Mechanism of the Penning Discharge at Low Pressures

Abstract

Experiments have been conducted with a cold cathode Penning discharge in which the sputtering patterns, the direction of ion motion, the energy distribution of the ions, and the E×B electron rotation were studied. It is concluded that below 10⁻⁴ mm Hg the discharge consists essentially of a pronounced, rotating plasma sheath close to the anode, while most of the center portion of the discharge is depressed to near‐cathode potential. At 3000 v and 2500 gauss, an average rotation frequency of the sheath of 70 Mc has been observed. This corresponds to an average field strength in the sheath of 8800 v/cm. An approximate computation indicates that in the presence of fields of this magnitude, classical cross‐field mobility can well account for the anode currents observed. This result differs from the findings of Backus and of Chen and Bingham at higher pressures, where random‐field‐enhanced diffusion according to Bohm had to be postulated to explain the large anode currents.