Temperature measurements in a welding arc by Rayleigh scattering and emission spectroscopy

Consecutive Rayleigh scattering and emission spectroscopic measurements have been performed on an atmospheric argon gas tungsten arc welding (GTAW) torch, at 100 A and 5-mm electrode-gap spacing, burning over a copper chilled block. Rayleigh scattering measurements taken with a pulsed Nd:YAG laser at four different axial locations show scattering intensity profiles that decrease again to a peak value at the center of the arc. Assuming that the scattering signal is predominantly due to ground-state Rayleigh scattering cross-sections, heavy particle temperatures, calculated from Dalton's law, yield reasonable values up to 8000 K in the arc fringes; however, under the same assumption, high scattering intensity values in the arc center yield unrealistically low temperatures down to 400 K in the core of the arc. In emission spectroscopy, the absolute intensity of seven argon I lines has been recorded along with the spectral profile of the H_β line (1% hydrogen admixture), each at 5 axial locations. In the fringes of the arc there is general agreement between temperatures obtained from Rayleigh scattering and emission spectroscopy.