Arc-cold cathode interactions: parametric dependence on local pressure

Abstract

A numerical model describing the attachment of an electric arc on a vaporizing non-refractory cathode is developed and applied to a Cu cathode. The model describes the arc - cathode interaction zone by a combination of a quasi-stationary vacuum arc cathode spot model with a collisionless cathode sheath model for the current transfer in the cathode region. The conditions of pressure and electron temperature within the cathode spot plasma necessary to account for current densities ranging from A (upper limit for non-vaporizing cathode models) to A are presented. Results show that current densities higher than A can only be accounted for with metallic plasma pressures exceeding 35 atm and electron temperatures ranging from 1 to 2 eV within the cathode spots. The current transfer to the cathode is mainly assumed by the ions at low current densities ( A ) and by the thermo-field electrons for higher current densities. The heat flux to the cathode surface under the spots is mainly due to the flux of returning ions and ranges from to W for current densities ranging from to A . At low current densities (), the main heat loss is by conduction through the cathode while at high current densities, the Nottingham cooling associated with the thermo-field emission of electrons dominates. The model allowed us to define the upper and lower limits for the vacuum erosion rate by vaporization of the cathode. It is shown that the experimentally obtained vacuum erosion rate value for Cu falls between both limits for an electron temperature within the cathode spot of 1 to 2 eV.