Electron distribution in a gridded Penning discharge

Abstract

Electron distributions in a Penning discharge, consisting of two glow discharges separated by a cylindrical microwave cavity with gridded ends, were measured using the shift of resonant microwave frequencies and Langmuir probe data. The shape and magnitude of the distributions were determined for a range of pressure, discharge current, magnetic field, and for several cathode shapes. The microwave technique gives the distribution as a two-parameter function, and the probe data gives a more detailed profile but quantitatively less reliable. All data indicated that the plasmas were electron rich. The distributions were azimuthally symmetric with a maximum at the center at higher pressures; at lower pressures and higher magnetic fields, the densities were depressed in the center, indicating the development of a circulating sheath of electrons. The data indicates that the microwave diagnostic technique is quite suitable when the distribution is a maximum at the center, and it is a useful procedure for calibrating a probe when there is no suitable probe theory for a particular range of experimental parameters.