Temperature measurement in thermal plasmas by Rayleigh scattering

The range of temperatures over which Rayleigh scattering is a useful diagnostic of temperature in a noble gas thermal plasma at atmospheric pressure is examined. The various scattering processes that can occur a light passes through such a plasma are outlined. It is shown that while Rayleigh scattering by ground-state atoms is the dominant process at low temperatures, Thomson scattering becomes important at temperatures at which significant ionization occurs. Resonance scattering is also a significant source of scattered radiation when the laser wavelength is approximately equal to the wavelength of a transition of a species present in the plasma. Radial profiles of the polarized and depolarized radiation scattered from argon and helium arcs have been measured and analysed to confirm the relative importance of the respective scattering mechanisms. The occurrence of Thomson scattering is found to set a fundamental upper-temperature limit to the applicability of Rayleigh-scattering techniques to temperature measurement. For a laser wavelength of 514.532 nm and a detection system bandwidth of less than 0.1 nm, Thomson scattering becomes significant at approximately 9000 K in argon and 11000 K in helium. Above these values, the derivation of plasma properties from scattering measurements requires that all scattering mechanisms be taken into account. The temperature of the argon arc is calculated to exemplify this.