Cathode Drop

a relatively rapid potential drop near the cathode in an electrical discharge in a gas. It is usually brought about by an excess of positive ions at the cathode created by a positive space charge, which shields the cathode. However, in some forms of non-self-sustained electrical current in a gas where there is intense electron emission from the cathode, a cathode drop is developed by the negative space charge (an excess of electrons); such a cathode drop limits emission and inhibits further increase of the space charge.

The main processes that make possible the flow of electric current in a gas operate in the zone of cathode drop and its immediate vicinity. The essential differences among the various forms of gas discharge result from the particular features and differences of these cathode processes. The qualitative feature of the processes in the zone of cathode drop is manifested quantitatively in the value of the cathode drop that is specific for the particular form of discharge. For example, a low cathode drop, of the order of the ionization potential of the gas and lower (1–10 volts), is the most typical feature of an arc discharge, and a high cathode drop (many hundreds of volts) distinguishes a glow discharge from the other types of current in a gas. (As seen from opposite the cathode, the zone of cathode drop adjoins the quasi-neutral plasma interval called a positive column in the arc discharge and the area of the “negative glow” in a glow discharge.) The specific value of cathode drop is a function of the kind of gas, the cathode material, and the condition of the cathode surface. It is independent of the distance between the electrodes and the magnitude of the discharge current over a wide range of values. However, at sufficiently high currents it increases sharply (anomalous cathode drop), to many dozens of volts in an arc discharge and to several thousand volts in a glow discharge.