Laser-scattering temperature measurements of a free-burning arc in nitrogen

Measurements of the temperature of a 200 A free-burning arc in nitrogen, performed using a laser-scattering technique that does not require frequency resolution of the scattered signal, are presented. The technique is applicable to the full temperature range observed in free-burning arcs. However, signal-to-noise problems limited the present measurements to temperatures below about 16000 K. The laser-scattering results show excellent agreement with temperatures measured by emission spectroscopy. The spectroscopic measurements are shown not to be affected by deviations from local thermodynamic equilibrium at temperatures as low as 8000 K, below which the emission was too weak to allow temperatures to be determined. This is contrasted with the case of free-burning arcs in argon, for which departures from local thermodynamic equilibrium, thought to be caused mainly by resonance radiation trapping, render spectroscopic measurements unreliable below about 11000 K. An explanation for the differences between arcs in the two gases, in terms of different rates of excitation of high-energy states by resonance radiation trapping, is suggested.