Matter with a high energy density (>10(5) joules per cm(3)) is prevalent throughout the Universe, being present in all types of stars and towards the centre of the giant planets; it is also relevant for inertial confinement fusion. Its thermodynamic and transport properties are challenging to measure, requiring the creation of sufficiently long-lived samples at homogeneous temperatures and densities. With the advent of the Linac Coherent Light Source (LCLS) X-ray laser, high-intensity radiation (>10(17) watts per cm(2), previously the domain of optical lasers) can be produced at X-ray wavelengths. The interaction of single atoms with such intense X-rays has recently been investigated. An understanding of the contrasting case of intense X-ray interaction with dense systems is important from a fundamental viewpoint and for applications. Here we report the experimental creation of a solid-density plasma at temperatures in excess of 10(6) kelvin on inertial-confinement timescales using an X-ray free-electron laser. We discuss the pertinent physics of the intense X-ray-matter interactions, and illustrate the importance of electron-ion collisions. Detailed simulations of the interaction process conducted with a radiative-collisional code show good qualitative agreement with the experimental results. We obtain insights into the evolution of the charge state distribution of the system, the electron density and temperature, and the timescales of collisional processes. Our results should inform future high-intensity X-ray experiments involving dense samples, such as X-ray diffractive imaging of biological systems, material science investigations, and the study of matter in extreme conditions.

We have used the Linac Coherent Light Source to generate solid-density aluminum plasmas at temperatures of up to 180 eV. By varying the photon energy of the x rays that both create and probe the plasma, and observing the K-α fluorescence, we can directly measure the position of the K edge of the highly charged ions within the system. The results are found to disagree with the predictions of the extensively used Stewart-Pyatt model, but are consistent with the earlier model of Ecker and Kröll, which predicts significantly greater depression of the ionization potential.

This review is focused on free-electron lasers (FELs) in the hard to soft x-ray regime. The aim is to provide newcomers to the area with insights into: the basic physics of FELs, the qualities of the radiation they produce, the challenges of transmitting that radiation to end users and the diversity of current scientific applications. Initial consideration is given to FEL theory in order to provide the foundation for discussion of FEL output properties and the technical challenges of short-wavelength FELs. This is followed by an overview of existing x-ray FEL facilities, future facilities and FEL frontiers. To provide a context for information in the above sections, a detailed comparison of the photon pulse characteristics of FEL sources with those of other sources of high brightness x-rays is made. A brief summary of FEL beamline design and photon diagnostics then precedes an overview of FEL scientific applications. Recent highlights are covered in sections on structural biology, atomic and molecular physics, photochemistry, non-linear spectroscopy, shock physics, solid density plasmas. A short industrial perspective is also included to emphasise potential in this area.

An accurate description of the ionization potential depression of ions in plasmas due to their interaction with the environment is a fundamental problem in plasma physics, playing a key role in determining the ionization balance, charge state distribution, opacity and plasma equation of state. Here we present a method to study the structure and position of the continuum of highly ionized dense plasmas using finite-temperature density functional theory in combination with excited-state projector augmented-wave potentials. The method is applied to aluminium plasmas created by intense X-ray irradiation, and shows excellent agreement with recently obtained experimental results. We find that the continuum lowering for ions in dense plasmas at intermediate temperatures is larger than predicted by standard plasma models and explain this effect through the electronic structure of the valence states in these strong-coupling conditions.

We investigate ultrafast (fs) electron dynamics in a liquid hydrogen sample, isochorically and volumetrically heated to a moderately coupled plasma state. Thomson scattering measurements using 91.8 eV photons from the free-electron laser in Hamburg (FLASH at DESY) show that the hydrogen plasma has been driven to a nonthermal state with an electron temperature of 13 eV and an ion temperature below 0.1 eV, while the free-electron density is 2:8 Â 10 20 cm À3 . For dense plasmas, our experimental data strongly support a nonequilibrium kinetics model that uses impact ionization cross sections based on classical free-electron collisions. The investigation of warm dense matter (WDM) is one of the grand challenges of contemporary physics [1]. WDM is a plasma state characterized by moderate-tostrong interparticle coupling which takes place at freeelectron temperatures of several eV and free-electron densities around solid density [1]. It is present in many physical environments, such as planetary interiors [2,3], gravitationally collapsing protostellar disks, laser matter interaction and particularly during the implosion of an inertial confinement fusion capsule [4]. While in the astrophysical context WDM exists under stable conditions, in the laboratory it is achieved only as a transient state bridging condensed matter and hot plasma regimes. Here, we report on the first investigation of the nonequilibrium transition of hydrogen from a liquid to a moderately coupled plasma on the fs time scale, induced by highly intense soft-x-ray irradiation. This is an important step towards the investigation of strongly-coupled plasmas which are within reach of current light sources such as the Linac Coherent Light Source (LCLS). Our measurement enables unprecedented direct tests of nonequilibrium statistical models beyond mean field theories in a regime where collision and relaxation processes are dominant [5][6][7].The use of x-ray scattering for the investigation of dense, strongly-coupled plasmas was successfully demonstrated in the past decade [5,[7][8][9][10][11]. This technique is the x-ray analog of optical Thomson scattering (TS) [12] and enables the experimental determination of plasma parameters in dense systems where optical light cannot penetrate. While previous experiments were carried out using highenergy laser facilities, the advent of soft-and hard-x-ray free-electron lasers (FELs) makes ultrashort high brightness beams available for this type of research [13,14]. This Letter reports on ultrafast heating of liquid hydrogen and TS measurement of dense plasma parameters using softx-ray FEL radiation. For the first time, nonequilibrium distributions are observed and the underlying relaxation dynamics are compared with kinetic models showing electron relaxation times in the order of 20 fs, thus, shorter than the pulse duration.The scattering taking place is collective TS, which is characterized by a spectrally blue and red shifted response due to collective electron motion, plasmons, and nearly elastic scattering due t...