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The time-resolved study of radiation transport in dense aluminium plasma awarded

The fifth summer school SFEL 2018 – School of XFEL and Synchrotron Radiation Users (27-31 May 2018) organized by the Pavol Jozef Šafárik University (Košice, Slovakia) in collaboration with the European XFEL (Schenefeld, SRN) and other institutions was held in a small village Liptovský Ján, which lies on a beautiful foothill of the Low Tatra mountains in Slovakia. The school organized a competition for the best presentation given by a young scientist. RNDr. Vojtěch Vozda, a PhD student at the Institute of Physics of Charles University in Prague and staff member of the Department of Radiation and Chemical Physics of the Institute of Physics of the Czech Academy of Sciences, won the first place in a strong competition of numerous advanced studies carried out at ESRF, Diamond, PETRA, FLASH and other large-scale facilities. He presented a successful study of absorption of soft x-ray radiation in the warm dense matter (WDM). The experiment was carried out at the FLASH (Free Electron LASer in Hamburg) facility and the experimental data was processed during Vojtěch‘s three-month stay at the Department of Physics of the University of Oxford. The work was supervised by Jaromír Chalupský (Institute of Physics, Czech Academy of Sciences), Jan Kunc (Institute of Physics of Charles University), and Sam M. Vinko (University of Oxford).

Inverse bremsstrahlung is the dominant absorption mechanism in aluminium for photons with an energy exceeding the plasma frequency (15 eV) and yet below the L-edge (73 eV). Even though several papers studying properties of solids heated by intense soft x-rays have been published, the ultrafast processes which follow the isochoric heating induced by short and intense soft x-ray pulse still remain to be clarified. In his talk, Vojtěch reported the first pump and probe measurements of free-free opacity in warm dense aluminium plasma performed at the FLASH facility in Germany. A significant increase of the opacity was registered in a thin Al foil heated by the soft x-rays with a wavelength of 32 nm to electron temperatures around 20 eV. An increase in the opacity followed a rise in the energy density exceeding 1011 J/m3. Within the experimental error, first principles calculations exhibit a very good agreement with the data obtained by the measurement.

The Study of the radiation transport in dense plasmas is very important for astrophysics, planetology and inertial confinement fusion.

A schematic drawing of the experimental setup: An energy of the incoming FEL pulse was measured with the gas monitor detector (GMD). Both split and delay of the FEL beam occur in the auto-correlator. Two parallel beams separated in space and time are then focused by a multilayer-coated off-axis parabola onto the target (i.e., a thin aluminium foil) behind which is placed an x-ray CCD camera measuring the soft x-ray radiation transmitted through the target.

More details can be provided by Jaromír Chalupský (E-mail address: chalatfzu [dot] cz; Phone: + 420 266052696).