Spectroscopic techniques have been used to measure the temperature distribution in free-burning arcs in argon at 1 atm pressure. The experiment provides evidence for departures from LTE in the arc, and demonstrates deficiencies in the theory describing continuum emission from high temperature plasmas. This paper describes the apparatus and data reduction procedures and compares the measurements with recent theoretical calculations.

Measured temperature profiles for various oxide-tungsten cathodes and for pure tungsten cathodes are presented for high-current arcs burning in argon at atmospheric pressure. Temperature profiles are also presented for thoriated tungsten cathodes with different cathode cone angles, are currents and composition of the gas provided to the arc. Evidence is also presented that the temperature and the behaviour of the cathode are sensitive to the oxygen concentration in the argon.

The temperatures of the cathode surface and of the plasma for an atmospheric-pressure high-current free-burning argon arc have been measured for a range of cone angles for cathodes of thoriated tungsten. These measurements have shown that both the surface temperature of the cathode and the temperature of the plasma depend strongly on the cathode shape. For cathodes with conical shape, plasma temperatures were found to be a maximum for a cathode cone angle of 60 degrees .

A Fowler-Milne technique using the relative intensity of the 746.8 nm N I line has been used to measure the plasma temperature of free-burning arcs in nitrogen at 1 atm pressure. Temperature profiles are presented for different arc currents. The maximum plasma temperature depends on the arc current and is 27000 K near the tip of a 3.2 mm diameter thoriated tungsten cathode with a 60 degrees cone angle for a 200 A, 5 mm long arc. Comparisons are made between high-current arcs burning in argon and in nitrogen.

A method using two-wavelength pyrometry has been developed for the measurement of the cathode surface temperature of high-current free-burning arcs. The method measures the total thermal radiation flux emitted from the cathode and the surrounding plasma and subtracts the derived plasma component using a modified Abel-inversion transformation. The effects associated with the plasma radiation reflected by the cathode are minimized by using measurements near the cathode edges. Experimental data are used to account for the dependency of the cathode emissivity on wavelength. In this paper the details of this technique are discussed and measurements of cathode surface temperature are presented for pure and thoriated tungsten cathodes.