Innovative XUV and X.ray Plasma Spectroscopy to explore Warm Dense Matter

Zusammenfassend widmet sich die vorliegende Arbeit der Erforschung definierter Zustaende warmer dichter Materie. Dieses Wissen ist von herausragender Bedeutung für die Laser-Fusions-Forschung, die Erforschung von Materiezustaenden mit hoher Dichte, sowie eine Vielzahl astrophysikalisch-relevanter Fragestellungen der Plasmaforschung. Die vorliegende Arbeit zeigt innovative Methoden der Erzeugung und Charakterisierung warmer dichter Materie auf. Dieser Nicht-Gleichgewichts-Zustand zwischen kaltem Festkoeorper und idealem Plasma wird durch Experimente mit hoher Energiedichte erzeugt. Seine Eigenschaften wurden mit hoher Praezision studiert. Die Erzeugung eines definierten Zustandes warmer dichter Materie mit Hilfe optischer Kurzpluslaser hoher Intensitaeten ist herausfordernd. Energiereiche Elektronen aus einem kleinen, heißen Plasma auf der Targetvorderseite heizen die kalte Materie durch Stoßionisation auf. Hohe spektrale sowie raeumliche Aufloesungen sind noetig, um nicht ueber verschiedene Plasmazustaende zu mitteln. Der Autor wendet hoch entwickelde Roentgenspektroskopie charakteristischer Emissionslinien, gefolgt von numerischer Datenverabeitung, an. Das Ergebnis enthaelt viele Informationen, da die Linienform durch die Plasmaparameter beeinflusst wird und die Roentgenstrahlen zudem transparent für Festkoerperdichte sind. Alternative Wege zur Erzeugung warmer dichter Materie sind momentan im Fokus der Hohe-Energiedichte-Forschung. In der vorliegenden Arbeit wird erstmals die Erzeugung warmer dichter Materie durch direkte Photoionisation gebundener Elektronen mit den Femtosekundenpulsen des weltweit ersten weichen Roentgenlasers FLASH demonstriert. Innerhalb der durch die internationale Peak-Brightness-Collaboration geschaffenen Rahmenbedingungen war der Autor verantwortlich für Pionierarbeiten. Schließlich gelang die erste vollstaendige Charakterisierung warmer dicher Materie an FLASH sowie die erste Demonstration saettigbarer Absorption im weichen Roentgenbereich

Bent crystal spectrometer for both frequency and wavenumber resolved x-ray scattering at a seeded free-electron laser

We present a cylindrically curved GaAs x-ray spectrometer with energy resolution $\Delta E/E = 1.1\cdot 10^{-4}$ and wave-number resolution of $\Delta k/k = 3\cdot 10^{-3}$, allowing plasmon scattering at the resolution limits of the Linac Coherent Light Source (LCLS) x-ray free-electron laser. It spans scattering wavenumbers of 3.6 to $5.2/$\AA\ in 100 separate bins, with only 0.34\% wavenumber blurring. The dispersion of 0.418~eV/$13.5\,\mu$m agrees with predictions within 1.3\%. The reflection homogeneity over the entire wavenumber range was measured and used to normalize the amplitude of scattering spectra. The proposed spectrometer is superior to a mosaic HAPG spectrometer when the energy resolution needs to be comparable to the LCLS seeded bandwidth of 1~eV and a significant range of wavenumbers must be covered in one exposure

Imaging Shock Waves in Diamond with Both High Temporal and Spatial Resolution at an XFEL

The advent of hard x-ray free-electron lasers (XFELs) has opened up a variety of scientific opportunities in areas as diverse as atomic physics, plasma physics, nonlinear optics in the x-ray range and protein crystallography. In this article, we access a new field of science by measuring quantitatively the local bulk properties and dynamics of matter under extreme conditions, in this case by using the short XFEL pulse to image an elastic compression wave in diamond. The elastic wave was initiated by an intense optical laser pulse and was imaged at different delay times after the optical pump pulse using magnified x-ray phase-contrast imaging. The temporal evolution of the shock wave can be monitored, yielding detailed information on shock dynamics, such as the shock velocity, the shock front width and the local compression of the material. The method provides a quantitative perspective on the state of matter in extreme conditions

Ultra-fast yttrium hydride chemistry at high pressures via non-equilibrium states induced by x-ray free electron laser

Controlling the formation and stoichiometric content of desired phases of materials has become a central interest for the study of a variety of fields, notably high temperature superconductivity under extreme pressures. The further possibility of accessing metastable states by initiating reactions by x-ray triggered mechanisms over ultra-short timescales is enabled with the development of x-ray free electron lasers (XFEL). Utilizing the exceptionally high brilliance x-ray pulses from the EuXFEL, we report the synthesis of a previously unobserved yttrium hydride under high pressure, along with non-stoichiometric changes in hydrogen content as probed at a repetition rate of 4.5\,MHz using time-resolved x-ray diffraction. Exploiting non-equilibrium pathways we synthesize and characterize a hydride with yttrium cations in an \textit{A}15 structure type at 125\,GPa, predicted using crystal structure searches, with a hydrogen content between 4.0--5.75 hydrogens per cation, that is enthalpically metastable on the convex hull. We demonstrate a tailored approach to changing hydrogen content using changes in x-ray fluence that is not accessible using conventional synthesis methods, and reveals a new paradigm in metastable chemical physics

The Speed of Sound in Methane under Conditions of the Thermal Boundary Layer of Uranus

We present the first direct observations of acoustic waves in warm dense matter. We analyze wavenumber- and energy-resolved X-ray spectra taken from warm dense methane created by laser-heating a cryogenic liquid jet. X-ray diffraction and inelastic free electron scattering yield sample conditions of 0.3$\pm$0.1 eV and 0.8$\pm$0.1 g/cm$^3$, corresponding to a pressure of $\sim$13 GPa and matching the conditions predicted in the thermal boundary layer between the inner and outer envelope of Uranus. Inelastic X-ray scattering was used to observe the collective oscillations of the ions. With a highly improved energy resolution of $\sim$50 meV, we could clearly distinguish the Brillouin peaks from the quasi-elastic Rayleigh feature. Data at different wavenumbers were used to obtain a sound speed of 5.9$\pm$0.5 km/s, which enabled us to validate the use of Birch's law in this new parameter regime.Comment: 7 pages, 4 figures with supplementary informatio

Spatially resolved X-ray Scattering from shock-compressed Carbon at the LCLS

The diversity of the electronic properties of carbon makes it of key interest to the material science community; nowhere is this more evident than in the myriad potential applications of structured allotropes like grapheme and nano tubes. By contrast, at the high pressures typical of planetary and stellar interiors, the behavior of carbon is poorly understood with large uncertainties in the conductivity and even the material phase. There is growing evidence of the abundance of diamond in the interiors of the ice giant planets Uranus and Neptune; the conductivity of which could potentially influence models for the origin of the unusual magnetic fields of these planets. In laboratory experiments, practical issues with gradients in the temperature and densityof shock compressed matter have hindered accurate measurement and further from distinguishing theoretical models. Here, we present spatially resolved x-ray scattering experiments using LCLS free electron laser to examine and understand the gradients of thermal properties under dynamic shock loading. We employed curved mosaic and perfect imaging crystals. Compared with hydro-dynamic simulations, we present time-resolved data on plasmon dispersion, axial compression gradients and finally carbon melting at shock coalescence