Session GO6: Fundamental High-Energy Density Science

Frontiers in plasma science: a high energy density perspective

The potential for ground-breaking research in plasma physics in high energy density (HED) regimes is compelling. The combination of HED facilities around the world spanning microjoules to megajoules, with time scales ranging from femtoseconds to microseconds enables new regimes of plasma science to be experimentally probed. The ability to shock and ramp compress samples and simultaneously probe them allows dense, strongly coupled, Fermi degenerate plasmas relevant to planetary interiors to be studied. Shock driven hydrodynamic instabilities evolving into turbulent flows relevant to the dynamics of exploding stars are being probed. The physics and dynamics of magnetized plasmas relevant to astrophysics and inertial confinement fusion are also starting to be studied. High temperature, high velocity interacting flows are being probed for evidence of astrophysical collisionless shock formation. Turbulent, high magnetic Reynolds number flows are being experimentally generated to look for evidence of the turbulent magnetic dynamo effect. And new results from thermonuclear reactions in dense hot plasmas relevant to stellar interiors are starting to emerge. A selection of examples providing a compelling vision for frontier plasma science in the coming decade will be presented.   

Exploding Pusher Targets for Electron-Ion Coupling Measurements

Over the past several years, we have conducted theoretical investigations of electron-ion coupling and electronic transport in plasmas. In the regime of weakly coupled plasmas, we have identified models that we believe describe the physics well, but experimental data is still needed to validate the models. We are currently designing spectroscopic experiments to study electron-ion equilibration and/or electron heat transport using exploding pusher (XP) targets for experiments at the National Ignition Facility. Two platforms are being investigated: an indirect drive XP (IDXP) with a plastic ablator and a polar-direct drive XP (PDXP) with a glass ablator. The fill gas for both designs is D$_{2}$. We propose to use a higher-Z dopant, such as Ar, as a spectroscopic tracer for time-resolved electron and ion temperature measurements. We perform 1D simulations using the ARES hydrodynamic code, in order to produce the time-resolved plasma conditions, which are then post-processed with CRETIN to assess the feasibility of a spectroscopic measurement. We examine target performance with respect to variations in gas fill pressure, ablator thickness, atom fraction of the Ar dopant, and drive energy, and assess the sensitivity of the predicted spectra to variations in the models for electron-ion equilibration and thermal conductivity. Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-675219   

Study of shear viscosity for dense plasmas by equilibrium molecular dynamics in asymmetric Yukawa ionic mixtures

We present molecular dynamics (MD) calculations of shear viscosity for asymmetric mixed plasma for thermodynamic conditions relevant to astrophysical and Inertial Confinement Fusion plasmas. Specifically, we consider mixtures of deuterium and argon at temperatures of 100-500 \textit{eV} and a number density of 10$^{25}$ \textit{ions/cc}. The motion of 30000-120000 ions is simulated in which the ions interact via the Yukawa (screened Coulomb) potential. The electric field of the electrons is included in this effective interaction. Shear viscosity is calculated using the Green-Kubo approach with an integral of the shear stress autocorrelation function, a quantity calculated in the equilibrium MD simulations. We study different mixtures with increasing fraction of the minority high-Z element (Ar) in the D-Ar plasma mixture. In the more weakly coupled plasmas, at 500 \textit{eV} and low Ar fractions, results from MD compare very well with Chapman-Enskog kinetic results. We introduce a model that interpolates between a screened-plasma kinetic theory at weak coupling and the Murillo Yukawa viscosity model at higher coupling. This hybrid kinetics-MD viscosity model agrees well with the MD results over the conditions simulated.   

