The Role of the Cathode in Discharge Instability

Abstract

It is shown that a highly stressed conditioned cathode in a gas will, by positive space charge action, produce an autocatalytic increase in current through a chain reaction leading to complete breakdown. Depending on conditions in the circuit this common tendency will explain a number of phenomena. In pure free electron gases, the discharge has a negative characteristic and must be controlled by external resistance or else lead to an arc. In the presence of adequate low field regions and moderate negative ion formation, a current controlled by the internal space charge resistance of the system results. Where negative ion formation is heavy and low field regions exist, the discharge will choke itself off and be intermittent. In cases where there are large electrically elastic columns of ionized gas at lower pressure that can be swept by ionizing potential waves, as in glow discharges, the cathode instability leads to pulsations or moving striations and is the cause of setting up and sustaining plasma oscillations. Among the consequences of such action where intermittent or pulsed discharges occur, there will be a Faraday dark space and perhaps striations. Faraday dark spaces will occur only where cathode instabilities cause current fluctuations and should not be observed in more stable discharges.