Particle in cell simulations of a Tom Martin diode

Summary form only given. The Tom Martin (TM) radiographic diode comprises of a cathode ball mounted on a stalk, separated by vacuum from a flat plate metal anode target. A 2D model of this configuration, as fielded on AWE pulsed power machines, has been created using the particle in cell (PiC) code, large scale plasma (LSP). Predicted outputs have been compared to measured diagnostics in the voltage range of 3 to 6.5 MV. Modelling the cathode ball as a space charge limited source of electrons, qualitative agreement between the predicted and measured impedance histories is found. The model output is shown to be highly dependent on the size of the cathode emission region, with a range in the predicted diode impedance of 100 Omega. Coupled with approximate analysis methods and large experimental uncertainties, this dependence results in a range of diode behaviours that are consistent with the measured data. In this model, ion creation and emission from the target surface is neglected based on experimental observations. This assumption is tested against LSP target surface temperature predictions and found to be consistent over the voltage range considered.