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Spectroscopie d'absorption X résolue en temps pour l'étude de la matière dense et tiède

Abstract : The possibility of producing, in a controlled way, matter at solid density (1-10 g/cc) while maintaining it at temperatures ranging from 0.1 to 100 eV has been for a long time a desired goal. At the boundary between condensed matter and plasma physics, the study of matter in the Warm Dense Matter (WDM) regime involves, nowadays, many laboratories worldwide. This state is still poorly understood and presents a very complex character, being partly degenerated (electrons) and partly correlated (ions). To explore this regime, we proposed to probe WDM by X-ray absorption fine spectroscopy with an ultra-fast X-ray source (ps), produced by ultra intense lasers. X-ray absorption fine spectroscopy, specifically XANES and EXAFS spectroscopy, is a diagnostic tool that allows the study of local atomic structures, even in non-crystalline states (solid, liquid, WDM, plasmas). It requires the use of broadband X-ray sources adjusted on the x-ray absorption edge of studied element. Several steps were necessary for the completion of this work. First, we had to develop an ultra-short X-ray source (a few picoseconds) created by laser and adapted to the X- ray absorption spectroscopy of aluminum. We developed such X-ray source by using the plasma M-shell emission of high Z elements, irradiated by a femtosecond laser. More precisely we used the broad and intense 4f − 3d transition arrays. The emission spectrum of this source has been studied around the aluminum K-edge which corresponds to the energetic range 1.50 - 1.75 keV. A series of experiments, consisting of temporal and spectral measurements of the X-ray emission was performed with a kHz laser (5 mJ, 30 fs) focused on different solid targets : Sm, Gd, Dy, Er and Yb. The results were compared with Averroes - Transpec simulations (collisional - radiative code with non local thermodynamic equilibrium, coupled with a superconfiguration treatment of atomic physics). After optimization, the X-ray emission presents a broadband spectrum in the energetic range of interest and a duration of ∼ 3 − 4 ps. The X-ray intensities are of the order of 1e7 photons per shot per eV per sr, representing a conversion rate of 1 ‰ of the laser energy emitted in the energy range 1.50 - 1.75 keV. In a second step, we developed a spectrometer adapted to X-ray absorption measurements with X-ray sources created by laser. This diagnostic involves the simultaneous measurement of the spectrum transmitted through an aluminum sample and the reference spectrum directly from the source, for each laser shot. This then allows us to calculate the transmission and therefore the absorption, corrected for X-ray source fluctuations. A specific effort has been made to minimize fluctuations and to obtain high quality spectra corrected for laser shot to shot variations and for spurious detector responses. X-ray absorption spectra were recorded for a cold aluminum sample (20 °C) by accumulation of data from a few thousand laser shots (i.e. a few seconds at 1 kHz). XANES structures are clearly identified and resolved with a noise level below 1 %. These high quality XANES spectra obtained with high repetition rate lasers provide opportunities to conduct pump - probe experiments on tabletop facilities. Finally, we performed an experiment in collaboration with the LULI laboratory (Ecole Polytechnique), the CEA and the INRS (Canada) to characterize a sample of aluminium isochoricaly heated by a laser accelerated proton beam. The ultra-intense (1e19 W/cm2), ultra-short (fs) laser pulse of the LULI 100TW facility was focused on a gold solid target to accelerate protons while a synchronised secondary laser beam is used to create the ultra-short X-ray backlighter, previously described. With this proton heating method, the sample is rapidly raised to relatively high temperatures (up to ten eV). Under these conditions, the expension of plasma becomes relevant only after a few tens of picoseconds and it is possible to probe the aluminum warm dense plasma when its temperature is uniform and its density is close to the solid one. In agreement with theoretical work, experimental results show a loss of XANES structures when the aluminum reaches a temperature of about 1 eV. We attributed this XANES structure vanishing to a loss of ion - ion correlation. In perspectives of this study, we propose to extend this work to the analysis of dense and warm plasmas by using different heating techniques such as laser heating (fs), laser shock propagation generated by laser (ns), or by X-ray or XUV FEL heating (fs). It is also planned to extend this study to other elements such as iron.
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Submitted on : Monday, July 23, 2012 - 7:20:44 PM
Last modification on : Monday, October 17, 2022 - 1:42:05 PM
Long-term archiving on: : Wednesday, October 24, 2012 - 2:55:10 AM


  • HAL Id : tel-00720159, version 1




Marion Harmand. Spectroscopie d'absorption X résolue en temps pour l'étude de la matière dense et tiède. Physique des plasmas [physics.plasm-ph]. Université Sciences et Technologies - Bordeaux I, 2009. Français. ⟨NNT : ⟩. ⟨tel-00720159⟩



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