The origin of current chopping in vacuum arcs

A semiempirical model is presented which relates the phenomenon of current chopping of vacuum arcs to rapid microscopic processes on the cathode surface. First, for a resistive circuit the approximate chopping current as a function of power-current amplitude was calculated using two DC arc lifetime parameters which were obtained experimentally. Secondly, it is shown that spontaneous arc extinction is the result of an accumulation of characteristic arc instabilities, each of which can be described as a randomly occurring transient arc resistance. Qualitatively, the 'severity' of these instabilities is demonstrated to depend on arc current, arc length, and circuit parameters. Thirdly, the presence of instabilities seems to be a manifestation of an ion deficiency in the near-anode region. This deficiency may be due to a discontinuity in the ionized mass flow into the plasma, occurring at a failure to initiate a new cathodic emission site in time. Based on these findings, the nature of the current-chopping phenomenon, hitherto explained by a minimum cathode spot operation current, can be coupled to the dynamics of the spot.<
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