Session M24: Matter at Extreme Conditions: High Energy Density Physics

50 TPa Hugoniot measurements using laser-driven Mach waves

Plasma equation of state models in the tens of TPa pressure regime are based almost entirely on quantum mechanical theories since prior experiments could not reach these conditions.  Achieving experimental data in this regime is important because the models tested at lower pressures cannot be simply extrapolated.  At high pressures the contribution to the free energy from thermally-excited electrons starts to become comparable to the cold and ionic contributions, and is predicted to drive significant changes in properties like compressibility, opacity and transport.  These models are important because these conditions are reached in the interiors of stars and large planets, and in ICF and HED plasma experiments.  In this talk we will describe the design, implementation, and development progress at the Omega and NIF laser facilities of a laser-driven Mach wave platform for generating ultra-high pressure planar shock waves to drive stepped physics packages that can be probed using interferometric velocimetry.  We will report results from impedance-matching measurements of the Au Hugoniot near 50 TPa.   

High Energy Density Materials Science on Lasers*

The field of high energy density (HED) materials science refers to regimes at energy densities greater than ~10¹² erg/cm³ or pressures greater then ~1 Mbar, accessible on high energy, pulsed laser facilities.  In this talk, a selection of experiments done on the NIF, Omega, and Omega EP lasers will be presented on a variety of materials ranging from deuterium to lead. [1-16]  Examples include high pressure equations of state measurements using VISAR diagnostics; and time resolved x-ray diffraction to determine the lattice structure at high pressures.  X-ray Thomson scattering has been used to characterize the ionization state, density, and temperature in HED plasma experiments.  Radiography has been used to characterize the sample density and optical depth on the Hugoniot at pressures from 25 – 800 Mbar.  Strength Rayleigh-Taylor instability experiments have been done in the solid state, plastic flow regime at high pressures and strain rates.  Water experiments at 1-4 Mbar have observed the superionic phase whereby the oxygen freezes into a crystalline fcc lattice, whereas the H remains itinerant.  And finally, EXAFS experiments for measuring sample temperatures at high pressures have been done on Omega, and are under development on NIF.  Examples from the above will be given.   

Ultrafast time-resolved measurement of the structural evolution in strongly excited liquid water

Understanding the structural properties of liquid water in warm dense conditions has numerous implications for areas including astrophysics, shock physics and solution-phase chemistry. Here we report the results of using femtosecond electron diffraction to directly image the structural change in warm dense water formed by strong optical excitation of liquid water. In this study, a 266 nm, 100 fs laser pulse was focused onto a liquid water sheet of ~650 nm thickness, reaching a maximum excitation energy density of ~5.4 MJ/kg. Structural change of the excited water was probed with time-resolved electron diffraction up to 500 ps after laser arrival. The diffraction data is converted to differential pair distribution function (dPDF) to resolve the dynamics of intermolecular O···O and O···H bonds of water. The dPDF results show that the structural change is dominated by OH(H₃O⁺) radical-cation pairs within 0.5 ps, followed by a steady state up to 50 ps and a transition to phase explosion regime on a 100-ps time scale. Molecular dynamics simulation results will be also presented.   

Predictions of novel features in x-ray Thomson scattering spectra for temperature diagnosis

Warm dense matter experiments often diagnose temperature by analyzing the relative prominence of red- and blue-shifted plasmon features in x-ray Thomson scattering spectra. However, this procedure loses sensitivity as the temperature exceeds the plasmon energy, and it does not provide information on the extent to which the sample is in thermal equilibrium. We summarize recent predictions from both time-dependent density functional theory (TDDFT) and a modified average atom model which show clear signatures of scattering into thermally depleted bound states in aluminum and iron heated to 20eV. These prominent bound-bound features can allow determination of electron temperature in conditions beyond the sensitivity range of plasmon-based approaches. The TDDFT results also contain signatures of atomic order within the bound-free portion of the scattering spectra, which may be used to distinguish isochorically heated and thermalized matter and perhaps even infer ion temperature in studies of ultrafast melting. These theoretical insights advance the capabilities of x-ray scattering diagnostics as more extreme conditions become experimentally accessible.   

