Hybrid Modeling Study On High-Power Microwave Propagation In The Atmosphere

High-power microwave has important applications in military and civil fields. breakdown threshold of background gas such as air. In this case, the gas breakdown occurs easily. Once the gas breakdown occurs, the generated plasma strongly hinders the propagation of the high-power microwave. Therefore, it is very important to study and understand the high-power microwave propagation in the atmosphere. In this dissertation, we use the electron fluid model combined with other models to study the interaction of the high-power microwave and the plasma formed in gas breakdown. The fluid model includes Maxwell’s equations, the continuity equation of the electron density, the balance equation of electron fluid momentum and the balance equation of the electron fluid energy. The electron fluid model has the charcteristics of speed and simplicity, and it is able to study the influence of high-density plasma formed in gas breakdown on the microwave propagation, which may be not carried out by using the Particle-in-cell-Monte Carlo collision (PIC-MCC) model.The one-dimensional electron fluid model and two-dimensional electron fluid model are solved numerically by the finite-difference time-domain method. It is worth emphasizing that, in the iterative comptations, the balance equation of the electron fluid energy and the continuity equation of the electron density are coupled nonlinearly, and the Aitken iteration method is adopted to solve the electron energy and electron density. At each time step, not only the electromagnetic and electron fluid components are calculated, but also the transport coefficients such as the ionization frequency are updated by introducing the mean electron energy into the corresponding formulas. To verify these algorithms, the predicted breakdown threshold is compared with the experiment data, showing good agreement.The electron fluid model requires specification of the electron energy distribution function (EEDF), by which the transport coefficients can be estimated. The EEDF is generally assumed to be the Maxwellian distribution that leads to inaccurate transport coefficients and results in errors in plasma parameter prediction when the EEDF deviates observably equilibrium. To improve the accuracy of the fluid model, we use other models to develop two methods for improving the EEDF. One is to introduce the effective formula of the EEDF, whose shape parameter is dependent on the gas composition and microwave frequency and can be determined based on the PIC-MCC results. The second is to find EEDF by directly solving the electron Boltzmann equation. The improved EEDFs obtained by the two different methods are introduced into the electron fluid model, respectively, and the breakdown time is predicted, which is very well matched with PIC-MCC results. We also confirm that the results of the fluid model with the Maxwellian distribution become poor compared with those of PIC-MCC when the EEDF deviates obviously equilibrium.By using the model and its algorithm mentioned above, several transisent problems of the high-power microwave propagation are studied. Results show that, the phenomenon of pulse shorting is absent due to the low saturation electron density produced in the breakdown of mixture of air and SF6-With the ratio of SF6 in the mixture increasing, the breakdown threshold at high pressures increases significantly. At low altitudes, the mean electron energy is low, and the air breakdown occurs difficultly. As the altitude enhances, the mean electron energy increases and occurrence of the air breakdown becomes easy. The produced plasma in air breakdown strongly reflects and absorbs the tail of the pulse. Through PIC-MCC simulations, it is confirmed that the EEDF for a rectangular microwave pulse directly derived from the Boltzmann equation solver Bolsig+can well approximate those for non-rectangular pulses. The time evolution of a non-rectangular pulse breakdown is analyzed and the effect of the shape of the incident pulse on breakdown is considered. Similar filamentary plasma arrays propagating toward the source are formed at atmospheric pressure at different microwave frequencies. With the microwave frequency decreasing, the ratio of the distance between two adjacent plasma filaments to the corresponding wavelength almost remains unchanged (on the order of 1/4), while the formation time of the new plasma filament in upstream increases significantly. The air breakdown mainly occurs near the dielectric window of the circular waveguide due to the high electric field, and the generated nonuniform plasma causes significant attenuation in the radiated field. The wave reflection by the air plasma, far larger than the plasma absorption, attenuates the transmitted power significantly at low pressures, while at high pressures, the plasma absorption plays a major role in the transmitted power loss.