Ionic Transport in High Energy-Density Matter

Ionic transport coefficients for dense plasmas have been numerically computed using an effective Boltzmann approach. We have developed a simplified effective potential approach that yields simple fits for all of the relevant cross sections and collision integrals. We have validated our new results with molecular dynamics simulations. Molecular dynamics has also been used to examine the underlying assumptions of the Boltzmann approach through a categorization of behaviors of the velocity autocorrelation function. Implications of these results on Coulomb-logarithm approaches are also discussed. The impact of our new results on material mixing in high energy-density environments, including interdiffusion near an interface and viscous corrections to Rayleigh-Taylor instability growth, is examined.   

Stopping Power and Transport in Warm and Hot Dense Matter

Stopping power is not only of direct relevance to the heating of fusion-burning plasmas and fast ignition inertial confinement fusion, but also serves as a velocity-resolved probe of the many-body response of plasma. The accuracy of a model for a set of plasma conditions and projectile energy and charge serves as a detailed test of collision operators and their predicted transport coefficients. Classical molecular dynamics studies [1] can tell us much about the relative importance of strong scattering, nonlinear screening, and inter-particle correlations of a uniform plasma. The dominant quantum correction for hot dense matter is quantum diffraction, which can be experimentally confirmed [2]. However, the presence of bound states and inhomogeneous electronic structure in warm dense matter requires more sophisticated models. These models fall into two main classes: the local density approximation [3] and bound-free splitting [3]. High-precision experiments ($\sim$ 3\%) can now confirm such approximations [3], but a full survey of parameter space must be done. I will put these models in a unified framework and discuss their relationship. \\[4pt] [1] Grabowski et al., PRL 111, 215002 (2013).\newline [2] Frenje et al., submitted.\newline [3] Zylstra et al., PRL, 114, 215002 (2015).   

First-Principles Investigations on Thermal Conductivity and Average Ionization of CH Ablators Under Extreme Conditions

A plastic CH ablator (polystyrene) is often used for inertial confinement fusion (ICF) target designs. Upon intense laser or x-ray ablations, a CH ablator can be shocked to warm-dense-matter (WDM) conditions. Many-body coupling and quantum electron degeneracy are expected to play an essential role in determining the properties of such warm dense plasmas. Using \textit{ab initio} methods of quantum molecular dynamics (QMD), we have performed investigations on the principal Hugoniot of a CH ablator,\footnote{S. X. Hu \textit{et al}, Phys. Rev. E \textbf{89}, 063104 (2014).} the first-principles equation-of-state table of CH, and its effect on ICF simulations.\footnote{ S. X. Hu \textit{et al.,} submitted to Physical Review E.} In this presentation, we focus on the thermal conductivity and average ionization of CH-ablators under a wide range of plasma temperatures and densities. The resulting thermal conductivity ($\kappa$) and average ionization ($\langle $Z$\rangle$) show large differences from the usual model predictions in the WDM regime. These results, being fitted with analytical functions of plasma density and temperature, have been incorporated into radiation$-$hydrodynamics codes. Their effects on the ICF implosion simulations will be presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944 and the Scientific Campaign 10 at LANL under Contract No. DE-AC52-06NA25396.   

Radiative properties measurements of photoionized plasmas on Z

Physical descriptions of accretion-powered objects such as black holes, x-ray binaries, or AGN are informed through the interpretation of emergent spectra from the photoionized plasmas that surround them. Line formation in photoionized plasmas is dependent on the details of the radiation transport treatment and the so-called Resonant Auger Destruction hypothesis typically required to interpret the relativistically broadened Fe K$\alpha$ emitted from near the black hole event horizon. The Z facility at Sandia National Laboratories can produced such photoionized plasmas producing 1.6MJ of x-rays from the z-pinch dynamic hohlraum. The extended suite of diagnostics allows for a detailed characterization of plasmas conditions through absorption spectroscopy. present accurate and high-resolution emergent intensity observed from a photoionized silicon plasma for a discrete set of column densities that will help us evaluate understanding for radiation transport in accretion powered objects. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.   

