Pulsed high current, high pressure, SF6 arcs, 100 mm long, burning between Cu/W electrodes in an axial supersonic Mach 1.5 gas flow have been investigated. The current range examined was 1 kA to 45 kA, the pulses were half sine waves with durations of 6 ms to 12ms, and the gas pressures were 0.65 MN m-2 (6.5 bar) upstream (anode) and 0.31 MN m -2 (3.1 bar) downstream (cathode). Time and space-resolved photoelectric and photographic emission spectroscopy was used to determine the arc temperature (by ion/atom and atom/atom line intensity ratio techniques), the electron density (by quadratic Stark broadening of gas and metal lines) and the metal vapour impurity concentration from Saha calculations and relative line intensities. L.t.e. assumptions were confirmed for the current range 1 kA to 20 kA. The arc radius and shape were determined by high speed photography. Specific column features, a vapour-rich core consisting of thin vapour filaments, were revealed using interference filters tuned to characteristic emission lines. Variations in the thermal zone surrounding the arc were examined with a laser shadowgraph technique. A convection-loss dominated arc model has been extended and, in the middle current range, the arc temperature, radius and voltage distributions have been accurately predicted using equilibrium plasma properties and a single independent geometrical parameter - the axial pressure distribution. Using a two-zone arc representation it is shown that the input heat fraction removed by convection within the electrical zone exhibits a maximum (75%) at 10 kA to 15 kA. At low currents radial losses to the molecular thermal zone are dominated by atomic sulphur and fluorine v.u.v. line radiation. The effective transport coefficient is some two orders of magnitude larger than the electron diffusion coefficient. At very high currents resonance radiation from the metal core escapes completely from the arc and dominates the energy balance in this range. Formation of the core is attributed to the self-magnetic pinch effect. The excitation temperature increases from 20,000 K to 30,000 K upon formation of the core and, together with a high (1.3 MN m-2 - 13 bar), spectroscopic pressure accounts partly for the high electrical conductivity. Non-equilibrium effects are suspected of enhancing this conductivity at very high currents.
|Date of Award||1977|