Evidence of bound electron contribution to soft X-ray laser interferograms of dense plasmas.

Abstract. We present clear experimental evidence showing that the index of refraction of laser created plasmas at soft x-ray wavelengths can be dominated by the contribution of bound electrons. We report picosecond resolution soft x-ray laser interferograms of Al laser-created plasmas that late in their evolution display negative fringe shifts in the plasma periphery. The comparison of measured and simulated interferograms show that this results from the dominant contribution of low charge ions to the index of refraction. The observed ion contribution to the index of refraction has broad significance, as it affects the propagation of soft x-ray radiation in plasmas and the interferometric diagnostics of plasmas for many elements.