Warm Dense Matter Studies Research Articles

X-ray scattering from warm dense iron

We have carried out X-ray scattering experiments on iron foil samples that have been compressed and heated using laser-driven shocks created with the VULCAN laser system at the Rutherford-Appleton Laboratory. This is the highest Z element studied in such experiments so far and the first time scattering from warm dense iron has been reported. Because of the importance of iron in telluric planets, the work is relevant to studies of warm dense matter in planetary interiors. We report scattering results as well as shock breakout results that, in conjunction with hydrodynamic simulations, suggest the target has been compressed to a molten state at several 100 GPa pressure. Initial comparison with modelling suggests more work is needed to understand the structure factor of warm dense iron.

Uniform electron gas at warm, dense matter conditions

A simple, practical model for computing the equilibrium thermodynamics and structure of the uniform electron gas (jellium) by classical strong-coupling methods is proposed. Conditions addressed are those of interest for recent studies of warm dense matter: solid densities and temperatures from zero to plasma states. An effective pair potential and coupling constant are introduced, incorporating the ideal gas, low density, and weak-coupling quantum limits. The resulting parameter-free, analytic model is illustrated by the calculation of the pair correlation function via strong-coupling classical liquid state theory. The results compare favorably with the first finite-temperature restricted path integral Monte Carlo simulations reported recently.

Theoretical treatments of the bound-free contribution and experimental best practice in X-ray Thomson scattering from warm dense matter

By comparison with high-resolution synchrotron x-ray experimental results, we assess several theoretical treatments for the bound-free (core-electron) contribution to x-ray Thomson scattering (i.e., also known as nonresonant inelastic x-ray scattering). We identify an often overlooked source of systematic error in the plane-wave form factor approximation (PWFFA) used in the inference of temperature, ionization state, and free electron density in some laser-driven compression studies of warm dense matter. This error is due to a direct violation of energy conservation in the PWFFA. We propose an improved practice for the bound-free term that will be particularly relevant for XRTS experiments performed with somewhat improved energy resolution at the National Ignition Facility or the Linac Coherent Light Source. Our results raise important questions about the accuracy of state variable determination in XRTS studies, given that the limited information content in low-resolution XRTS spectra does not strongly constrain the models of electronic structure being used to fit the spectra.

用于温密物质研究的强流电子束加载技术

用于温密物质研究的强流电子束加载技术

Temperature-dependent behavior of confined many-electron systems in the Hartree-Fock approximation

Many-electron systems confined at substantial finite temperatures and densities present a major challenge to density functional theory. In particular, there is comparatively little systematic knowledge about the behavior of free-energy density functionals for temperatures and pressures of interest, for example, in the study of warm dense matter (WDM). As with ground-state functionals, development of approximate free-energy functionals is faced with significant needs for reliable assessment and calibration data. Here we address, in part, this need for detailed results on well-characterized systems. We present results on a comparatively simple, well-defined, computationally feasible but previously unexplored model, the thermal Hartree-Fock approximation. We discuss the main technical tasks (defining a suitable basis and evaluation of the required matrix elements) and give an illustrative initial application that probes both the content of the model and the solution techniques: a system of eight one-electron atoms with nuclei at fixed, arbitrary positions in a hard-walled box. Even this simple system produces physical behavior different from that produced by simple ground-state density functionals used at finite temperature (a common approximation in the study of WDM).

Warm dense matter at the bench-top: Fs-laser-induced confined micro-explosion

We report the experimental evidence for creation of Warm Dense Matter (WDM) in ultrafast laser-induced micro-explosion inside a sapphire (Al 2O 3) crystal. We show that the WDM can be formed by a 100 nJ fs-pulse if the following conditions are satisfied: (1) the laser pulse is tightly focused to inside of the bulk of transparent material so the intensity at focus is two orders of magnitude higher than the optical breakdown threshold; (2) the pulse duration is shorter than the electron–ion energy exchange time; and, (3) the absorbed energy density is above the Young’s modulus for the material studied. The empty void created inside a sapphire crystal surrounded by a shell of compressed material provides the direct evidence of the maximum pressure above the Young’s modulus of sapphire (∼400 GPa). Synchrotron X-ray diffraction (XRD) analysis of the shell revealed the presence of novel super-dense bcc-Al crystalline phase predicted at pressures above ∼380 GPa theoretically, which has never been observed experimentally before neither in nature in laboratory experiments. These results show that confined micro-explosion induced by tightly focussed fs-laser inside a transparent solid opens new routes for synthesis of new materials and study of WDM at a laboratory bench-top.

Evidence of superdense aluminium synthesized by ultrafast microexplosion.

