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Modeling of Plasma Dynamics and Pattern Formation During High Pressure Microwave Breakdown in Air

Guo-Qiang Zhu

出版	2012
URL	http://books.google.com.hk/books?id=kZz_kQEACAAJ&hl=&source=gbs_api

註釋

In this thesis, a model for the plasma dynamics after microwave breakdown at atmospheric pressure in air has been developed. The model has been able to explain for the first time the formation and dynamics of self-organized structures during microwave breakdown. Microwave breakdown in air at atmospheric pressure has been recently observed at MIT with high power microwave sources and fast CCD cameras. The measurements show that a self-organized multi-streamer array forms and propagates towards the incident microwave source with a high velocity (several km/s) during the discharge. The detailed dynamics of the self-organized streamer structures during microwave breakdown is still not well understood and the objective of this thesis was to clarify the physics of the plasma dynamics and self-organization during and after microwave breakdown. In order to study the plasma dynamics in microwave breakdown, Maxwell's equations have been coupled to a simple plasma model and solved numerically. The plasma model assumes quasineutrality and describes the evolution of the plasma due to diffusion, ionization, attachment and recombination. Ionization and attachment are supposed to depend on the local effective field. The electron mean velocity is obtained from a simplified momentum equation. The diffusion coefficient must be ambipolar in the plasma bulk but should be equal to the free electron diffusion on the edge of the plasma since the plasma density decays to zero in the front. A heuristic expression of the transition from ambipolar diffusion in the bulk plasma to free diffusion at the edges has been derived and validated with a non-neutral one-dimensional (1D) model based on drift-diffusion and Poisson's equations. The 1D and 2D plasma-Maxwell models have been used to study the plasma dynamics after breakdown in the conditions of the MIT experiments. The numerical results show the formation of self-organized structures or patterns that are in excellent qualitative agreement with the MIT measurements. The formation of the self-organized dynamical pattern can be attributed to the scattering of the microwave field by the plasma. New filaments continuously form in the plasma front due to diffusion-ionization mechanisms. The model shows that the formation of the filamentary plasma array is associated with the standing wave pattern formed by the microwave field scattered by the plasma. In the last part of the thesis we analyze the formation of a single, isolated microwave filament or streamer at the antinode of a standing wave formed at the intersection of two microwave beams. The microwave streamer stretches in a direction parallel to the electric field because of polarization effects. The model results show that the field is strongly enhanced at the tips of the microwave filament and that the field intensity is modulated in time as the streamer length increases. This modulation is associated with resonant effect when the filament length reached values that are close to multiples of the half wavelength.