Spectroscopic investigation of the plasma boundary layer in front of HID-electrodes

Abstract

To investigate the properties of the plasma boundary layers of an argon arc in front of high intensity discharge (HID) electrodes a special spectroscopic measuring system was set-up in front of a model lamp. It is characterized by a high spatial resolution and a high degree of automation of the measuring procedure. Measurements were performed at the boundary layers in front of anodes and cathodes made of pure and thoriated tungsten with diameters from 0.6 to 2 mm operated with currents from 0.5 to 10 A at pressures from 0.1 to 0.3 MPa. The electron temperatures Te were deduced from Boltzmann-plots of population densities of a couple of excited states just below the ionization limit, which were obtained from measured line intensities. Electron densities ne were determined from measured continuum intensities. In front of the anodes, maxima of Te and ne were found with Te of the order of 12 000 K and ne of the order of 5 × 1021 m−3, indicating strong deviations from local thermal equilibrium. In the diffuse mode of cathodic arc attachment, steep gradients are measured with values of Te of the order of 12 000 K and of ne of the order of 5 × 1022 m−3. The measured temperatures in the boundary of the spot mode of cathodic arc attachment exceed 22 000 K. The electron temperatures in front of the cathode are in accordance with the results of modelling.