Kiel 2011 – wissenschaftliches Programm

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SYCP: Symposium Charged particles in ultra-short fields

SYCP 1: Charged particles in ultra-fast fields

SYCP 1.2: Hauptvortrag

Donnerstag, 31. März 2011, 14:30–15:00, HS D

High Energy Density Experiments at the Free-Electron Laser Facility FLASH at DESY — •Sven Toleikis — Deutsches Elektronen-Synchrotron DESY, Hamburg

FLASH is a unique source of extremely bright, coherent, and ultrashort pulses of extreme-ultraviolet radiation and soft X-rays enabling researchers to explore the temporal evolution of physical, chemical, and biochemical processes happening in femtoseconds or picoseconds. FLASH produces laser light of short wavelengths from the extreme ultraviolet down to soft X-rays (wavelength range of the fundamental: 4.5 - 60 nm). The light comes in pulses, as in an electronic flashlight, but the pulses are a 100 billion times shorter (pulse duration 20-200 fs) [1]. Since 2005 FLASH operates as a user facility. Since then, the unprecedented shortness and intensity of the soft X-ray pulses as well as their coherence has opened entirely new fields of research and led to new discoveries. The unique radiation properties of FLASH allows when focussed to *1 *m sizes to reach record intensities over 1016 W/cm* in the soft X-ray wavelength regime. Employing these intensities in an experiment, it became possible to saturate the absorption of an L-shell transition in aluminium, hereby the Al sample becomes transparent for soft X-rays (at 92 eV photon energy) [2]. This has never been observed in core-electron transitions owing to the short lifetime of the exited states involved and the high intensities of soft X-rays needed. Considering the relevant lifetimes, one can infer that after the soft X-rays have passed, the sample is an exotic state, where all irradiated aluminium atoms have an L-shell hole, and the valence band has been heated to ~9 eV temperature, whereas the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to the warm dense matter regime, at ~25 eV temperatures. This method is ideal to create homogenous warm dense matter (WDM), which is highly relevant to planetary science, astrophysics and inertial confinement fusion. The short pulse duration and high intensity of FLASH soft x-ray radiation also allows to heat and probe highly homogeneous warm dense non-equilibrium hydrogen within a single light pulse. By analyzing the spectrum of the Thomson scattered photons around 13.5 nm (92 eV photon energy), one can determine the plasma temperature and electron density. These results have been compared via simulations with different models for impact ionization, which is the main interaction on this early femtosecond time scale. From the comparison one finds that classical models of this interaction describe our dense plasma conditions better than state of the art theories [3].

References [1] W. Ackermann et al., "Operation of a free-electron laser from the extreme ultraviolet to the water window", Nature Photon. 1, 336 (2007). [2] B. Nagler et al., "Turning solid aluminium transparent by intense soft X-ray photoionization", Nature Phys. 5, 693 (2009). [3] R.R. Fäustlin et al., "Observation of Ultrafast Nonequilibrium Collective Dynamics in Warm Dense Hydrogen", Phys. Rev. Lett. 104, 125002 (2010).