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It has been shown that the source of current and erosion plasma in a unipolar arc is explosive electron emission, which occurs as ejection of individual portions of electrons named ectons. This phenomenon is responsible for the numerous microcraters left by unipolar arcs on metal surfaces. An arc of this type is self-sustained due to the interaction of the erosion plasma with the electrode surface. The duration of an arc is determined by the conditions of its initiation: the higher the arc current, and hence, the number of cells in the spot formed on arc initiation, the longer the arc operation.

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The conditions under which runaway electrons are generated in a gas diode with a strongly nonuniform electric field created by electrodes of specific geometry have been investigated. For an edge cathode, the equation of motion for electrons has been solved analytically. According to the solutions, for electrons to run away at the periphery, in the low field region, it is necessary that the applied potential difference be greater than a certain threshold determined by the electrode gap spacing and by the parameters of the gas. This condition supplements the classical electron runaway condition according to which the field strength near the emitting edge of the cathode should exceed some value depending only on gas parameters. It turned out that for a sharp-edged cathode, the new condition imposes more stringent requirements on the field strength compared with the classical one. Our calculations are supported by experiments in which electron runaway conditions were determined for a set of cathodes with different edge radii.