Simulations of paraxialdiode operation on the 10mv rits-6 accelerator

Summary form only given. Paraxial electron diodes with gas transport cells have been used to focus intense electron beams onto a high-Z target producing bremsstrahlung radiation. The quality of the resulting radiographic X-ray source increases linearly with the dose and inversely with the square of the spot size. This configuration is being fielded on the recently commissioned RITS-6 accelerator producing 35-kA current and 10-MeV energy electron beams. Direct ionization of the beam and avalanche from the electron secondaries drive a break down of the gas that rapidly increases the gas cell conductivity. Because of the delay and incompleteness of the gas breakdown, the gas cells typically operate with a small but finite and slowly increasing net current (sum of the plasma and beam currents). These non-ideal effects result in an axial sweep of the beam focus position that ultimately limits the radiographic spot. Other factors limiting the spot include intrinsic beam emittance, foil and gas scattering, and nonlinear magnetic field evolution in the gas cell. The gas breakdown is being studied with hybrid particle-in-cell simulations using several different gas chemistry models. In this paper, we numerically optimize the beam spot for radiographic utility. In addition, we assess the accuracy of these models via comparison with measured time-dependent beam spot and radiation production rates