Laboratory-Produced X-Ray Photoionized Plasmas for Astrophysics Exploration

X-ray photoionized plasmas are rare in the laboratory, but of broad importance in astrophysical objects such as active galactic nuclei, x-ray binaries. Indeed, existing models are not yet able to accurately describe these plasmas where ionization is driven by radiation rather than electron collisions. Here, we describe an experiment on the LULI2000 facility whose versatility allows for measuring the X-ray absorption of the plasma while independently probing its electron density and temperature. The bright X-ray source is created by the two main beams focused inside a gold hohlraum and is used to photoionise a Neon gas jet. Then, a thin gold foil serves as a source of backlit photons for absorption spectroscopy. The transmitted spectrum through the plasma is collected by a crystal spectrometer. We will present the experimental setup used to characterize both plasma conditions and X-ray emission. Then we will show the transmitted spectra through the plasma to observe the transition from collision dominated to radiation dominated ionization and compare it to model predictions. This work was performed under the auspices of the U.S.Department of Energy by Lawrence Livermore Natl Lab under Contract No. DE-AC52-07NA27344.   

Creation of optically-thin solid-density plasmas using LCLS

The advent of X-ray free-electron-lasers such as LCLS provides the capability to truly isochorically heat solid-density matter on femtosecond time-scales [1]. K-shell emission from such plasmas has provided new information on ionization potential depression [2] and collisional ionisation rates [3]. However, in previous work the targets were 1-$\mu$m thick, resulting in high-opacity on the K-shell transitions. We report here results of a detailed study of K-shell emission from exactly solid-density Mg plasmas with thicknesses ranging from 500 down to 25 nm -- just over 100 atoms across. A curve-of-growth analysis exhibits text-book behavior, and confirms peak optical depths for the thinnest targets well below unity, in excellent agreement with simulations. The rich data-set provides information on line-widths, collisional dynamics, and radiation transfer in solid density plasmas. \\[4pt] [1] S.M. Vinko {\it et al.}, Nature, {\bf 482}, 59 (2012)\\[0pt] [2] O. Ciricosta {\it et al.}, Phys. Rev. Lett., {\bf 109}, 065002 (2012)\\[0pt] [3] S.M. Vinko {\it et al.}, Nat. Comm., {\bf 6}, 6397 (2015)   

Measuring Ionization at Extreme Densities

A precise knowledge of ionization at given temperature and density is crucial in order to properly model compressibility and heat capacity of ICF ablator materials for efficient implosions producing energy gain. Here, we present a new experimental platform to perform spectrally resolved x-ray scattering measurements of ionization, density and temperature in imploding CH or beryllium capsules on the National Ignition Facility. Recording scattered x-rays at 9 keV from a zinc He-alpha plasma source at a scattering angle of 120 degrees, first experiments show strong sensitivity to k-shell ionization, while at the same time constraining density and temperature. This platform will allow for x-ray Thomson scattering studies of dense plasmas with free electron densities up to 10$^{25}$ cm$^{-3}$, giving the possibility to investigate effects of continuum lowering and Pauli blocking on the ablator ionization state right before stagnation of the implosion.   

Saturable Absorption of an X-Ray Free-Electron-Laser Heated Solid-Density Plasma

High-intensity $\approx$10$^{17}$ Wcm$^{-2}$, short duration (100 fsec) x-ray pulses from the LCLS x-ray free-electron laser, with photon energies ranging from below to above the K-edge of cold Al (1560 eV), are used to generate and probe a solid-density aluminum plasma. The photon-energy-dependent transmission of the heating beam is studied through the use of a photodiode. Saturable absorption is observed, with the resulting transmission differing significantly from the cold case, with the increased transmission being due to the K-edge energy of the dominant ion species shifting in time as the solid-density target is heated, in good agreement with atomic-kinetics simulations [1]. \\[4pt] [1] D.S. Rackstraw {\it et al.}, Phys. Rev. Lett., {\bf} 114, 015003 (2015)   

