Spectrally and temporally resolved reflectivity measurement for studying the temporal evolution of a dense plasma of interest for the Warm Dense Matter problematic...
The properties of Warm Dense Matter ( WDM - solid density and temperature of several eV's ) are a subject of strong interest among a wide scientist community ranging from astrophysicists to solid-state physicists. A major difficulty to characterize the ultra-fast dynamics of such dense plasmas results from their opacity to visible light. Thus, only a few sets of experimental data are available making the validation of theories and numerical simulations difficult.
We have suggested to study the temporal variation of a XUV probe beam reflected by a plasma created on metallic target. The IR pump-XUV probe beam experiment has been set up on LUCA laser facility (CEA-Saclay) lien vers SLIC. An IR has been focused on Au target at intensity ranging from 7x1013W/cm2 to 1015W/cm2. XUV beam is HHG from argon gas jet. As the XUV spectrum is made of odd harmonics of the fundamental laser frequency, we record the wavelength dependant reflected signal in one single shot.
The temporal variation of reflectivity starts to exhibit different slopes, depending on the laser intensity. These results can be interpreted in an obvious way: the higher the intensity on target, the faster the plasma is created and expands. At 1015W/cm2, the plasma is created very rapidly and the reflectivity falls down on less than 2 ps, whereas at an intensity close to the optical damage of the cold solid (7x1013W/cm2) the reflectivity drops down in 10ps. This temporal scale is consistent with preliminary hydrodynamic simulations performed in 1D, that predicts variations on the same time scale.