Recent experimental studies of 110 GHz microwave air volume breakdown have shown that as pressure is decreased from atmosphere to a few torr, the discharge transitions from a well-defined array to a smeared-out array and finally to a diffuse plasma. In this letter, a two-dimensional model coupling plasma fluid equations and Maxwell equations is used to simulate the pressure dependence of plasma structure and formation under conditions similar to the experiments. The modeling pressure dependence of plasma structures in H plane and E plane is qualitatively consistent with the experimental observation. The quantitative discrepancy between modeling and experimental results is explained. The simulations allow a good understanding of plasma structure at different pressures in microwave volume breakdown.

Plasma cutting arc characteristics are investigated for different turbulence models, i.e. the Reynolds stress model (RSM), the k–ϵ model and its variants, the renormalization group (RNG) k–ϵ model, the RNG k–ϵ model taking into account the low Reynolds number effect and the realizable k–ϵ model. The results of the RSM and the RNG k–ϵ model taking into account the low Reynolds number effect are in reasonable agreement with experiment. They both predict very close voltage, shock wave location and temperature variation along the axis to experiment. On the other hand, the other three models overestimate the turbulence effects and predict much lower velocity and temperature, especially the standard k–ϵ model, which predicts that the temperature is about 10 000 K lower than the experiment in certain plasma jet regions.

Some interesting experiment phenomena of light patterns that appear in microwave plasma have been investigated. The wavelength of surface plasmon polaritons (SPPs) at the dielectric-plasma interface as the functions of incident wave frequency and plasma density has been calculated. The comparison of the experiment results with the calculating results has been carried out. By experimentally and theoretically analyzing, the phenomena are considered to be caused by the surface wave of SPPs at the interface between Pyrex wall and plasma.

The influence of nozzle length and two process parameters (arc current, mass flow rate) on the plasma cutting arc is investigated. Modeling results show that nozzle length and these two process parameters have essential effects on plasma arc characteristics. Long nozzle torch can provide high velocity plasma jet with high heat flux. Both arc voltage and chamber pressure increase with the nozzle length. High arc current increases plasma velocity and temperature, enhances heat flux and augments chamber pressure and thus, the shock wave. Strong mass flow has pinch effect on plasma arc inside the torch, enhances the arc voltage and power, therefore increases plasma velocity, temperature and heat flux.