Numerical investigation of regimes of current transfer to anodes of high-pressure arc discharges

Abstract

Unified 1D numerical modeling of high-pressure high-current arc discharges is revisited. Two regimes of current transfer to anodes are investigated. The “passive anode” regime occurs for low and moderate anode surface temperatures Ta. The energy flux from the plasma to the anode surface, qpl, depends on Ta rather weakly in this regime and may be conveniently expressed in terms of the local current density jc, and the so-called anode heating voltage Uh. Uh is independent of the arc length and the cathode surface temperature, although it weakly varies with jc between approximately 6 and 8.5 V for jc in the range from 105 to 108 A m−2. In the “active anode” regime, qpl is higher than in the passive anode regime and varies with Ta. The active anode regime may occur on hot refractory anodes, such as those of high-intensity discharge lamps, when Ta exceeds approximately 3000 K and the thermionic electron emission from the anode comes into play. The latter causes an increase in the electron density near the anode. One consequence is the increase in the electron energy transport from the bulk plasma to the near-anode layer by electron heat conduction. The other effect contributing to increase in qpl is the formation of a negative near-anode space-charge sheath with a positive voltage drop. In non-stationary simulations, the active regime occurs via the development of a thermal instability similar to that causing the appearance of spots on thermionic arc cathodes. The occurrence of the active regime is strongly affected by parameters, in particular, by the distance between the anode surface and the cooling fluid.