At extreme pressures and temperatures, such as those inside planets and stars, common materials form new dense phases with compacted atomic arrangements and unusual physical properties. The synthesis and study of new phases of matter at pressures above 100 GPa and temperatures above 104 K—warm dense matter—may reveal the functional details of planet and star interiors, and may lead to materials with extraordinary properties. Many phases have been predicted theoretically that may be realized once appropriate formation conditions are found. Here we report the synthesis of a superdense stable phase of body-centred-cubic aluminium, predicted by first-principles theories to exist at pressures above 380 GPa. The superdense Al phase was synthesized in the non-equilibrium conditions of an ultrafast laser-induced microexplosion confined inside sapphire (α-Al2O3). Confined microexplosions offer a strategy to create and recover high-density polymorphs, and a simple method for tabletop study of warm dense matter.

Simulations for experimental study of warm dense matter and inertial fusion energy applications on NDCX-II

The Neutralized Drift Compression Experiment II (NDCX II) is an induction accelerator planned for initial commissioning in 2012. The final design calls for a ∼3 MeV, ∼30 A Li+ ion beam, delivered in a bunch with characteristic pulse duration of 1 ns, and transverse dimension of order 1 mm. The purpose of NDCX II is to carry out experimental studies of material in the warm dense matter regime, and ion beam/hydrodynamic coupling experiments relevant to heavy ion based inertial fusion energy. In preparation for this new machine, we have carried out hydrodynamic simulations of ion-beam-heated, metallic solid targets, connecting quantities related to observables, such as brightness temperature and expansion velocity at the critical frequency, with the simulated fluid density, temperature, and velocity. We examine how these quantities depend on two commonly used equations of state.

Beam dynamics of the Neutralized Drift Compression Experiment-II, a novel pulse-compressing ion accelerator

Intense beams of heavy ions are well suited for heating matter to regimes of emerging interest. A new facility, NDCX-II, will enable studies of warm dense matter at ∼1 eV and near-solid density, and of heavy-ion inertial fusion target physics relevant to electric power production. For these applications the beam must deposit its energy rapidly, before the target can expand significantly. To form such pulses, ion beams are temporally compressed in neutralizing plasma; current amplification factors of ∼50–100 are routinely obtained on the Neutralized Drift Compression Experiment (NDCX) at the Lawrence Berkeley National Laboratory. In the NDCX-II physics design, an initial non-neutralized compression renders the pulse short enough that existing high-voltage pulsed power can be employed. This compression is first halted and then reversed by the beam’s longitudinal space-charge field. Downstream induction cells provide acceleration and impose the head-to-tail velocity gradient that leads to the final neutralized compression onto the target. This paper describes the discrete-particle simulation models (one-, two-, and three-dimensional) employed and the space-charge-dominated beam dynamics being realized.

X-ray absorption for the study of warm dense matter

A time-resolved ultrafast x-ray spectrometer is developed in order to extract the x-ray absorption near-edge spectroscopy (XANES) structure of an Al sample in the warm dense matter regime. In this context, an intense, broadband, short (ps) x-ray source based on the M-band emission from high-Z plasmas is optimized to maximize the photon flux around the Al K-edge. An experiment is reported, devoted to probe a solid Al foil isochorically heated by laser-produced protons up to 3 eV. The experimental x-ray spectra lead to an estimation of the electron temperature with an accuracy of 15%. In good agreement with two different theoretical approaches, the observed progressive smoothing of the XANES structures is clearly related to a significant loss of ion–ion correlation.

Toward a physics design for NDCX-II, an ion accelerator for warm dense matter and HIF target physics studies

The Heavy Ion Fusion Science Virtual National Laboratory (HIFS-VNL), a collaboration of LBNL, LLNL, and PPPL, has achieved 60-fold pulse compression of ion beams on the Neutralized Drift Compression eXperiment (NDCX) at LBNL. In NDCX, a ramped voltage pulse from an induction cell imparts a velocity “tilt” to the beam; the beam's tail then catches up with its head in a plasma environment that provides neutralization. The HIFS-VNL's mission is to carry out studies of warm dense matter (WDM) physics using ion beams as the energy source; an emerging thrust is basic target physics for heavy ion-driven inertial fusion energy (IFE). These goals require an improved platform, labeled NDCX-II. Development of NDCX-II at modest cost was recently enabled by the availability of induction cells and associated hardware from the decommissioned advanced test accelerator (ATA) facility at LLNL. Our initial physics design concept accelerates a ∼ 30 nC pulse of Li + ions to ∼ 3 MeV , then compresses it to ∼ 1 ns while focusing it onto a mm-scale spot. It uses the ATA cells themselves (with waveforms shaped by passive circuits) to impart the final velocity tilt; smart pulsers provide small corrections. The ATA accelerated electrons; acceleration of non-relativistic ions involves more complex beam dynamics both transversely and longitudinally. We are using an interactive one-dimensional kinetic simulation model and multidimensional Warp-code simulations to develop the NDCX-II accelerator section. Both LSP and Warp codes are being applied to the beam dynamics in the neutralized drift and final focus regions, and the plasma injection process. The status of this effort is described.

