The basic experimental arrangement used for wavefront sensing of high density targets. The thin foil target is heated an XFEL pulse at a shallow angle, and the plasma causes a gradient in the refractive index between the cold solid and plasma region, bending the probe beam with a deflection proportional to the gradient (Eq. (1) ).
The electron-ion collision frequency, , as a function of electron temperature for a photon energy of 40 eV (solid line), electron-ion coupling from XFEL heating (dotted-dash line), and the limiting collision frequency (dashed line). 46
The real refractive index, , and absorption co-efficientt, , for a photon energy of 40 eV for solid density aluminium as a function time according to the temperature evolution of that in Figure 2 for . The Drude values are calculated using Eq. (3) and the collision frequency, , given by the solid line in Figure 3 . The FLYCHK absorption values are calculated from the opacity spectrum, and the real values using Kramers-Kronig analysis. The arrows indicate the cold solid values of refraction and absorption. 43
The refractive index changes for equilibrium temperatures up to 10 eV according to the RPA, following from the model proposed by Vinko et al. 34
Values of the expected probe beam deflections, [mrad], for photon energy [eV] assuming a linear temperature gradient from calculations and tabulated values, and the method used for the refractive index gradient calculation, over a spot diameter of , with a refractive index gradient through a thin foil plasma with a thickness of 300 nm.
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