Less than half of the heat generated in a TIG welding arc (typically 1600 W at 16 V, 100 A) is transferred to the workpiece (anode). Convection, conduction and radiation from the gas occur over the whole of the arc region, but they represent relatively minor contributions to the total heat balance. The principal anode heating and cooling mechanisms involve electron and space-charge effects at the surface. These electron effects are evident in the workfunction (typically 4·5 V) which is dominant, the electron thermal energy transfer (1 V) and the anode fall (2 V), and they are concentrated in the restricted anode current spot so they may be considered as a localized heat source.Evaporation is most intense from the anode spot region and carries away some heat. However, most of the vapour condenses on the cooler, outer regions of the anode surface covered by the arc. In this way, heat is redistributed and diffused over a wider area. Vaporization effects explain the differences between previous measurements with cooled copper anodes (in which 80% of the arc power is transferred to the metal) and ones with practical, molten-steel welds (less than 50% heat transfer).

The electrostatic probe is an attractive diagnostic tool for DC welding arcs. The current-voltage characteristic of such a probe in a high-current (100 A), high-pressure (1 bar), DC argon (TIG) welding arc is found to give a very flat ion saturation region. Sheath thicknesses are estimated to be much smaller than 1 μm, ie less than the mean free path, so that conventional collisionless probe theory should be applicable - in contrast to the flame plasmas dealt with by Thomas (1969) and others. By using an Abel inversion technique the measured probe current can be converted to a radial distribution of current. The temperature variation across the arc can then be derived from this current variation. By measuring the radial temperature profile at several points along the length of the arc column, a complete isothermal map of the TIG arc has been obtained which is important for basic calculations of energy transfer.

Radial temperature profiles for pulsed SF6 arcs burning in high-pressure, approximately 5 bar, supersonic gas flow have been measured in the range 10 kA-1.0 kA. The temperature distribution was obtained from the emission intensities of spectral lines due to excited fluorine and ionized sulphur. The results for currents above 3.5 kA show that the axis temperature is 20000K+or-1000K and is independent of the arc current. Also the temperature profile is essentially parabolic, and any increase in arc current is accompanied by a corresponding increase in arc cross section to maintain a constant current density. Below 3.5 kA the axis temperature falls rapidly with current down to 15500K+or-1000K at kA and the temperature profiles show very steep temperature gradients less than 1 mm from the arc axis.

Conduction and breakdown have been studied in the boundary regions of wall-stabilized arc columns for arc currents up to 50 A in N2, Ar, He, air and SF6 at atmospheric pressure, and for stabilizing hole diameters of 5 and 4 mm. In the case of SF6 no electrons could be drawn to a positively charged disk, and this is shown to be due to the unusual property of an SF6 plasma below 4000°K, namely that it comprises only positive and negative ions (S+ and F−). Estimates of temperature on the plasma side of the boundary layer, based on the magnitudes of the positive-ion currents, vary from 2800°K in SF6 to 4800°K in N2 for 25 A arcs in 5 mm diameter holes. Some characteristics of unipolar arcs, which are defined as arcs which join a metallic electrode to a plasma, are given for Ar and SF6.

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