Towards a fully kinetic 3D electromagnetic particle-in-cell model of streamer formation and dynamics in high-pressure electronegative gases

Abstract

Streamer and leader formation in high pressure devices is dynamic process involving a broad range of physical phenomena. These include elastic and inelastic particle collisions in the gas, radiation generation, transport and absorption, and electrode interactions. Accurate modeling of these physical processes is essential for a number of applications, including high-current, laser-triggered gas switches. Towards this end, we present a new 3D implicit particle-in-cell simulation model of gas breakdown leading to streamer formation in electronegative gases. The model uses a Monte Carlo treatment for all particle interactions and includes discrete photon generation, transport, and absorption for ultra-violet and soft x-ray radiation. Central to the realization of this fully kinetic particle treatment is an algorithm that manages the total particle count by species while preserving the local momentum distribution functions and conserving charge [D. R. Welch, T. C. Genoni, R. E. Clark, and D. V. Rose, J. Comput. Phys. 227, 143 (2007)]. The simulation model is fully electromagnetic, making it capable of following, for example, the evolution of a gas switch from the point of laser-induced localized breakdown of the gas between electrodes through the successive stages of streamer propagation, initial electrode current connection, and high-current conduction channel evolution, where self-magnetic field effects are likely to be important. We describe the model details and underlying assumptions used and present sample results from 3D simulations of streamer formation and propagation in SF6.

Kinetic simulation studies of laser-triggering in the Z gas switch

“…The basic PIC Monte Carlo (MCC) model 14,15 utilized here is described in detail for breakdown of SF 6 in Rose et al 6 This model, including electron elastic, inelastic, and ionization processes with neutrals, was initially benchmarked for noble gases, such as He and Ar, in which no significant electron sink is present. For an electronegative gas such as SF 6 , electron attachment processes play a key role in inhibiting breakdown at low field values.…”

“…Relatively high energy photons created by relaxation (using 0.3 ns decay time suggested in Ref. 6) of the excited state neutrals can then be propagated through a photon transport model. These photons, propagating at the speed of light, generate electron-ion pairs by neutral photoabsorption and provide a non-local source of plasma generation in the vicinity of the streamer front.…”

“…We note that there are many dissociative neutral and ionic fragments which can occur in SF 6 reactions. 16 For computational efficiency, however, a relatively simple chemistry model for SF 6 was developed. The model can easily be extended to include additional reaction channels and particle species beyond those described here.…”

“…The simulations presented here extend a recently developed Monte Carlo model of electronegative gas breakdown and streamer propagation. 6 Here, the model is applied to the laser-triggered gas switches used on the Z pulsed-power facility at Sandia National Laboratories, 1,7 where SF 6 is used as the working gas in order to provide higher voltage switch operation. The goal is to develop 2D and 3D models capable of treating the switch performance through the successive stages of laser-initiation of one or more thin plasma filaments between the electrodes, the subsequent streamer/leader propagation that provides the initial current contact between the electrodes, and finally the formation of a high-current conduction channel.…”

“…Within the framework of the LSP PIC code, 12 a Monte Carlo treatment for all particle interactions is used that includes discrete photon generation, transport, and absorption for ultra-violet (UV) and soft x-ray radiation. 6 The laser-triggered gas switch operation relies on a sequence of conditions in the trigger gap:…”