Measuring Femtosecond Collisional Ionization Rates in Solid-Density Plasmas

The rate at which atoms and ions within a plasma are further ionized by collisions with free electrons is a fundamental parameter that dictates the dynamics of plasma systems at intermediate and high densities. While collisional ionization rates are well known experimentally in a few dilute systems, similar measurements for non-ideal plasmas at densities approaching or exceeding those of solids remain elusive. Here we illustrate a spectroscopic method capable of measuring rates of collisional ionization dynamics in solid-density plasmas by clocking them to Auger recombination processes. We have recently employed this technique on the LCLS X-ray free-electron laser at SLAC and will present the first experimental results for optically-thin, solid-density magnesium plasmas at peak temperatures exceeding 200 eV. \\[4pt] [1] S.M. Vinko {\it et al.}, Nature {\bf 482}, 59 (2012). \\[0pt] [2] S.M. Vinko {\it et al.}, Nature Communications {\bf 5}, 6397 (2015).   

Measurements of Continuum Lowering in Strongly Coupled Plasmas of Elements and Compounds

We have used the X-ray pulse of the Linac Coherent Light Source to perform a charge-resolved measurement of continuum lowering in solid-density plasmas, at temperatures up to 200 eV, from Mg, Al, Si, alumina, silica and mica. The comparison between Al or Si and their respective compounds shows that the K-edges for the same element in different plasma environments is unaffected by the considerable density differences, contrary to the predictions of any analytical continuum lowering model. Conversely, the K-edges for all of the materials can be approximated surprisingly well by pure atomic-physics calculations, consistent with recent DFT predictions [1]. The results provide strong evidence that the ion-sphere models, used to describe continuum lowering as well as to calculate the equation of state of materials, may need to be revisited for strongly coupled systems. \\[4pt] [1] S.M. Vinko {\it et al.}, Nat. Comm. {\bf 5}, 3533 (2015).   

Kinetic Modeling of Ultraintense X-Ray Laser-Matter Interactions

High-intensity XFELs have become a novel way of creating and studying hot dense plasmas. The LCLS at Stanford can deliver a millijoule of energy with more than 10$^{12}$ photons in a $\sim$ 100 femtosecond pulse [1]. By tightly focusing the beam to a micron-scale spot size, the XFEL can be intensified to more than 10$^{18}$ W/cm$^{2}$, making it possible to heat solid matter isochorically beyond a million degrees (\textgreater 100 eV). Such extreme states of matter are of considerable interest due to their relevance to astrophysical plasmas. Additionally, they will allow novel ways of studying equation-of-state and opacity physics under Gbar pressure and strong fields. Photoionization is the dominant x-ray absorption mechanism and triggers the heating processes. A photoionization model that takes into account the subshell cross-sections has been developed in a kinetic plasma simulation code, PICLS, that solves the x-ray transport self-consistently [2]. The XFEL--matter interaction with several elements, including solid carbon, aluminum, and iron, is studied with the code, and the results are compared with recent LCLS experiments. \\[4pt] [1] S. M. Vinko \textit{et al.}, \textit{Nature} \textbf{482}, 59-62 (2012).\\[0pt] [2] Y. Sentoku \textit{et al.}, \textit{Phys. Rev. E} \textbf{90}, 051102 (2014).   

Atomic rate coefficients in a degenerate plasma

The electrons in a dense, degenerate plasma follow Fermi-Dirac statistics, which deviate significantly in this regime from the usual Maxwell-Boltzmann approach used by many models. We present methods to calculate the atomic rate coefficients for the Fermi-Dirac distribution and present a comparison of the ionization fraction of carbon calculated using both models. We have found that for densities close to solid, although the discrepancy is small for LTE conditions, there is a large divergence from the ionization fraction by using classical rate coefficients in the presence of strong photoionizing radiation. We have found that using these modified rates and the degenerate heat capacity may affect the time evolution of a plasma subject to extreme ultraviolet and x-ray radiation such as produced in free electron laser irradiation of solid targets.