A Study of the Cathode-Spot Operation on Tungsten Point Cathodes in the Range of Threshold Arc Currents

The paper reports some experimental results on measuring the lifetime and the current density of the cathode spot initiated on the tip of point cathodes. The point taper angle was varied in the range of 13-80°, the arc current was 0.5-4 A. The generator total voltage was not above 50 V. It is shown that the cathode-spot lifetime is independent of both the point taper angle and the current and is in the range of 50-250 ns. By the moment of arc extinction, the current density amounts to (0.3-2) × 108 A/cm2 at the voltage across the arc gap 20-30 V. The experimental results are compared with those calculated using a Joule model that takes into account the displacement of molten metal from the emission region under the action of the pressure of evaporating atoms. Some possible reasons are given for the disagreement of experimental results with theoretical predictions.