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.

A simple model is reported for copper electrode erosion in electric arc heaters as a result of electrode surface fusion by the action of the arc spot. In the analysis we substitute the real arc spot by an intensive moving surface heat source. The time evolution and the distribution of the electrode surface temperature within the arc spot is studied in coordinates coupled with the moving heat source. The electrode erosion is attributed in the model to the unbalance between the heat supply and the heat removal due to the development of an arc spot fusion zone. An erosion effective enthalpy is applied in the analysis, including all physical processes for the material transformation from the solid state into the plasma state under the action of the intensive heat flux. Formulae were obtained that enable one to calculate the electrode erosion as a function of arc velocity, electrode temperature, current, near-electrode thermal volt-equivalent, current density and electrode material properties. We show briefly that the theory agrees reasonably well with experimental results. The present model enables us to reveal the relative significance of the different parameters in the erosion process and to predict the erosion behaviour in copper electrode electric arc heaters.

Two important copper electrode parameters, namely, the volt-equivalent of the arc spot heat flux and the arc spot effective current density have been measured experimentally in magnetically driven arcs in air. The measurements were performed for currents in the range 0.025 - 1 kA, magnetic field strengths 0.133 - 0.95 T and air pressures 1 - 40 atm. The volt-equivalent of the arc spot heat flux is proposed to account for the total heat flux entering the electrode surface through the arc spot. An unsteady thermal method was applied in the measurements. To obtain the effective current density, the thermophysical experiment was combined with the application of equations of the erosion model presented in a previous companion paper. We show that the volt-equivalent of the cathode arc spot heat flux increases with increasing magnetic field. The sum of the cathode and the anode voltage drops is shown to be approximately equal to the sum of the corresponding volt-equivalents of the arc spot heat fluxes. Good agreement with the measurements of other authors is obtained. In particular, the effective arc spot current density allows one to confirm the thermophysical model for the erosion of copper electrodes indirectly.

This paper presents an approximate formula for the determination of local axis temperatures of an inhomogeneous, axisymmetric, optically thin, local thermodynamic equilibrium plasma column without using the Abel inversion technique. The proposed method is straightforward and is based on recording the spatially integrated radiances of spectral lines. The formula is useful for high gradients thermal plasma columns of the type found in DC electric arcs or plasma jets of gaussian shaped temperature profile. The formulation permits a simple experimental arrangement for rapid monitoring or control of parameters in industrial plasma devices or for the determination of emissivities and line transition probabilities. We also show that the method can be applied to spatially unstable arc discharges, where traditional techniques are difficult and cumbersome to apply.

Experimental results are presented on copper-cathode erosion for magnetically driven arcs in air, for currents in the range 0.025 - 1 kA, magnetic induction values 0.03 - 0.24 T, arc velocities 19.3 - 344 m and electrode surface temperatures 300 - 1073 K. From experimental data we obtained the important characteristic parameters for the copper-cathode's erosion process; namely the effective enthalpy of electrode material ablation in the arc spot, , and the specific microerosion value, . The experiments demonstrated that, just like the near-cathode thermal volt equivalent U, the arc spot current density j is also a monotonic ascending function of the magnetic field strength. We have shown a good agreement between the theory and the experimental data for a wide range of arc-heater operating regimes. A mathematical analysis and diagrams are provided for predicting the best conditions for an arc heater to work with the minimum erosion level. The minimum erosion is shown to depend on the temperature attained by the electrode and on the degree of influence of the gas velocity on the electrode temperature. It is predicted that the erosion decreases with increasing arc velocity, then stabilizes at a given minimum level and then increases as one increases the value of the arc velocity.