Noninvasive observations of primary (beam) electrons near the electrode of a low pressure discharge

Summary form only given. Using a millimeter wave (150 GHz) interferometer, we observe primary electrons, indirectly, in discharges with sufficiently fast time varying current (e.g., discharges in fluorescent lamps driven at several kilohertz or greater). Measurements of electron density along a path that passes near the electrode reveal variations in the bulk electron density that are synchronous with variations in the driving current. Similar measurements in the positive column show no such variations at these frequencies. Presumably, these density variations near the electrode are due mainly to variations in the primary electron impact ionization rate which, in turn, are due to variations in the sheath voltage. Variations in sheath voltage are a consequence of the time varying discharge current because they allow for the time varying emission current at the electrode surface. Thus, interferometer measurements near the electrode offer a way of noninvasively probing the sheath voltage in time varying discharges. We use such measurements to understand the physical mechanisms occurring in the negative glow plasma and at the electrode/plasma interface.