Microwave diagnostics of the electron density in molecular mixture plasmas

The electron density in the positive column of DC glow discharges has been determined by means of an improved method of the microwave diagnostics. Thi s method is based on the measurement of both the resonance frequency shift of the resonator cavity and the change of the inverse resonator quality It makes explicit use of the associated phase delay between the microwave fiel d and the induced HF electric current. To evaluate the applicability of the method and of a simplified version of it to complex molecular mixture plas mas, the electron density in CO2/CO/O-2 mixture plasmas has been measured f or larger ranges of gas pressure and discharge current. The applicability c heck is made by a comparison between measured and calculated results for th e phase delay. The good agreement between experimental and theoretical resu lts supplies evidence for a far-reaching applicability of the simplified ve rsion of the improved microwave diagnostics method that allows the determin ation of the electron density without detailed information about the plasma composition and the electron energy distribution. The theoretical determination of the phase delay has been done by solving t he time-dependent electron Boltzmann equation. Two different techniques hav e been employed to solve this kinetic equation. In particular, the conventi onal two-term treatment has been analysed with respect to its suitability t o molecular gas mixtures and has been found to be sufficient for the determ ination of the phase delay.