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.

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).

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