Arc attachment at HID anodes: measurements and interpretation

Abstract

Anodes for high intensity discharge lamps made of cylindrical tungsten rods and the plasma in front of them are investigated in a special lamp filled with argon and other noble gases at pressures of 0.1–1 MPa. The arc attachment on these anodes takes place in a constricted mode. The temperature is measured pyrometrically along the electrode axis and the anode fall electrically. The electron temperature, Te, and the electron density, ne, within the anodic boundary layer are determined spectroscopically with high spatial resolution. It is found that the power input into the anode increases nearly linearly with the arc current. The proportionality constant is mainly determined by the work function of the electrode material and Te but is independent of the electrically measured anode fall and scarcely dependent on the electrode dimensions. The constriction is more pronounced in cold anodes, with maxima of Te and ne in front of the electrode surface, than on hot anodes with thermionic electron emission and vaporization of the electrode material. The distances of the Te- and ne-maxima from the anode surface are increased and Te is reduced in front of the anode with increasing anode temperature. The experimental findings may be explained by a model of the anodic boundary layer consisting of a thin sheath in front of the surface and a more extended constriction zone. The current and voltage are anti-parallel within the sheath. The power which is needed to sustain the sheath is supplied by an enhanced electrical power input into the constriction zone.