Analysing the dramatic impact of the bound electron contribution to the index of refraction in plasmas

For decades the plasma physics community has relied on the approximation that the index of refraction of a plasma is due solely to the free electrons. This makes the index of refraction less than one and is an essential assumption used in energy deposition in the plasma as well as plasma diagnostics such as interferometers. Recent measurements of Al plasmas using X-ray laser interferometers have observed anomalous results with the index of refraction being larger than one. Subsequent analysis of the experiments showed that the bound electrons could have a larger contribution to the index of refraction with the opposite sign than the free electrons. This effect extends far from the absorption edges and lines of the bound electrons. Utilizing a new average atom code we calculate the index of refraction in C, Al, Ti and Pd plasmas and show that there are many conditions over which the bound electron contribution dominates as we explore photon energies from the optical to 100 eV (12 nm) soft X-rays. During the next decade X-ray free electron lasers and other sources will be available 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. PACS codes: 51.70.+f , 52.38.-r, 52.70.-m, 52.70.La