Measured Index of Refraction for Argon Plasma

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Abstract
The index of refraction of argon plasma in the temperature range 9500 to 19 000°K was measured at a wavelength of 6328 Å. 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.