Method of time resolved refractive index measurements of x-ray laser heated solids

Abstract

International audienceWith the advent of new x-ray light-sources worldwide, the creation of dense, uniformly heated plasma states arising from intense x-ray irradiation of solids has been made possible. In the early stages of x-ray solid heating, before significant hydrodynamic motion occurs, the matter exists in a highly non-equilibrium state. A method based on wavefront sensing is proposed to probe some of the fundamental properties of these states. The deflection and absorption of a high harmonic probe beam propagated through the plasma can be measured with a wavefront sensor, and allow for the determination of the complex refractive index (RI) of the plasma, giving a 2D map of the optical properties as function of time in a pump-probe arrangement. A solid heating model has been used to estimate the expected temperatures of x-ray heated thin foils, and these temperatures are used in three separate models to estimate the changes in the refractive index. The calculations show the changes induced on an extreme ultra-violet (XUV) probe beam by a solid density thin foil plasma are significant, in terms of deflection angle and absorption, to be measured by already existing XUV Hartmann wavefront sensors. The method is applicable to a wide range of photon energies in the XUV (10s to several 100s of eV) and plasma parameters, and can add much needed experimental data to the fundamental properties of such dense plasma states. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794964

Optimization and Characterization of High-Harmonic Generation for Probing Solid Density Plasmas

“…Following [13], the largest deflection of an HH probe beam-larger than the plasma created at WDM conditions-is between the cold Al foil and warmest part of the solid plasma at a temperature of 10 eV. For a plasma with a diameter of 20 µm, a maximum beam deflection of 0.93 mrad is expected under these conditions.…”

“…In [13] values for the absorption and deflection of a XUV probe pulse in Al heated to warm-dense conditions are estimated. The referenced values result from heating Al with an FEL, however WDM can also be produced with optical lasers as shown in [28][29][30].…”

“…In order to use XUV pulses from HHG for different applications, it is important to optimize the photon number and to characterize the stability and spatial properties. For experiments like plasma probing [7,13], holography [14], coherent diffraction imaging [12], and X-ray laser seeding [15], it is necessary to have spatially characterized and energy-optimized XUV pulses. Optimization in photon number and wavefront is achieved by adjusting the phase matching conditions of the different high-harmonic orders [16][17][18][19].…”

Ab initio simulations and measurements of the free-free opacity in aluminum

“…Dense plasmas, in turn, have proven far more challenging both to model and to investigate experimentally [9][10][11][12][13][14]. Perhaps surprisingly, similar difficulties are encountered in condensed matter systems such as ground-state and liquid metals.…”

EUV and Hard X-ray Hartmann Wavefront Sensing for Optical Metrology, Alignment and Phase Imaging

“…Plasma probing with a Hartmann sensor has been proposed long ago [ 69 ]. It regains interest with the advent of X-ray Free Electron Lasers [ 70 ]. A demonstration of the use of Hartmann WFS for plasma probing has been performed at LCLS Free Electron Laser.…”