Nonideal effects in the operation of a paraxial diode with gas cell focusing

Paraxial electron diodes with gas transport cells have been used to focus intense electron beams onto a high-Z target producing bremsstrahlung radiation. 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. Non-ideal effects result in an axial sweep of the beam focus position that ultimately limits the radiographic spot. These effects include cathode plasma evolution, stimulated emission of anode ions, and magnetic diffusion in the gas cell. The gas breakdown is studied with hybrid particle-in-cell simulations using several different gas chemistry models including kinetic Monte Carlo interaction. In addition, we test the validity of these models against measured time- dependent beam spot and radiation production rates.