Diode and final-focus simulations for DARHT

Beam dynamics calculations for the injector and final-focus region of a 4 kA, 20 MeV linear induction accelerator are presented. The injector is a low-emittance 4 MeV thermionic or photocathode diode designed to produce four 70 ns pulses over 2 μsec. Due to the long total pulse length, we have kept the field stress to <200 kV/cm over the cathode electrode, and to ≈50 kV/cm on the radial insulator stacks. The normalized edge emittance produced by the diode is only≈0.019 cm-rad. In the final-focus region, we have modelled the effect of ion emission from the target. The intense electric field of the beam at the 1-mm-diameter focal spot produces substantial ion velocities, and, if the space-charge-limited current density can be supplied, significant focal spot degradation may occur due to ion space charge. Calculations for the existing Integrated Test Stand, which has a larger focal spot, show that the effect should be observable for H⁺ and C⁺ ion species. The effect is lessened if there is insufficient ion density on the target to supply the space-charge-limited current density, or if the ion charge-to-mass ratio is sufficiently small