Microwave diagnostics of a repetitive, short-pulse-sustained, weakly ionized, air plasma under the influence of a magnetic field

摘要

A microwave-transmission-based diagnostic method is presented here, applicable to plasmas having electron collision frequencies up to about twice their electron plasma frequency, and under the influence of an applied magnetic field. This technique is capable of measuring both electron number density and collision frequency. By varying the intensity of the applied magnetic field, the frequency of the upper hybrid resonance for transmission of extraordinary waves, as predicted by the Asher-Appleton-Hartree dispersion relation, is scanned through the microwave diagnostic frequency. Qualitatively, the zero transmission location of the resonant band depends on the electron number density, and the existence of the band depends on the collision frequency. Because there is essentially zero transmission through the resonant band, the measurement is accomplished by determining the ranges of microwave frequencies and magnetic field intensities that yield no transmission. In contrast to more standard techniques, the results presented here do not rely on accurate measurement of the transmission fraction or phase shift. The technique relies instead on measuring the applied field strengths and diagnostic frequencies that yield an opaque plasma. The opacity of the plasma can be robust with respect to the refraction, diffraction, multiple reflections, and impedance matching that can plague accurate measurements of microwave transmission fraction and phase shift, particularly in the case of small plasmas with near field geometries.