Comparison of electron densities measured with Langmuir probes and with two different microwave devices

Abstract

Electron densities in the positive column of a low-pressure helium discharge are measured 1) with cylindrical and spherical Langmuir probes; 2) with a precision microwave interferometer and 3) with a microwave cavity. The resulting electron densities agree very closely for the two microwave diagnostics; the results obtained with the probes are about 30% lower. The origin of this difference is discussed. Electron mobilities are calculated from the measured electron densities and electric field. The agreement with theoretical and experimental results of other authors is good for results obtained at 0.5 torr; at lower pressures a gradually increasing difference is found