Session VO05: HED: Warm Dense Matter Experiments

Live

Understanding the physics of ion stopping power in warm dense matter (WDM) at low projectile velocity is of great interest both for fundamental science and inertial confinement fusion. This regime where vp (ion velocity) \textasciitilde vth (electron thermal velocity) is theoretically and experimentally challenging. We report a first measurement of the energy loss of 500 keV protons in WDM at a low ratio vp / vth $=$ 3. A novel platform for proton stopping power measurements by laser driven ion sources was developed for the experiment at the 30 fs 200 TW system CLPU - VEGA II (Spain). A low energy proton beam with small bandwidth and time spread probed WDM with Te $=$10$-$20 eV, generated via fs laser heating of a thin carbon foil. XUV spectroscopy and SOP provided the characterization of target temperature, which is used to benchmark 2D hydrodynamic simulations. The measured proton energy loss is enhanced in WDM compared to the same cold target, as expected from the theoretical modeling, and motivates further development of this measurement technique.   

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Silicon dioxide is a major constituent of the earth's crust and mantle as well as a common material for optics. X-ray absorption spectroscopy measurements have indicated that warm dense SiO2 is either a semimetal or has a significant metallic component. Ultrafast electron diffraction was performed on amorphous silicon dioxide foils irradiated by femtosecond optical pulses. From the electron diffraction patterns, pair distribution functions were calculated showing a reduction of the Si-O peak. The results are compared with molecular dynamics calculations, which predict structures with broken Si-O bonds. In addition, the time scale for the formation of the warm dense state was observed indicating a non-thermal phase transition.   

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Recent experiments investigating dense plasmas have shown significant discrepancies with continuum lowering predictions from standard plasma models. Much of the theoretical difficulties encountered in plasmas where Debye lengths are comparable to inter-particle spacings can be traced back to the difficulty of defining valence states as either purely bound, or purely free. Here we describe an approach to resolve this difficulty using finite-temperature density functional theory. By looking at the inverse participation ratio for valence states in highly ionized plasmas we propose a method to measure the ``boundness'' of a state in terms of its spatial localization. We apply this technique to help interpret spectroscopic experimental measurements of continuum lowering conducted at the LCLS free-electron laser at SLAC.   

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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 the Fermi energy. Plasma heating and opacity-enhancement is observed on ultrafast time scales, 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.   

Live

The OMEGA EP short pulse lasers have been used to heat and characterize Si in the Warm Dense Matter (WDM) regime. The primary laser, with 1100J in 10ps produced a proton beam with \textasciitilde 50J of total energy that was focused into a Si wafer face-on 500$\mu $m away, heating it to \textasciitilde 50eV in a timespan of \textless 100ps. The second laser, with 700J in 5ps, irradiated the tip of a Zn wire, producing a bremsstrahlung-like X-ray strobe to backlight the Si at various delays. Absorption measurements show the evolution of Si K-shell features throughout the heating. The Si was initially 0.9 or 1.8$\mu $m thick for adequate absorption, and it was tamped with 1.1$\mu $m CH layers to limit expansion. The expansion has been modeled with the radiation-hydrodynamics code HELIOS, and the X-ray transmission of the expanded target has been modeled with the atomic-radiative code PrismSpect. We present the spectroscopy alongside the modeling and compare the fit conditions to those predicted by LSP particle-in-cell. This proton source could be applied to thicker targets as a way to create uniform, near-solid WDM targets for opacity testing.   

Live

Resonant inelastic x-ray scattering (RIXS), while prevalent in atomic and condensed matter physics, has not been used to date in high-energy-density physics research. This can be traced to three important requirements: the x-ray source must be spectrally bright, stable, and have a tunable (narrow-bandwidth) wavelength so as to match the required resonance condition. Such requirements can now be met with the advent of X-Ray FELs such as LCLS. We show how the electronic structure of nickel, heated to form a solid-density plasma at temperatures of tens of eV on femtosecond timescales, can be studied by RIXS using LCLS. [1] We present single-shot measurements of the valence density of states in the x-ray-heated transient system, tuned over a range of incident photon energies, and extract simultaneously electron temperatures, ionization, and ionization potential energies. The RIXS spectrum provides a wealth of information on the valence structure of this solid-density plasma that goes beyond what can be extracted from x-ray absorption or emission spectroscopy alone. \ [1] O.S. Humphries, R.S. Marjoribanks, Q. van den Berg, E.C. Galtier, M.F. Kasim, H.J. Lee, A.J.F. Miscampbell, B. Nagler, R. Royle, J.S. Wark, S.M. Vinko, https://arxiv.org/abs/2001.05   

Live

X-ray Free Electron Lasers can produce solid-density plasmas by ultrafast isochoric heating and at the same time the brilliant X-rays allow for in situ diagnostics of unparalleled precision. Here we present experiments performed at the Linac Coherent Light Source and the European XFEL that for the first time combine X-ray Thomson scattering and X-ray Raman spectroscopy to study isochorically heated carbon allotropes and other carbon-containing sample materials. Our results provide unique insights into the isochoric heating process and the irradiance-dependent evolution of the electronic structure, while at the same time constraining temperature and ionization.   

