Investigation of a low pressure penning discharge

Abstract

A Penning discharge at low pressure ( P < 10 -4 torr) with long anode cylinder is investigated, both theoretically and experimentally. A classification and short description are given of the various Penning modes and corresponding characteristics are presented. An extensive treatment is given of the two most important modes at low pressure: one at low magnetic field (LMF) and the other at high magnetic field (HMF). The theory for the LMF-mode is based on Poisson's equation and the continuity equation for electrons, supplemented with expressions for the first Townsend coefficient α and the ionization frequency vi. The limits of the mode, viz. the extinction at low magnetic field and the transition to the HMF-mode, are described and calculated. The analogous theory for the HMF-mode is only valid in a small stable region of the discharge. A formula is derived for the ion density in the anode sheath, which plays a dominant part in the transition to the high-pressure mode. The transition pressure and the extinction voltage are also evaluated. The apparatus and the domain of experimental parameters used in it are briefly described. Operating characteristics, extinction and transition characteristics are presented, which show a good agreement with theory. In particular, the dominating influence of the thermal velocity of neutral particles on the transition to the high- pressure mode is demonstrated.