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Register a new userAn investigation into the approximations used in wave packet molecular dynamics for the study of warm dense matter
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Wave packet molecular dynamics (WPMD) has recently received a lot of attention as a computationally fast tool to study dynamical processes in warm dense matter beyond the Born-Oppenheimer approximation. These techniques, typically, employ many approximations to achieve computational efficiency while implementing semi-empirical scaling parameters to retain accuracy. We investigate three of the main approximations ubiquitous to WPMD: a restricted basis set, approximations to exchange, and the lack of correlation. We examine each of these approximations in atomic and molecular hydrogen in addition to a dense hydrogen plasma. We find that the biggest improvement to WPMD comes from combining a two Gaussian basis with a semi-empirical correction based on the valence-bond wave function. A single parameter scales this correction to match experimental pressures of dense hydrogen. Ultimately, we find that semi-empirical scaling parameters are necessary to correct for the main approximations in WPMD. However, reducing the scaling parameters for more ab-initio terms gives more accurate results and displays the underlying physics more readily.
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Using numerical simulations of the time-dependent Schrodinger equation, we study the full quantum dynamics of the motion of an atomic ion in a linear Paul trap. Such a trap is based on a time-varying, periodic electric field, and hence corresponds to a time-dependent potential for the ion, which we model exactly. We compare the center of mass motion with that obtained from classical equations of motion, as well as to results based on a time-independent effective potential. We also study the oscillations of the width of the ions wave packet, including close to the border between stable (bounded) and unstable (unbounded) trajectories. Our results confirm that the center-of-mass motion always follow the classical trajectory, that the width of the wave packet is bounded for trapping within the stability region, and therefore that the classical trapping criterion are fully applicable to quantum motion.
Warm dense matter (WDM) -- an exotic state of highly compressed matter -- has attracted high interest in recent years in astrophysics and for dense laboratory systems. At the same time, this state is extremely difficult to treat theoretically. This is due to the simultaneous appearance of quantum degeneracy, Coulomb correlations and thermal effects, as well as the overlap of plasma and condensed phases. Recent breakthroughs are due to the successful application of density functional theory (DFT) methods which, however, often lack the necessary accuracy and predictive capability for WDM applications. The situation has changed with the availability of the first textit{ab initio} data for the exchange-correlation free energy of the warm dense uniform electron gas (UEG) that were obtained by quantum Monte Carlo (QMC) simulations, for recent reviews, see Dornheim textit{et al.}, Phys. Plasmas textbf{24}, 056303 (2017) and Phys. Rep. textbf{744}, 1-86 (2018). In the present article we review recent further progress in QMC simulations of the warm dense UEG: namely, textit{ab initio} results for the static local field correction $G(q)$ and for the dynamic structure factor $S(q,omega)$. These data are of key relevance for the comparison with x-ray scattering experiments at free electron laser facilities and for the improvement of theoretical models. In the second part of this paper we discuss simulations of WDM out of equilibrium. The theoretical approaches include Born-Oppenheimer molecular dynamics, quantum kinetic theory, time-dependent DFT and hydrodynamics. Here we analyze strengths and limitations of these methods and argue that progress in WDM simulations will require a suitable combination of all methods. A particular role might be played by quantum hydrodynamics, and we concentrate on problems, recent progress, and possible improvements of this method.
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.
In a recent experiment on the Trident laser facility, a laser-driven beam of quasi-monoenergetic aluminum ions was used to heat solid gold and diamond foils isochorically to 5.5 eV and 1.7 eV, respectively. Here theoretical calculations are presented that suggest the gold and diamond were heated uniformly by these laser-driven ion beams. According to calculations and SESAME equation-of-state tables, laser-driven aluminum ion beams achievable on Trident, with a finite energy spread of (delta E)/E ~ 20%, are expected to heat the targets more uniformly than a beam of 140 MeV aluminum ions with zero energy spread. The robustness of the expected heating uniformity relative to the changes in the incident ion energy spectra is evaluated, and expected plasma temperatures of various target materials achievable with the current experimental platform are presented.
Warm dense matter is present at the heart of gas giants and large exo-planets. Yet, its most basic properties are unknown and limit our understanding of planetary formation and evolution. In this state, where pressure climbs above 1 Mbar, matter is strongly coupled and quantum degenerate. This combination invalidates most theories capable of predicting the equation of state, the viscosity or heat conductivity of the material. When such properties are missing, understanding planetary evolution becomes an arduous endeavor. Henceforth, research in this field is actively growing, using high power laser or heavy ion beams to produce samples dense enough to overcome the 1 Mbar limit. These samples are not actively confined and tend to expand rapidly, precluding the existence of any thermodynamically stable equilibrium. However, a mega-ampere-class pulsed-power generator can produce confined matter in the Mbar range, providing two conditions are being met. First, the sample needs to be compressed cylindrically, to maximize magnetic pressure (compared to slab compression). Second, a damper must be used to preclude the formation of a corona around the sample. This corona robs the main sample from valuable current and limits the homogeneity of the compression. According to numerical simulations, the setup proposed here, and called a magnetic anvil cell, can reach pressure on the order of 1 Mbar using a mega-ampere pulsed power driver. These samples span several millimeters in length. Unlike diamond anvil cell, which pressure is limited below 1 Mbar due to materials strength, the magnetic anvil cell has virtually no pressure limit. Further, the current heats the sample to several eV, a temperature well beyond diamond anvil cell capabilities.
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