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Over the last decade the electron density of plasmas has been measured using X-ray laser interferometers in the 14 to 47
nm wavelength regime. With the same formula used in decades of experiments with optical interferometers, the data
analysis assumes the index of refraction is due only to the free electrons, which makes the index less than one. Over the
last several years, interferometer experiments in C, Al, Ag, and Sn plasmas have observed plasmas with index of
refraction greater than one at 14 or 47 nm and demonstrated unequivocally that the usual formula for calculating the
index of refraction is not always valid as the contribution from bound electrons can dominate the free electrons in
certain cases. In this paper we search for other materials with strong anomalous dispersion that could be used in X-ray
laser interferometer experiments to help understand this phenomena. An average atom code is used to calculate the
plasma properties. This paper discusses the calculations of anomalous dispersion in Ne and Na plasmas near 47 nm and
Xe plasmas near 14 nm. With the advent of the FLASH X-ray free electron laser in Germany and the LCLS X-FEL
coming online at Stanford in 2 years the average atom code will be an invaluable tool to explore plasmas at higher X-ray
energy to identify potential experiments for the future. During the next decade X-ray free electron lasers and other
X-ray sources will be used to probe a wider variety of plasmas at higher densities and shorter wavelengths so
understanding the index of refraction in plasmas will be even more essential.
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