Particle simulation of a virtual cathode in bipolar flow*

Virtual cathode formation is studied in a bipolar flow relevant to the High Current Experiment parameter regime. A 174 mA potassium ion beam with beam energy of 972 keV, confined in a conducting drift tube, is incident on a stainless steel plate. The ion impacts generate secondary electrons with an energy and angular dependent yield large compared to 1, resulting in a current sufficient for formation of a virtual cathode. Using enhanced ion-induced secondary models in the PIC codes XOOPIC and WARP, the formation of the virtual cathode and its effect on the bipolar flow is investigated, including time-dependent effects. Numerical issues including resolution of the virtual cathode scale length $l_{vc} \sim \lambda_{De}$ and time scale $t \sim 1 / \omega_{pe}$ are characterized. Results indicate an improvement in temporal and spatial fidelity leads to a lower and more stable virtual cathode potential as well as significant impact on neutrality in the bipolar flow.