Plasma focused electron beam for the pasotron

The pasotron (plasma-assisted slow-wave microwave source) is a high power microwave source which uses a plasma lens in place of external magnetic fields to focus a beam within a slow wave circuit. The pasotron operates in the vicinity of 40-55 kV, at 40-50 A, with power output as high as 1 MW with 40% efficiency. Pulse lengths of up to 100 microseconds have been sustained. The plasma is formed via electron impact with an expanding background gas, where the gas density profile decays with distance from the plasma cathode due to diffusive effects. The present model includes the self consistent generation of the plasma lens via electron impact. The focusing mechanisms include self-consistent electron and ion space charge as well as the self magnetic field of the beam electrons. The XOOPIC code, employing a particle-in-cell Monte-Carlo collision model is compared to theoretical and experimental results. The radial profile of the beam along the axial distance from the cathode varies with operating parameters such as voltage, current, and gas pressure profile and gas constituent. Also examined are the noise properties of the beam in the absence of RF.