Thomson scattering of laser light from a high pressure air arc discharge and its application to electron density measurement

Abstract

Thomson scattering of laser light, which has been adopted for diagnosis of fusion plasmas, etc., is applied here to high pressure arc discharges as found in power circuit breakers, for measuring the electron density in the columns. The scattered light spectrum from the electrons in the high temperature air at 1 atm is theoretically calculated in the electron density range from 5.0*1019 to 1.8*1023 m-3. The notable aspect of light scattering under such high pressure conditions is that the scattered light spectrum has sharp satellite peaks on both sides of the incident laser wavelength, which are brought about by the plasma oscillation between electrons and ions. Actual measurements of the Thomson scattered light have been carried out on DC arcs of 10 to 40 A which are stabilised by axial air flow. The satellite peak is distinctly detected, as expected from calculations, at the wavelength 5.7 nm from the laser wavelength for 40 A arc current. The electron density around the arc axis is determined as 9.2*1022 m-3 from its profile analysis. This value of electron density shows a good agreement with that estimated from the spectroscopic diagnosis.