Decay of wall stabilized arcs in SF6-N2 mixtures

The variations of temperature and conductance are calculated during the extinction of nonblown, atmospheric pressure, wall-stabilized arcs. The validity of the computation method based on the assumption of local thermodynamic equilibrium (LTE) has first been tested by comparing the computed values of electron density with experimental values obtained by laser interferometry in SF6-N2 mixtures during the first tens of microseconds in the extinction. The computed values of temperature and conductance have also been compared with experimental and theoretical values from the literature, in the case of pure Ar, SF6, and N2. In spite of discrepancies, due to nonequilibrium phenoma, the relative computed variations of conductance in the case of pure SF6 and N2 are comparable with the experimental variations. In the case of SF6-N2 mixtures, and assuming LTE, the computation shows that the evolution of the mixture conductance is not systematically intermediate between those of pure SF6 and N2. For example, with a mixture containing 20% SF6 and 80% N2, the calculated conductance of a 100 A extinguishing arc after 130 μs is 40 times higher than of pure SF6 and about 15 times that of pure N2. This phenomenon is due to the combination of two properties of the SF6-N2 plasma for temperatures below 7500 K: low thermal conductivity and high electrical conductivity.