Ion beam heated target simulations for warm dense matter physics and inertial fusion energy

Hydrodynamic simulations have been carried out using the multi-physics radiation hydrodynamics code HYDRA and the simplified one-dimensional hydrodynamics code DISH. We simulate possible targets for a near-term experiment at LBNL (the Neutralized Drift Compression Experiment, NDCX) and possible later experiments on a proposed facility (NDCX-II) for studies of warm dense matter and inertial fusion energy-related beam-target coupling. Simulations of various target materials (including solids and foams) are presented. Experimental configurations include single-pulse planar metallic solid and foam foils. Concepts for double-pulsed and ramped-energy pulses on cryogenic targets and foams have been simulated for exploring direct drive beam-target coupling, and concepts and simulations for collapsing cylindrical and spherical bubbles to enhance temperature and pressure for warm dense matter studies.

Experiments at FLASH

FLASH, the free electron laser at DESY in Hamburg, is the first facility of its kind providing intense, femtosecond radiation pulses in the vacuum ultraviolet and soft X-ray range for user experiments. A broad science program has been started in summer 2005 with applications in atomic and molecular physics, cluster physics, the study of warm dense matter and surface dynamics, and diffraction imaging of small structures and biological samples with nanometer resolution. A selection of first experimental results is highlighted in this paper.

US heavy ion beam research for high energy density physics applications and fusion

Key scientific results from recent experiments, modeling tools, and heavy ion accelerator research are summarized that explore ways to investigate the properties of high energy density matter in heavy-ion-driven targets, in particular, strongly-coupled plasmas at 0.01 to 0.1 times solid density for studies of warm dense matter, which is a frontier area in high energy density physics. Pursuit of these near-term objectives has resulted in many innovations that will ultimately benefit heavy ion inertial fusion energy. These include: neutralized ion beam compression and focusing, which hold the promise of greatly improving the stage between the accelerator and the target chamber in a fusion power plant; and the Pulse Line Ion Accelerator (PLIA), which may lead to compact, low-cost modular linac drivers.

Laser-produced proton beams as a tool for equation-of-state studies of warm dense matter

The direct determination of equation-of-state data in the warm dense matter regime using a laser-produced proton beam is proposed. The careful choice of the target parameters provides a planar isentropic expansion of the heated matter. A wide region of the phase diagram becomes available by changing the target thickness. Hydrodynamic simulations show that energy and angular spread of the proton beam contribute less than 10% to the error of measured equation-of-state data. The requirements on the target diagnostics arising from the simulation results are discussed.

Progress in the study of warm dense matter

In the last few years, high power lasers have demonstrated the possibility to explore a new state of matter, the so-called warm dense matter. Among the possible techniques utilized to generate this state, we present the dynamic compression technique using high power lasers. Applications to planetary cores material (iron) will be discussed. Finally new diagnostics such as proton and hard-x-ray radiography of a shock propagating in a solid target will be presented.

Semi-Empirical Approach to Pulsed Wire Discharges in Water as a Method for Warm Dense Matter Studies

Hydrodynamic behaviors accompanied by a pulsed thin wire discharge in water have been observed via a fast framing/streak camera, together with the basic electric characteristics. Results show that the discharge plasma is tamped and stabilized by the surrounding water and it evolves through a warm dense state with high degree of symmetry and reproducibility up to a 2 μs discharge time. Numerical simulations show that the shock wave trajectories driven in the water are strongly dependent on equation of state (EOS) models of the plasma. Those results indicate that a semiempirical fitting of the shock traces to the experimental observation is a useful method for studying the EOS models of matter in a warm dense state.

Dynamic confinement of targets heated quasi-isochorically with heavy ion beams.

Isochoric heating of matter by intense heavy ion beams promises to become a fruitful approach to warm dense matter studies. For heating times that are long on the hydrodynamic time scale of the target response a tamped target is essential. The proposed dynamic confinement provides homogeneous target heating by a low-Z tamper, which allows one to apply powerful x-ray scattering diagnostics. To demonstrate the potential of the method, heating of a hydrogen sample with the SIS-18 beam at GSI Darmstadt is investigated numerically. The intense x-ray bursts for diagnostics can be provided by the PHELIX laser currently installed at GSI. In the optimized heating regime, density variations can be reduced to a level of 15% from the initial density value.