Observation of a Multiply Ionized Plasma with Index of Refraction Greater than One

Abstract

We present clear experimental evidence showing that the contribution of bound electrons can dominate the index of refraction of laser-created plasmas at soft x-ray wavelengths. We report anomalous fringe shifts in soft x-ray laser interferograms of Al laser-created plasmas. The comparison of measured and simulated interferograms shows that this results from the dominant contribution of low charge ions to the index of refraction. This usually neglected bound electron contribution can affect the propagation of soft x-ray radiation in plasmas and the interferometric diagnostics of plasmas for many elements.

Impact of anomalous dispersion on the interferometer measurements of plasmas

“…Since a typical interferometer experiment [4,5] uses 0.1 cm long plasma, this means the plasma will be optically thin to the X-rays. Keep in mind that the energy scale needs to be shifted about 5 eV to higher energy to agree with the experiments, so even the 89 eV Ag Xray laser used in the experiments in Ref.…”

“…From the anomalous results in the interferometer experiments [4,5] of the Al plasmas it is clear that the traditional technique used to analyze the interferometer experiments is incomplete.…”

“…Since experiments typically measure an electron density that is much less than the critical density the formula above is approximated by n = 1 -(N elec / 2N crit ). In typical interferometer experiments [3][4][5][6][7] that probe plasma of length L using a source with wavelength λ, the number of fringe shifts is equal to…”

“…Since the anomalous index of refraction results were first measured in Al plasmas [4,5] using X-ray laser interferometers we utilize the average atom code to analyze Al plasmas and compare against OPAL calculations. To simplify the analysis, the calculations are done for different temperatures but with a constant ion density of 10 20 cm -3 .…”

“…The original analysis [9] of the index of refraction for Al plasmas was done only for a single wavelength, 14.7 nm, which is the wavelength of the Ni-like Pd X-ray laser used in experiments at LLNL [5]. That analysis combined individual calculations done for each isoelectronic sequence of Al and assumed that all the population was in the ground state of each ionization stage.…”