Electron-Beam Probing of a Penning Discharge

Abstract

An electron beam probe has been used to study the distribution of electric fields within the cold-cathode Penning discharge. Three basically different potential distributions have been observed, in addition to the high pressure mode, which could not be studied with this technique. At the lowest pressures, the potential is roughly parabolic. As the pressure increases, a region develops in the center of the discharge, in which there is no electric field, that is, the potential profile is flat. Upon further increase in pressure, this central portion enlarges until it fills the majority of the discharge. At a pressure in the vicinity of 10−4 mm Hg the mode abruptly changes to one in which there is very little electric field throughout the discharge, although there are reasons to suspect a narrow region of high electric field near the anode. At still higher pressures (about 10−3 mm Hg) there is another abrupt change into the high pressure mode which acts essentially like a magnetically confined glow discharge. Due to instrumentation difficulties, and end effects of considerable magnitude, it was not possible to measure numerical values for the electric field and potential, but their qualitative behavior has been plotted over a range of pressure, voltage, and magnetic field which is characteristic of the device.