a type of stable, self-maintained electrical discharge in a gas. A glow discharge occurs at a low cathode temperature and is characterized by a comparatively low current density at the cathode and a high cathode drop U, which may be of the order of a few hundred volts. Although such discharges may occur at gas pressures p as high as atmospheric pressure, most investigations of the glow discharge have been carried out at pressures ranging from a few hundredths of a millimeter to several millimeters of mercury.

In a glow discharge electrons are emitted by the cathode primarily as a result of bombardment by positive ions and fast electrons; the photoelectric effect and the energy of metastable atoms, however, also play a certain role in electron emission. At

Figure 1. Appearance and parameter distribution of a normal glow discharge at a relatively low pressure: (1) cathode, (2) Aston dark space, (3) cathode glow, (4) cathode dark space, (5) negative glow, (6) Faraday dark space, (7) positive column, (8) anode region, (9) anode

pressures of the order of tenths of a millimeter of mercury or higher, a glow discharge in a long cylindrical tube is typically divided into a number of regions that differ markedly in appearance (Figure 1). The formation of these regions can be explained by reference to the characteristics of the elementary processes of ionization and excitation of atoms and molecules. The most important region, which determines the very existence of the glow discharge under the given conditions, is the cathode dark space. In this region collision ionization by electrons results in the formation of positive ions, which cause the emission of electrons by the cathode.

The voltage between the electrodes depends primarily on two parameters: the current density j at the cathode and the product pi of the pressure and the distance l between the electrodes. A general classification of the various forms of glow discharge was established in the investigations of the Soviet scientist B. N. Kliarfel’d and his students. The classification extends to the case of very small values of pl and j, where no space charge is present between the electrodes and the field is practically uniform. In Kliarfel’d’s terminology this case is called the simplest (prosteishii) glow discharge. Here, the separate regions mentioned above are absent, and the gas is ionized by electrons throughout the space between the electrodes. If pl and j are increased, two forms of glow discharge can be obtained: the normal glow discharge and the abnormal glow discharge. In the case of the normal glow discharge, electrons are emitted from only a part of the cathode surface, and j and U remain constant. An increase in current causes emission to occur from a larger part of the cathode. The abnormal glow discharge is observed at high values of j. It is characterized by a rapid increase in the voltage between the electrodes as the current is increased.

A special type of glow discharge can be obtained with a hollow cathode (a cathode consisting of a hollow cylinder or two parallel plates). In such a discharge the electrons undergo multiple oscillations between the walls of the cathode and intensely ionize the gas. Hollow-cathode glow discharges exhibit much higher current densities and much brighter glow regions than do ordinary glow discharges.

The properties and characteristics of glow discharges are made use of in technology in, for example, voltage regulator tubes.