Factors affecting the glow-to-arc transition at the cathode of an electric discharge at atmospheric pressure

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Abstract
The glow-to-arc transition has been studied in an electric discharge at atmospheric pressure. Although this transition is a cathode effect, the cathode and the positive column have both been seen to affect the current at which the transition occurs. If the cathode is relatively small, the discharge passes through the abnormal glow region before the glow to arc transition takes place. If the cathode is sufficiently large, increasing the current causes the positive column to contract and the transition takes place directly from the normal glow without passing through the abnormal region. The oscillation phenomenon is explained in terms of the Steenbeck minimum principle and related to the power dissipated at the cathode root, which decreases when the glow changes to an arc. If the power dissipated at the cathode in the arc regime is insufficient to sustain electron emission the arc will revert to a glow. Variation of the slope of the load line alters the frequency of oscillation of the glow-to-arc transition. The current at which the transition occurs can be increased by increasing the area available for the cathode up to a current at which the discharge column begins to constrict the cathode root. The frequency of oscillation is increased by increasing the resistance in series with the discharge. The results are relevant to the minimum condition required to ignite arcs so as to obtain transitions from glow to arc, and to applications of glow discharges in which the maximum current is limited by the glow to arc transition, such as gas lasers and discharge lamps.

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