA, 3.5 MV, low-emittance electron beam source. The injector is being designed at LBNL and consists of a large-area thermionic cathode mounted atop a vertical column. The 90° bend between the horizontally emitted beam and the column produces dipole and higher-pole fields which must be corrected. In addition, the fast rise of the current flowing into the vacuum rank excites RF modes which cause transverse oscillations of the beam centroid. We have modeled these effects with the 3D electromagnetic code LSP. The code has models for pulsed power transmission-lines attached to boundaries, space-charge-limited emission and transport of charged particles, externally applied magnetic fields, and frequency-dependent absorption of RF. We calculate the transverse displacement of the beam as a function of time during the current pulse, and the positioning and thickness of ferrite absorber needed to damp the RF modes. The numerical results are compared to analytic calculations