The paper deals with calculation of parameters in the near-cathode plasma layer, on the cathode surface and in the body of a cathode in high-pressure arc discharges. These parameters can be calculated independently of the arc column if the heat flux coming from the column to the edge of the near-cathode layer does not play a decisive role in the energy balance of the layer, which, according to the estimates presented, is a likely case. The physics of the near-cathode layer is reconsidered in view of major contradictions that have appeared in the literature recently, in particular with regard to the role of the near-cathode space charge sheath. A model of a near-cathode layer is developed that is based on a multifluid description of the plasma and takes into account multiply charged ions. The model is employed to calculate parameters of the layer as functions of the voltage drop in the layer and of the local value of the surface temperature. By means of these data, an approximate asymptotic theory of arc spots is extended to cathode spots in high-pressure plasmas. Calculated spot parameters are presented for the following combinations cathode/plasma: tungsten/argon, thoriated-tungsten/argon, thoriated-tungsten/nitrogen, and zirconium/nitrogen. The obtained results agree with the recent measurements of the spot temperature.

Considerable advances have been attained during the last decade in the theoretical and experimental investigation of electrode phenomena in high-pressure arc discharges, in particular, in low-current arcs that occur in high-intensity discharge lamps. The aim of this paper is to deliver a concise review of the understanding achieved and modelling methods developed.

A model of the near-cathode plasma layer in a plasma under a pressure of the order of one or several bars is reconsidered on the basis of recent theoretical results. Physics of the near-cathode layer is analysed in the range of near-cathode voltage drops of up to 50 V, in accord to recent experimental results which have shown that the near-cathode voltage drop in high-pressure arc discharges may be that high. It is found that a non-monotony of the dependence of the energy flux density on the surface temperature at fixed values of the near-cathode voltage drop is caused by one of the three mechanisms: overcoming of the increase of combined ion and plasma electron heating by the increase of thermionic cooling as the plasma approaches full ionization; non-monotony of the dependence of the ion current on the electron temperature which is caused by the deviation of the ion current from the diffusion value; rapid increase of the plasma electron heating which is subsequently overcome by thermionic cooling. A closed description of the plasma-cathode interaction is obtained by numerically solving the nonlinear boundary-value problem for the temperature distribution inside the cathode body. Results of numerical modelling of the diffuse discharge under conditions of a model arc lamp are given and a good agreement with the experimental data is shown.

A model of a near-cathode region in high-pressure arc discharges is developed in the framework of the hydrodynamic (diffusion) approximation. Governing equations are solved numerically in 1D without any further simplifications, in particular, without explicitly dividing the near-cathode region into a space-charge sheath and a quasi-neutral plasma. Results of numerical simulation are reported for a very high-pressure mercury arc and an atmospheric-pressure argon arc. Physical mechanisms dominating different sections of the near-cathode region are identified. It is shown that the near-cathode space-charge sheath is of primary importance under conditions of practical interest. Physical bases of simplified models of the near-cathode region in high-pressure arc discharges are analysed. A comparison of results given by the present model with those given by a simplified model has revealed qualitative agreement; the agreement is not only qualitative but also quantitative in the case of an atmospheric-pressure argon plasma at moderate values of the near-cathode voltage drop. The modelling data are compared with results of spectroscopic measurements of the electron temperature and density in the near-cathode region.

No abstract