Measurements of the radiation source strength in argon at temperatures between 5000 and 10 000 K

The volumetric radiative source strength has been measured for argon using a 50 kW rf induction plasma torch operated at atmospheric pressure with temperatures ranging between 5000 and 10 000 K. At temperatures below 7500 K the results differ by over an order of magnitude from the only previously published measurements [Phys. Fluids 10, 1125 (1967)]. Spatially resolved total radiation measurements were made with a calibrated pyroelectric detector system of flat response in the wavelength range 250 to 2500 nm. The corresponding temperature measurements were made through spectroscopic examination of relative and absolute line intensities, and absolute continuum intensities. The data were interpreted in terms of a partial equilibrium model in which the free and excited bound electrons are found to be mutually in equilibrium, irrespective of departures from equilibrium with the ground state. The differences from the previous radiation source strength measurements are interpreted in terms of unaccounted for nonequilibrium effects in that study. The present measurements are in satisfactory agreement with theoretical predictions. A simple yet complete set of calorimetric measurements taken independently supports these results.