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.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The free-free opacity in plasmas is fundamental to our understanding of energy transport in stellar interiors and for inertial confinement fusion research. However, theoretical predictions in the ...challenging dense plasma regime are conflicting and there is a dearth of accurate experimental data to allow for direct model validation. Here we present time-resolved transmission measurements in solid-density Al heated by an XUV free-electron laser. We use a novel functional optimization approach to extract the temperature-dependent absorption coefficient directly from an oversampled pool of single-shot measurements, and find a pronounced enhancement of the opacity as the plasma is heated to temperatures of order of the Fermi energy. Plasma heating and opacity enhancement are observed on ultrafast timescales, within the duration of the femtosecond XUV pulse. We attribute further rises in the opacity on ps timescales to melt and the formation of warm dense matter.
The thermal decomposition on silicon carbide (SiC) is one of the most used growth techniques for fabrication of epitaxial graphene. However, it significantly diminishes graphene’s otherwise ...exceptional carrier mobility. Reduction of the substrate influence is therefore essential for keeping conductivity at high levels. Here we present a novel technique where a sample with epitaxial graphene grown on SiC was exposed to intense 21.2 nm radiation. A sub-nanosecond pulse at low fluence in an interval 0.4–0.7 J/cm2 was used to break covalent sp3 bonds between the SiC substrate and buffer (the first graphene layer) which remains, except for release of its intrinsic strain, almost unaffected. A detailed analysis of the irradiated area examined by several microscopic and spectroscopic methods such as white-light interferometry and micro-Raman spectroscopy shows a clear evidence of a graphene layer detached from the substrate. Higher fluences induce damage to SiC substrate which expands due to the amorphization process. Damage thresholds were obtained by an advanced method of ablative imprints and compared with those calculated by the hybrid code XTANT.
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We theoretically investigate which experimental observations enable discrimination between thermal and nonthermal melting in femtosecond laser pulse-irradiated semiconductors. We identify that ...coherent phonon excitation, visible in the oscillations of various diffraction peaks, provides an opportunity to observe ongoing modifications of interatomic potential. Decoupling the effects of anharmonicity, caused by thermal heating, from the effects of evolution of the interatomic potential, due to electronic excitation, potentially enables differentiation between the two damage channels.
By use of high intensity XUV radiation from the FLASH free-electron laser at DESY, we have created highly excited exotic states of matter in solid-density aluminum samples. The XUV intensity is ...sufficiently high to excite an inner-shell electron from a large fraction of the atoms in the focal region. We show that soft-x-ray emission spectroscopy measurements reveal the electronic temperature and density of this highly excited system immediately after the excitation pulse, with detailed calculations of the electronic structure, based on finite-temperature density functional theory, in good agreement with the experimental results.
X-ray lasers represent a powerful tool to explore matter under extreme conditions. A rigorous characterization of their output parameters is, therefore, of substantial importance for the purposes of ...the experiments being conducted at these sources. A profound knowledge of the spatial, temporal, spectral, statistical, coherence, and wavefront beam properties may protect us from an unwanted misinterpretation of the experimental data. We present an experimental technique of the spatial (transverse and longitudinal) characterization of the beam profile. Investigating ablative imprints in various materials, we evaluate the spatial properties of the incident beam, namely, the beam waist radius and position, the Rayleigh range,
M
2 parameter, and divergence. In this paper, we recall briefly our recent work at the transverse beam profile reconstruction. A newly developed method of the longitudinal beam profile characterization is the main subject of this work.
We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in ...situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference-polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources.
We have focused a beam (BL3) of FLASH (Free-electron LASer in Hamburg: lambda = 13.5 nm, pulse length 15 fs, pulse energy 10-40 microJ, 5 Hz) using a fine polished off-axis parabola having a focal ...length of 270 mm and coated with a Mo/Si multilayer with an initial reflectivity of 67% at 13.5 nm. The OAP was mounted and aligned with a picomotor controlled six-axis gimbal. Beam imprints on poly(methyl methacrylate) - PMMA were used to measure focus and the focused beam was used to create isochoric heating of various slab targets. Results show the focal spot has a diameter of < or =1 microm. Observations were correlated with simulations of best focus to provide further relevant information.
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