Current density at the refractory cathode of a high-current high-pressure arc (two modes of cathode spot attachment)

The attachment of a high-pressure arc at a refractory cathode was investigated theoretically and experimentally. Simple model considerations showed that an isolated cathode spot (CS) could function in two different modes. At relatively low currents and pressures, the CS mode (first mode) corresponds to the existing model (Bade W L and Yos Y M 1963 Technical Documentary Report No ASD-TDR-62-729 vol 1 (part II); Neumann W 1969 Beitr Plasmaphysik 9 499–526; Benilov M S 1993 Phys. Rev. 48 506–15, 1994 IEEE Trans. Plasma Sci. 22 73–7, 1999 J. Phys. D: Appl. Phys. 32 257–62 ; Benilov M S and Marotta A 1995 J. Phys. D: Appl. Phys. 28 1869–82 ; Benilov M S and Cunha M D 2002 J. Phys. D: Appl. Phys. 35 1736–50 , 2003 J. Phys. D: Appl. Phys. 36 603–14 ). In this mode current density does not depend on the arc current and is directly proportional to the gas pressure. At higher currents and/or higher pressures the CS exists in a different mode (second mode). In this mode current density does not depend on pressure and decreases as the current increases. If the product of the arc current, I , and the gas pressure, p , is lower than some critical value, then the first mode is realized; at a higher Ip product, the second one is realized. For discharges in nitrogen, the critical value was estimated as ∼7 × 10 7 A Pa. In the experimental part of this work, we investigated the footprints that the arc left at the cathode after it was terminated. Cathodes were made of pure and thoriated tungsten and the gases were nitrogen and argon. We have observed both modes. At 3 × 10 5 Pa, the second mode was observed at currents higher than ∼300 A; at 200 A, it was observed at pressures higher than ∼3 × 10 5 Pa. The CS footprint appearances are quite different. In the first mode the footprint has a smooth central part and a heavily eroded periphery. We believe that the cathode temperature is maximal not at the CS centre but at its periphery in this mode. With a thoriated tungsten cathode and swirling gas flow, the CS footprint exhibits a very specific crown-like pattern.