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. This 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 field and the induced HF electric current. To evaluate the applicability of the method and of a simplified version of it to complex molecular mixture plasmas, the electron density in CO 2 /CO/O 2 mixture plasmas has been measured for larger ranges of gas pressure and discharge current. The applicability check is made by a comparison between measured and calculated results for the phase delay. The good agreement between experimental and theoretical results supplies evidence for a far-reaching applicability of the simplified version of the improved microwave diagnostics method that allows the determination 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 the time-dependent electron Boltzmann equation. Two different techniques have been employed to solve this kinetic equation. In particular, the conventional two-term treatment has been analysed with respect to its suitability to molecular gas mixtures and has been found to be sufficient for the determination of the phase delay.