Challenges in numerical simulation of nanosecond-pulse discharges

Nanosecond-pulse electrical discharges offer an efficient means of plasma generation in applications, but accurate numerical simulation of these discharges remains extremely challenging. The continuing difficulties lie in an enormous separation of space and time scales, a lack of transport and kinetic data, and extreme nonequilibrium physics. In the face of these challenges, we present an example of good practice in selecting the physical model and comprehensively checking numerical accuracy. We focus on a particular discharge experiment, and illustrate how simulations can provide useful guidance for ongoing experimental work, despite the difficulty of the simulations. The target experiments were carried out in a planeto-plane electrode configuration with a 20mm gap in 400 Pa (3 Torr) argon using 3 ns, 850V pulses with a 30 kHz pulse repetition frequency. The model employed the drift-diffusion approximation for species motion, and the self-consistent electric field was obtained through the solution of the Poisson equation. The baseline physical model utilized the local field approximation. In an extended model, non-local-field effects on the electron temperature were investigated by solving a simplified electron energy equation. Calculations were carried out for both a pure argon kinetic model and an argon-water model. The model generally underestimated the measured electron number densities, but the inclusion of additional physical effects helped to reduce the discrepancy with experiment. These results represent a step toward efficient modeling of pulsed electrical discharges for applications to combustion enhancement, flow control, and plasma antennas.