Critical parameters of the cathode region of a vacuum arc

An attempt has been made to determine the limiting values of the main physical parameters that define the current transfer process through the cathode spot plasma: current density, temperature, and the concentration of charge particles. The values obtained reveal the upper bound of these parameters. The evaluations are carried out on the basis of balance correlations and with consideration of the fact that conductivity of an ideal copper vapor plasma in the given range of temperature and concentrations is naturally more than that of a nonideal one. On the basis of the experimental and theoretical reasons discussed, it is concluded that there are two stages in the development of a nonstationary cathode spot, namely, the stage of initiation and the regular stage. During initiation, the current density achieves the values at which the explosion emission becomes possible while the cathode drop is up to 100-200 V. This stage lasts no longer than 1-3 ns. The medium providing the transport of current in the regular stage is generated mostly during the initiation stage. In the regular stage, which lasts 5-7 mu s in the case of a copper cathode, the cathode drop is about 15 V and no considerable erosion of cathode material occurs. The theoretical estimates verify the existence of the two stages in the development of a nonstationary cathode spot and demonstrate that there are some parameters that are critical from the viewpoint of whether the initiating on regular current density takes place.<
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