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X-ray Thomson scattering is a widely recognized technique for measuring physical properties and parameters of dense plasmas. In this presentation we show the development of inelastic plasmon scattering in the forward collective scattering geometry to demonstrate first-principles temperature measurements from detailed balance and evaluate the extent of contributions from collisional damping to evaluate electrical conductivity in near-degenerate plasma conditions. Here we resolve the x-ray scattering spectrum from a spectrally narrow, intense laser-produced 9 keV Zn He-alpha x-ray probe on hohlraum-driven compressed beryllium targets with a total laser energy approaching 1 MJ. Importantly, we use a copper backlight filter to reduce the x-ray source bandwidth and deliver a narrow x-ray source to resolve the much smaller plasmon shift compared to Compton scattering.   

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Recently, we developed the capability to directly measure the evolution of DC electrical conductivity of high intensity laser excited solid density plasmas using ultrafast terahertz radiation that overcomes the high frequency (\textgreater 1PHz) electron shielding. Our measurements resolve the DC conductivity in free-electron laser-irradiated gold at lattice heating rate exceeding 10\textasciicircum 14K/s where melting occurs under superheating conditions that produce pressures of the order of 10 GPa [1]. The experimental results further allow us to determine the electron scattering between electron-electron and electron-ion. These unprecedented data will help to test and improve models of warm dense matter. [1] Z. Chen et al., PRL. 121, 075002 (2018)   

Live

Understanding the equation of state (EOS) of warm dense matter (WDM) is of particular importance to fields ranging from astrophysics to high energy density physics. Experiments have been done to characterize either the Hugoniot [1, 2] or the isentrope [3] of WDM. We demonstrate the measurement of release isentrope of materials heated isochorically up to 10 eV by a proton beam generated by the OMEGA EP short-pulse laser. Three EP long-pulse beams were used to heat an X-ray backlighter to provide an X-ray source for streaked X-ray radiography, which recorded time-dependent density profiles of the expanding target. We have used a 1D zone plate to significantly improve the spatial resolution and signal-to-noise ratio of the radiograph, so that following a method derived by Foord et al. [4], we can analyze the results accurately enough to obtain a pressure-density isentrope curve for benchmarking various EOS models. [1] T. Do\textunderscore ppner et al., Phys. Rev. Lett. 121, 025001 (2018). [2] A. L. Kritcher et al., HEDP 10, 27 (2014). [3] R. F. Smith et al. Nature 511, 330 EP (2014). [4] M. E. Foord et al., RSI 78, 2586 (2004).   

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Transport processes in warm dense matter such as diffusion remain poorly understood with considerable scatter between models and the absence of experimental data. We present results of modeling of diffusive interface broadening between diffusion couple materials in plasma generated in x-ray free electron laser (XFEL) experiments. A novel x-ray scattering technique developed recently at LLNL to measure the interdiffusion in liquids leverages the high intensity of XFELs to provide highly sensitive measurements on picosecond time scales. It will be extended~to provide highly sensitive measurements on picosecond time scales, matching warm-dense-matter diffusion processes which are directly accessible to atomistic simulation. For the experimental design we have used existing interdiffusion models relevant to warm dense matter conditions. In a few cases we directly simulate these experiments using a fully atomistic approach at the actual length and time scales. The motion of ions is simulated via the screened Coulomb potential based on a Thomas-Fermi-Poisson description of the electrons. The simulation results will be used to compare to the experiment and to improve upon existing theories and to test our atomistic model.   

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The accurate description of the nonequilibrium dynamics of correlated electrons is crucial for warm dense matter and dense quantum plasmas. Among others, the nonequilibrium Green functions (NEGF) method has proven to be a powerful tool to reliably predict the quantum dynamics. However, NEGF simulations are computationally expensive due to their $T^3$ scaling with the simulation duration $T$ . With the introduction of the generalized Kadanoff--Baym ansatz (GKBA)\footnote{P. Lipavsk\'y \textit{et al.}, Phys. Rev. B \textbf{34}, 6933 (1986)}, $T^2$ scaling could be achieved for second-order Born (SOA) selfenergies\footnote{S. Hermanns \textit{et al.}, Phys. Scr. \textbf{2012} 014036 (2012)} which has substantially extended the scope of NEGF simulations. Recently\footnote{N. Schl\"unzen \textit{et al.}, \textit{Phys.~Rev.~Lett.} {\bf 124}, 076601 (2020)}, we could show that GKBA-NEGF simulations can be performed with order $T^1$ scaling for SOA, $GW$, and T-matrix selfenergies, and even for the screened ladder approximation\footnote{J.-P. Joost \textit{et al.}, Phys. Rev. B \textbf{101}, 245101 (2020)}. Here, we show numerical results for various many-body approximations and demonstrate that a tremendous computational speed-up can be achieved in practice.   

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Understanding ultrafast melting of metals is crucial for applications ranging from laser micro-machining to warm dense matter (WDM) experiments. Here we report results of using femtosecond electron diffraction to study structural evolution of Cu as it underwent ultrafast laser-induced solid-liquid phase transition. In our experiments, 40-nm-thick polycrystalline Cu films were irradiated by 400nm, 130fs laser pulses to produce WDM states. Structural evolution of irradiated target was measured with 3.2MeV, 350fs electron pulses. We observed homogeneous melting that occurs within 10ps at absorbed energy densities of \textasciitilde 1.2-2.5 MJ/kg. The measured melting times are understood with two-temperature model simulations. The experimental results are consistent with TTM simulations using electron-ion coupling strength G$_{\mathrm{ei}}$ that is inferred from the T$_{\mathrm{i}}$ evolution from Laue peak dynamics. The inferred G$_{\mathrm{ei}}$ has a much weaker T$_{\mathrm{e}}$ dependence and is about a factor of four lower than the T$_{\mathrm{e}}$-dependent value calculated by density functional theory [Z. Lin, PRB (2008)].