The Characteristics and Function of Anode Spots in Glow Discharges

Abstract

Brilliant hemispherical anode spots appeared on a small disk or spherical anode mounted at the center of a large copper spherical discharge chamber which served as cathode during an investigation of nitrogen glows. Such an arrangement permits large anode current densities with a normal cathode fall in potential. At a critical current density at the anode, the spots form and arrange themselves in beautiful geometrical patterns. In nitrogen the spots appeared only in the pressure range of 0.15 to 1.50 mm Hg. Space potentials, electron temperatures, electron and ion densities determined by a Langmuir probe analysis revealed that the spots are regions of intense ionization, and that the region just above them contains a net positive space charge. The potential fall through a spot is nearly equal to the first ionization potential of nitrogen. A method was developed for growing anode spots at will on a small disk probe mounted flush with the anode surface but insulated from it. The current-voltage characteristics for this probe, or auxiliary anode, exhibit peaks which account for the stability and size of the spots. An investigation of the current-voltage relationships of the entire discharge showed that anode spots occur chiefly in the region in which the current increases with small change in voltage. At sufficiently great current densities the spots move about because of the magnetic field accompanying the discharge. The function of the spots is to furnish the positive ions necessary for the maintenance of a stable discharge in the plasma extending from anode to negative glow and to aid in the collection of electrons at the anode. A mechanism for the formation of the spots as a combined space charge and bipolar current phenomenon is proposed.