Characteristics of the Glow to Arc Transition in Mercury Vapor

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Abstract
The probability of the transition from glow discharge to arc discharge at graphite electrodes has been investigated under carefully controlled cathode conditions. The transition probability was found to increase in an approximately linear fashion with an increase in cathode current density and glow voltage. The curve of transition probability as a function of cathode temperature was found to have a distinct maximum at a temperature of about 400° or 450°C. The probability of the transition occurring at temperatures 50° lower or higher was not more than about one-fourth the maximum. It was also found that the frequency with which the glow to arc transitions occurred depended greatly on the previous history of the discharge and the amount of high voltage positive ion bombardment experienced by the cathode under test. A possible explanation of these results has been based on the hypothesis that it is never possible to de-gas completely the cathode electrode even after prolonged operation in a high voltage discharge. It has been shown that the nonuniform emission of this gas from the cathode in the form of bursts from minute patches can lead to a greatly increased probability for transition from glow to arc.

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