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.
A model has been developed for the explosive electron emission cell pulse of a vacuum discharge cathode spot that describes the ignition and extinction of the explosive pulse. The pulse is initiated due to hydrodynamic tearing of a liquid-metal jet which propagates from the preceding cell crater boundary and draws the ion current from the plasma produced by the preceding explosion. Once the jet neck has been resistively heated to a critical temperature (∼1 eV), the plasma starts expanding and decreasing in density, which corresponds to the extinction phase. Numerical and analytical solutions have been obtained that describe both the time behavior of the pulse plasma parameters and their average values. For the cell plasma, the momentum per transferred charge has been estimated to be some tens of g cm/(s C), which is consistent with the known measurements of ion velocity, ion erosion rate, and specific recoil force. This supports the model of the pressure-gradient-driven plasma acceleration mechanism for the explosive cathode spot cells. The ohmic electric field within the explosive current-carrying plasma has been estimated to be some tens of kV/cm, which is consistent with the known experimental data on cathode potential fall and explosive cell plasma size. This supports the model that assumes the ohmic nature of the cathode potential fall in a vacuum discharge.
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