Damian C. Swift

Nonequilibrium melting and crystallization of a model Lennard-Jones system

Nonequilibrium melting and crystallization of a model Lennard-Jones system were investigated with molecular dynamics simulations to quantify the maximum superheating/supercooling at fixed pressure, and over-pressurization/over-depressurization at fixed temperature. The temperature and pressure hystereses were found to be equivalent with regard to the Gibbs free energy barrier for nucleation of liquid or solid. These results place upper bounds on hysteretic effects of solidification and melting in high heating- and strain-rate experiments such as shock wave loading and release. The authors also demonstrate that the equilibrium melting temperature at a given pressure can be obtained directly from temperatures at the maximum superheating and supercooling on the temperature hysteresis; this approach, called the hysteresis method, is a conceptually simple and computationally inexpensive alternative to solid-liquid coexistence simulation and thermodynamic integration methods, and should be regarded as a general method. We also found that the extent of maximum superheating/supercooling is weakly pressure dependent, and the solid-liquid interfacial energy increases with pressure. The Lindemann fractional root-mean-squared displacement of solid and liquid at equilibrium and extreme metastable states is quantified, and is predicted to remain constant (0.14) at high pressures for solid at the equilibrium melting temperature.

High strain-rate plastic flow in Al and Fe

Thin Fe and Al foils were ramp-compressed over several to tens of ns timescales to study the time-dependence associated with the onset of plastic flow. Peak stress states of 15–200 GPa were achieved through laser ramp-compression where the strain rate was varied, shot-to-shot, between 106 to 108 s−1. Our data combined with data from other dynamic compression platforms reveals a strong correlation between the peak elastic precursor stress, σE, and the strain rate at the onset of plastic flow, ɛ·p. In fcc Al, phonon drag dislocation flow dominates above ɛ·p~103s-1 and σE ∼ 0.03 GPa where σE scales as ɛ·p0.43. By contrast, the Al alloy 6061-T6 exhibits a relatively weak dependency of σE with ɛ·p up to strain rates of ∼107 s−1. Our Fe data, reveals a sharp increase in σE at ɛ·p>5×106s-1. This is consistent with a transition in plastic flow to a phonon drag regime.

Laser-launched flyer plates for shock physics experiments

The TRIDENT laser was used to launch Cu, Ga, and NiTi flyers from poly(methylmethacrylate) (PMMA) substrates, coated with thin (∼micron) layers to absorb the laser energy, confine the plasma, and insulate the flyer. The laser pulse was ∼600ns long, and the flyers were 50 to 250μm thick and 4 mm in diameter. With an energy of 10–20 J, speeds of several hundred meters per second were obtained. Simulations were performed of the flyer launch process, using different models. The simulations reproduced the magnitude of the flyer speed and qualitative variations with drive energy and design parameters, but systematically overpredicted the flyer speed. The most likely explanation is that some of the laser energy was deposited in the transparent substrate, reducing the amount available for acceleration. The deceleration of the flyer was measured on impact with a PMMA window. Given the equation of state and optical properties of PMMA, the deceleration allowed points to be deduced on the principal Hugoniot of Cu. Th...

Production of neutrons up to 18 MeV in high-intensity, short-pulse laser matter interactions

The generation of high-energy neutrons using laser-accelerated ions is demonstrated experimentally using the Titan laser with 360 J of laser energy in a 9 ps pulse. In this technique, a short-pulse, high-energy laser accelerates deuterons from a CD2 foil. These are incident on a LiF foil and subsequently create high energy neutrons through the 7Li(d,xn) nuclear reaction (Q = 15 MeV). Radiochromic film and a Thomson parabola ion-spectrometer were used to diagnose the laser accelerated deuterons and protons. Conversion efficiency into protons was 0.5%, an order of magnitude greater than into deuterons. Maximum neutron energy was shown to be angularly dependent with up to 18 MeV neutrons observed in the forward direction using neutron time-of-flight spectrometry. Absolutely calibrated CR-39 detected spectrally integrated neutron fluence of up to 8 × 108 n sr−1 in the forward direction.

Laser-induced shock waves in condensed matter: some techniques and applications

Laser-induced shock waves in condensed matter have important applications in dynamic material studies and high pressure physics. We briefly review some techniques in laser-induced shock waves, including direct laser drive, laser-launched flyer plate, quasi-isentropic loading, point and line imaging velocity interferometry, transient X-ray diffraction, spectroscopy and shock recovery, and their applications to study of equation of state, spallation, and phase transitions.

