A simplified 2-D fluid model of plasma formation under pulsed high power microwaves in atmospheric gases

Summary form only given. The mechanism of plasma formation under high power pulse (HPM) excitation in gases (nitrogen and argon) at atmospheric pressures is studied utilizing a fluid model verified against experimental data. A 2-D approximation was introduced to model the cylindrically shaped gas volume and the associated electric fields. Thus, the fluid equations and Poisson equation for space charge effects are updated in x and y directions only (setting the gradient in the z-direction to 0). The goal of this numerical simulation was to accurately predict the plasma formation delay time under different gas types and pressures based on the calculated evolution of the plasma conductivity.An S-band TE111 resonator with a built-in quartz gas isolation tube in the center was fabricated1. A 4 MW, 4 μs pulse in the dominant TE10 mode provided by a 2.85 GHz magnetron propagates through it. In the fluid model, the effect of HPM on plasma formation is modelled with particle heating as well as elastic and inelastic collisions driven by the incident electric field in the vertical (normal) coordinate. The amplitude distribution of the HPM electric field is obtained from numerical simulation of the resonator using commercial EM software. Since the focus is on the onset of plasma formation, simulation is stopped before a large plasma density develops up to the moment when the transmitted power is reduced by 10% (-0.5 dB), which corresponds to an average plasma conductivity of 0.02 S/m. Both measurement and simulation cover a gas pressure range from 25 to 700 torr with delay times for N₂ and Ar from 22 to 204 nanoseconds. The development of the particle densities and temperatures during the simulation is presented to reveal the dominant mechanism of plasma formation in atmospheric gases. The delay times for different gas types and pressures are in good agreement for low gas pressures (<; 200 torr). Deviations between the model and experiments at higher pre- sure are found to be primarily due to the onset of plasma filamentation.