Effects of electron-ion collisions on stopping powers in warm dense matter

Collisions between electrons and ions affect the response of high energy density (HED) plasmas to external perturbations, governing transport properties such as conductivities and stopping powers. Here, we present stopping power calculations based on a collisionless random phase approximation to a warm, dense plasma, and extend this picture by including electron-ion collision frequencies calculated using an average atom code. Moreover, we show that going beyond a simple Born description of collisions by using T-matrix cross sections further modifies stopping powers in the temperature and density regimes relevant to warm dense matter (WDM). We compare these results with Ehrenfest time-dependent density functional theory (TDDFT), which extracts stopping powers directly from electron-ion dynamics, and show that the inclusion of T-matrix collisions improves agreement with TDDFT. Our simplified atomic model provides a relatively computationally inexpensive and accurate approach to computing various atomic-scale quantities of interest to the HED and astrophysical communities.   

Continuum Lowering in a Solid under Extreme Pressure and Temperature: Fermi Surface Rising and Re-entrant Semimetallicity

Continuum lowering is an important spectroscopic diagnostic of extreme conditions in experiments where the X-ray transition energies are usually reduced with increasing pressure and/or temperature in plasmas. The corresponding effects in solids under those extreme conditions have not been yet established. We perform density functional theory calculations on diamond-structure silicon as a representative solid, and compute the absorption onset of soft x-rays up to 17× compression (~340 Mbar) and high electronic temperatures. The potential shows continuum lowering like in plasmas but the absorption onset rises with pressure unlike for typical plasmas, because of Fermi surface rising and quantum degeneracy. Additionally, silicon becomes semimetallic and then metallic at low compressions, but then again semimetallic at high compressions, which is correlated with delocalization of the electrons out of the atoms.  We compare our results to common ionization models, verify our pseudopotential approximation, and validate the quality of the absorption spectra with GW/Bethe-Salpeter calculations. Our studies provide insight into the relative effects of pressure and temperature in continuum lowering of solids for design and interpretation of high energy density spectroscopy.   

Ultrafast thermometry of nonequilibrium electrons via the X-ray core-hole photoelectron and X-ray absorption spectroscopies

Pump-probe spectroscopy is extensively applied to study ultrafast relaxation processes in the electron systems out of equilibrium. But we still lack an ultrafast means to perform thermometry. From our previous work on the time-resolved X-ray photoemission spectroscopy (tr-XPS), we found a strong correlation between the integrated weights of the peaks on the spectra, with the thermal occupancies of the appropriate many-body states. This approach can serve as a probe of the energy (and therefore the effective temperature) of the excited electrons. The advantage of X-ray absorption spectroscopy (XAS) is that it is photon-in-photon-out experiment, which is easier to set up. XPS and XAS spectroscopies are closely connected, so we expect there is a similar correlation between the peaks of a tr-XAS spectra and the energy of the excited electrons that can be exploited as another effective thermometer of the nonequilibrium state. We illustrate how this works in the metallic and Mott-insulator phases of the spinless Falicov-Kimball model within nonequilibrium dynamical mean-field theory.   

Color center formation in silicon crystals with intense ion pulses from laser-plasma acceleration

We report on studies of defect evolution and color center formation in silicon single crystals during the interaction with intense ion pulses from laser-plasma acceleration.  Ion pulses, including protons and carbon ions with energies up to several MeV, are generated when fs-laser pulses impinge on thin foils with intensities of ~10^19 W/cm^2 [1].  We characterize ion species and energy distributions and correlate resulting defects and color centers in photoluminescence measurements.  W- and G-center type color centers are formed with relative intensities that vary as a function of local ion flux conditions.  We use density functional theory to calculate color center linewidths as a function of local disorder.  Color centers form when a proton pulse pre-heats a sample, followed a few ns later by implantation and drive-in diffusion of low energy carbon ions.  We discuss potential applications of this form of local color center synthesis for the development of spin-photon qubits.  