Vibrational density of states and Lindemann melting law

We examine the Lindemann melting law at different pressures using the vibrational density of states (DOS), equilibrium melting curve, and Lindemann parameter delta(L) (fractional root-mean-squared displacement, rmsd, at equilibrium melting) calculated independently from molecular dynamics simulations of the Lennard-Jones system. The DOS is obtained using spectra analysis of atomic velocities and accounts for anharmonicity. The increase of delta(L) with pressure is non-negligible: delta(L) is about 0.116 and 0.145 at ambient and extreme pressures, respectively. If the component of rmsd normal to a reflecting plane as in the Debye-Waller-factor-type measurements using x rays is adopted for delta(L), these values are about 0.067 (+/-0.002) and 0.084 (+/-0.003), and are comparable with experimental and calculated values for face-centered-cubic elements. We find that the Lindemann relation holds accurately at ambient and high pressures. The non-negligible pressure dependence of delta(L) suggests that caution should be exerted in applying the Lindemann law to obtaining the high pressure melting curve anchored at ambient pressure.

Time-dependence of the alpha to epsilon phase transformation in iron

Iron was ramp-compressed over timescales of 3 ≤ t(ns) ≤ 300 to study the time-dependence of the α→e (bcc→hcp) phase transformation. Onset stresses (σα→e)  for the transformation ∼14.8-38.4 GPa were determined through laser and magnetic ramp-compression techniques where the transition strain-rate was varied between 106 ≤μα→e(s−1) ≤ 5×108. We find σα→e= 10.8 + 0.55 ln(μα→e) for μα→e   106/s. This μ response is quite similar to recent results on incipient plasticity in Fe [Smith et al., J. Appl. Phys. 110, 123515 (2011)] suggesting that under high rate ramp compression the α→e phase transition and plastic deformation occur through similar mechanisms, e.g., the rate limiting step for μ > 106/s is due to phonon scattering from defects moving to relieve strain. We show that over-pressurization of equilibrium phase boundaries is a common feature exhibited under high strain-rate compression of many materials encompassing many orders of magnitude of strain-rate.

The α→ϵ phase transition in iron at strain rates up to ∼109 s−1

We have used a table-top scale laser to dynamically compress iron at strain rates in excess of 109 s−1. Using an embedded ultrafast interferometer, we have measured corresponding free surface histories with a time resolution of approximately 10 ps. We have analyzed the surface histories using a method that accounts for nonsteady wave propagation and time-dependent material behavior. We show that at these strain rates, the α→ϵ polymorphic transition begins within 100 ps after an initial very large (∼10 GPa) and mostly elastic compression and appears largely complete within a similar time thereafter. The corresponding deviatoric stress before the transition begins can exceed 3 GPa, while the transition stress itself is up to 25 GPa, nearly twice the value measured at low strain rates. We use these results to propose a systematic variation with loading time of the normal-stress/relative-volume curve followed by iron during rapid compression.

Analysis of laser shock experiments on precompressed samples using a quartz reference and application to warm dense hydrogen and helium

Megabar (1 Mbar = 100 GPa) laser shocks on precompressed samples allow reaching unprecedented high densities and moderately high ∼103–104 K temperatures. We describe here a complete analysis framework for the velocimetry (VISAR) and pyrometry (SOP) data produced in these experiments. Since the precompression increases the initial density of both the sample of interest and the quartz reference for pressure-density, reflectivity, and temperature measurements, we describe analytical corrections based on available experimental data on warm dense silica and density-functional-theory based molecular dynamics computer simulations. Using our improved analysis framework, we report a re-analysis of previously published data on warm dense hydrogen and helium, compare the newly inferred pressure, density, and temperature data with most advanced equation of state models and provide updated reflectivity values.

Laser generated neutron source for neutron resonance spectroscopy

A neutron source for neutron resonance spectroscopy has been developed using high-intensity, short-pulse lasers. This technique will allow robust measurement of interior ion temperature of laser-shocked materials and provide insight into material equation of state. The neutron generation technique uses laser-accelerated protons to create neutrons in LiF through (p,n) reactions. The incident proton beam has been diagnosed using radiochromic film. This distribution is used as the input for a (p,n) neutron prediction code which is validated with experimentally measured neutron yields. The calculation infers a total fluence of 1.8×109 neutrons, which are expected to be sufficient for neutron resonance spectroscopy temperature measurements.