Plasma expansion as a cause of metal displacement in vacuum-arc cathode spots

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Abstract
The pressure on the molten surface of a vacuum-arc cathode due to the recoil from outward-directed ion jets is calculated for copper arcs. The calculation uses current density data and electrode force per unit current data from experiment. In addition, the force per unit current measured by Tanberg is shown to be consistent with a calculation of the same quantity based on the energy of outgoing ions measured by Davis and Miller and the so-called saturated positive-ion current measured by Kimblin. The ion recoil pressure is shown to be sufficient to remove molten metal from a cathode-spot crater in time of the order of 25–250 nsec with a velocity of 2×103−2×104 cm sec−1. It is shown that motion of molten metal in the cathode-spot crater must be considered as a first-order effect in rigorous calculations of surface temperature and heat flow in the metal in contact with the cathode-spot plasma. It is suggested that the rapid removal of metal by the plasma pressure causes molten droplets to be ejected, as has been observed experimentally, and causes a preference for the cathode to operate, after the liquid is ejected, on the rim of the crater or nearby on the surface where hotter metal may exist due to liquid-metal overflow and spatter.