51ÁÔÆæÈë¿Ú

Abstract:

We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ∼15fs pulse of containing >10 ¹² photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 10 ¹⁸ W/cm 2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given. © 2009 SPIE.

Abstract:

We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.

Abstract:

The beam of the Free-Electron Laser in Hamburg (FLASH) was focused by a grazing incidence elliptical mirror (32.5 nm and 13.5 nm) and an off-axis parabolic mirror coated with a Si/Mo multilayer (13.5 nm) to 20-micron and 1-micron spot, respectively. The grazing incidence and normal incidence focusing of ∼25-fs pulses with an energy of 10 μJ resulted in an irradiance of 10¹³ W/cm² and 10¹⁶ W/cm², respectively, at the surface of various solids (Si, Al, Ti, Ta, Si 3N4, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate) - PMMA, stainless steel, etc.). The optical emission of the plasmas produced under these conditions was registered by a grating (1200 lines/mm and/or 150 lines/mm) spectrometer MS257 (Oriel) equipped with iCCD head (iStar 720, Andor). Surprisingly, only lines belonging to the neutral atoms were observed at intensities around 10¹³ W/cm². No spectral lines of ions have been identified in UV-vis spectra emitted from the plasmas formed by the FLASH beam focused in a 20-micron spot. At intensities around 10¹⁶ W/cm², the OE spectra are again dominated by the atomic lines. However, a weak emission of Al⁺ and Al²⁺ was registered as well. The abundance ratio of Al/Al⁺ should be at least 100. The plasma has an excitation temperature equivalent to 0.8 eV found by a computer simulation of the aluminum plasma OE spectrum. A broadband emission was also registered, both from the single-element plasmas (typical is for carbon; there were no spectral lines) and the scintillating slabs (on Ce:YAG crystal, both the luminescence bands and the line plasma emission were recorded by the spectrometer). © 2009 SPIE.

Abstract:

We present calculations of the free-free opacity of warm, solid-density aluminum at photon energies between the plasma frequency at 15 eV and the L-edge at 73 eV, using both density functional theory combined with molecular dynamics and a semi-analytical model in the RPA framework which includes exciton contributions. As both the ion and electron temperature is increased from room temperature to 10 eV, we see a marked increase in the opacity. The effect is less pronounced if only the electron temperature is allowed to increase, while the lattice remains at room temperature. The physical significance of these increases is discussed in terms of intense light-matter interactions on both femtosecond and picosecond time scales. © 2009 Elsevier B.V. All rights reserved.

Abstract:

We present calculations of the free-free XUV opacity of warm, solid-density aluminum at photon energies between the plasma frequency at 15 eV and the L-edge at 73 eV, using both density functional theory combined with molecular dynamics and a semi-analytical model in the RPA framework with the inclusion of local field corrections. As the temperature is increased from room temperature to 10 eV, with the ion and electron temperatures equal, we calculate an increase in the opacity in the range over which the degree of ionization is constant. The effect is less pronounced if only the electron temperature is allowed to increase. The physical significance of these increases is discussed in terms of intense XUV-laser matter interactions on both femtosecond and picosecond time-scales.