Interferometric measurement of particle densities in cascaded arcs at atmospheric pressure

Two wavelength interferometry is applied to a cascaded arc burning in Ar, Xe and He at atmospheric pressure. The interferometer is of the Mach-Zehnder type with a He-Ne-laser as a light source. The refractive index of the plasma is measured at the wavelengths lambda 1=6328 AA and lambda 2=33912 AA and is compared to calculated data. A contribution of excited levels to the refractive index was not found in the temperature and pressure range studied here. Using experimentally determined refractive index profiles neutral and electron density distributions are derived and compared to spectroscopically measured values. The comparison shows that the diagnostic method applied here leads to accurate particle densities without using any assumption concerning the thermodynamic equilibrium state of the plasma.