Exploring dense plasmas using X-ray free-electron lasers

The advent of the first free-electron X-ray lasers (XFELs), FLASH in 2004 and LCLS in 2009, may prove to be the most profound development since the invention of the laser and, equally, the synchrotron. Sharp improvements in a number of laser parameters, most notably intensity and pulse duration, support this expectation. This brings scientific dreams within reach. Indeed, the unprecedented opportunities and expectations have triggered considerable research activities worldwide. The talk will yield an overview of the experimental application of today's XFELs to explore matter in extreme conditions with advanced x-ray spectroscopy.In the near future, the High Energy Density Science (HED) instrument at the European X-ray Free-Electron Laser Facility in Hamburg, Germany, will allow investigations of an even wider range of materials and systems at extreme conditions. For sample excitation a variety of high energy drivers will be installed. In particular, three separate optical laser systems will be available for warm- to hot-dense-matter creation, dynamic compression and laser-plasma interaction in electron-relativistic regime. These drivers will allow studying various phase space parameters with time-resolution down to 10 fs, pressures into the TPa regime, and electric field strength up to $10^{20}$ W/cm. This unique instrument is designed to enable the application of various x-ray probes including spectroscopic, diffraction and imaging methods. It will operate in the photon energy range from 3 to above 20 keV and will feature a variety of platforms facilitating the usage of different techniques in user-driven experiments. Future capabilities of the HED instrument, including the contributions from the Helmholtz International Beamline for Extreme Fields (HIBEF) will be presented along with selected science cases