Effect of ambient gas species on microwave breakdown pattern

The air, nitrogen, and helium breakdown caused by microwaves at atmospheric pressure are, respectively, investigated by solving the theoretical model, which consists of Maxwell's equations and plasma fluid equations. The transport coefficients (e.g. an ionization frequency) are predicted by the Boltzmann equation solver BOLSIG+. The effective diffusion coefficient that describes properly the transition from the free diffusion to the ambipolar diffusion is introduced into the plasma fluid equations. The two-dimensional simulation results show that a filamentary plasma array is produced in the air and nitrogen breakdown, but it disappears completely in the helium breakdown. This is because the transport coefficients and wave scattering by the plasma differ from gas to gas. The simulated plasma patterns in the three different gases agree qualitatively with the experimental results. In addition, the plasma propagation speed in the nitrogen breakdown from the simulations is close to the experimental data.

Automated summary

The breakdown phenomena of air, nitrogen, and helium by microwaves at atmospheric pressure were studied by solving the theoretical model. Simulation results showed that filamentary plasma arrays were generated in air and nitrogen breakdown, but disappeared completely in helium breakdown due to different transport coefficients and wave scattering behaviors. The qualitative agreement between simulated plasma patterns in the three gases and experimental results was observed, with the plasma propagation speed in nitrogen breakdown simulation matching closely with experimental data.