Measured Index of Refraction for Argon Plasma

The index of refraction of argon plasma in the temperature range 9500 to 19 000^∘{}K was measured at a wavelength of 6328 {}. A helium-neon laser was used as the light source for a Mach-Zehnder interferometer. A modified commercial plasma jet and a free-burning arc were used as plasma sources. The index of refraction was calculated using the Slater screening-constant method. The measured index of refraction lies between the theoretical curve which considers contributions from only ground-state atoms and free electrons and the curve which considers excited electrons as free. There appears to be a lowering of the ionization limit to a principle quantum number of 5 for the atom of 16 000^∘{}K, compared with Olsen's value of 6.