An R.F. Probe Technique for the Measurement of Plasma Electron Concentrations in the Presence of Negative Ions

Abstract

A method of measuring the electron population in the presence of both positive and negative ions has been developed. The method is based on the measurement of the local plasma electron resonance frequency, from which the free electron density may be calculated on the assumption that the relative influence of the ions is negligible. The validity of this technique has been confirmed by checking its results with those obtained by the Langmuir probe method in a d.c. discharge in noble gases; the limitations of the method and some factors which influence the measurements have also been noted. It was found that the method can be used in a continuous high-frequency electrodeless discharge where the normal wire probe methods are of doubtful validity. The technique has been applied to a study of the decay of electron density in the afterglow of a pulsed discharge in iodine vapour; the cross section of electron attachment σ was found to be 7.5±0.5×10-18 cm2 for electrons at an estimated temperature of 8000°K.