Professor Justin Wark

Abstract:

The analysis of the radiative properties of plasmas created by XUV and X-ray free electron laser radiations provides a tremendous challenge to researchers to investigate matter under extreme conditions. In the present work we report about the theoretical analysis of the radiation emission of Al heated by the interaction of 10 fs focused (1 μm) free electron laser radiation at 13.5 nm at intensities of about 1016 W/cm2. The data show strong resonance line emission 3l -2l′ from Ne-like Al but also numerous intense broad emission structures in the spectral range from 10-30 nm. Atomic structure analysis indicate that these emission structures might originate from multiple excited states with L-holes. By means of a genetic algorithm we analyze possible excitation channels driven directly by the FLASH free electron laser as well as by heated plasma electrons.

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:

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.