[1] S. Steinke, et al., “Acceleration of high charge ion beams with achromatic divergence by petawatt laser pulses”, Phys. Rev. Accel. and Beams 23,021302 (2020)   

Laser control of ultrafast nonthermal melting in silicon

Femtosecond-laser pulses can induce structural phenomena like solid-to-solid phase transitions and ultrafast melting in crystalline structures. The main reason for the appearance of such effects is the ultrafast modification of the bonding properties in the induced nonthermal state consisting of extremely hot electrons and nearly unaffected cold ions. Although melting is a stochastic process in thermodynamical equilibrium, we show that in the laser excited nonthermal case some coherences are preserved or created. Moreover, by performing ab initio molecular dynamics simulations of the excitation of silicon by a series of laser pulses, we demonstrate that it is possible to control nonthermal melting by light. Analyzing the energy flow in quasimomentum space, we found that the ultrafast disordering atomic motion can be stopped and redirected depending on the delay between the pulses. Essential for the controlling mechanism is the appearance of an intermediate state in the excitation process that shows a laser-induced coherent motion of the atoms. The appearing oscillation follows directly the bond softening of the material and can be connected to laser-changed thermal phonon frequencies.     

Ab initio investigation of two temperature warm dense gold.

Understanding the time evolution of the electronic and ionic structure of warm dense gold heated up to the warm dense matter regime by a femtosecond laser pulse is still challenging. Following laser illumination, electronic temperature increases and electron and ion temperatures are different for several ps. The time scale of the equilibration between the two temperatures is still needed. X-Ray absorption near edge spectroscopy (XANES) is a powerfull tool to investigate the electronic properties of two temperature metals and to link ionic and electronic properties. Following a prevoius study on copper, we present here XANES spectra near L-edge for two temperature gold computed using  DFT based  molecular dynamic. We demonstrate how the change in the spectra can be used to interpret future experiments.   

Ion stopping in a dense quantum plasma using quantum kinetic theory

Stopping of ions is a sensitive diagnostic of dense plasmas. A theoretical description is difficult because it requires to take into account correlation and quantum effects as well as dynamical screening.

We use Nonequilibrium Green's functions (NEGF)[1] with the Generalized Kadanoff-Baym Ansatz[2] to simulate ion stopping in a uniform warm dense plasma. Correlation effects are taken into account via the selfenergy Σ and the non-Markovian structure of the equations of motion for the one-particle NEGF. Time-dependent single-particle observables are available through the one-particle NEGF, so we can directly compute the stopping power from it. We explore the performance of different selfenergy approximations, such as the statically screened Second Order Approximation and the dynamically screened GW approximation, and compare the results to experiments[3] and linear response results with highly accurate dielectric functions[4], and the Markov limit[5].

[1] M. Bonitz, Quantum Kinetic Theory (Springer, 2016)

[2] P. Lipavský, V. Špička, B. Velický, Phys. Rev. B34, 6933 (1986)

[3] A. B. Zylstra et al, Phys. Rev. Lett. 114, 215002 (2015)

[4] Zh. A. Moldabekov, T. Dornheim, M. Bonitz, T. S. Ramazanov, Phys. Rev. E101, 053203 (2020)

[5] D. O. Gericke, M. Schlanges, Phys. Rev. E60 904 (1999)   

Time-Dependent Mixed Deterministic-Stochastic Kohn Sham Density Functional Theory for Matter in Extreme Conditions

The cubic scaling of computational costs with system size and temperature is a critical limitation for ab-initio simulations, based on Mermin density functional theory (DFT), of matter in extreme conditions. Additional real-time time-dependent DFT simulations scale linearly with the number of orbitals required to calculate the density. Our mixed-stochastic-deterministic Kohn Sham DFT algorithms can alleviate the burdon of these scaling laws. We apply this approach to the simulation of warm dense carbon system (up to 10 eV) for both electrical conductivity and electronic stopping power. We will compare calculated electronic stopping power to recent experimental measurements.