Yong-Jae Kim

The velocity interferometer for any reflector (VISAR) diagnostic is a key diagnostic at the National Ignition Facility (NIF) for characterizing how effectively kJ and MJ lasers compress and accelerate planar targets and spherical capsules. The active shock break out diagnostic is also a workhorse at the Omega Laser Facility. Both line-imaging Doppler-shift interferometers require targets with sufficient reflectivity and imaging resolution to collect suitable data. However, off-line target microscope inspection images have not been sufficient due to incompatibility of illumination wavelength, angle, and numerical aperture with those of the NIF and Omega VISAR. To provide a compatible imaging and reflectance measurement capability, we designed and commissioned a VISAR Target Qualification Station (VTQS) at NIF. This system delivers quantitative reflectance maps of target components and completed targets during and at the end of the fabrication process, enabling the shot responsible individual to assess target quality, plan shot operations and VISAR alignment, and support high-quality data collection.

Polymers are widely used in high energy density (HED) experiments, but rheological properties, such as viscosity and strength, are often assumed negligible in modeling despite limited experimental validation. We report initial measurements of rippled reflecting shocks in epoxy using the Sakharov shock perturbation decay technique supported by analytical formulation from Miller and Ahrens [Rev. Mod. Phys. 63(4), 919 (1991)]. Experiments at the Jupiter Laser Facility employed drive beams delivering a 10 ns, 800 J pulse across an ∼600 μm spot, launching a shock above 400 GPa into epoxy. The highly reflective shock in the epoxy sample was diagnosed using velocimetry and optical pyrometry. Velocimetry showed strong reflectivity and evidence of perturbation structure and streaked optical pyrometry clearly captured spatial evolution of the perturbed shock and showed amplitude growth, decay, and phase inversion, in agreement with hydrodynamic simulations using ARES. Our demonstration of technical viability and the observation of rippled reflecting shocks in epoxy establish a path toward future quantitative constraints on polymer rheology in HED experiments.

Equation of state (EOS) measurements in the 100 s Mbar range are needed to underwrite models employed in the simulation of high energy density plasmas. To this end, a platform has been developed for fielding on the National Ignition Facility, capable of producing high-quality impedance match EOS data, wherein a planar, high-pressure, steady shock is driven into a sample package, and sample and reference standard shock velocities are measured. This platform, dubbed planar high pressure, or PHP, was fielded with an initial proof-of-concept shot in January 2023. The first PHP shot, aiming to study gold, demonstrated a pressure close to 400 Mbar, two orders of magnitude higher than previously reported gold EOS data.

Iron (Fe) is a primary constituent of terrestrial planetary cores, yet its rheological properties under extreme conditions remain uncertain. Here we present direct measurements of Fe strength at 310-430 GPa pressures and 3700-5800 K temperatures, obtained using Rayleigh-Taylor (RT) instability experiments at the National Ignition Facility. Single-crystal α-Fe samples with [001] and [111] orientations are shock-ramp compressed past the α-ε transition along paths approaching Earth’s inner core conditions. We find that ε-Fe derived from [001] α-Fe is consistently stronger (11-20 GPa) than that from [111] α-Fe (8-18 GPa), contrary to the trend at ambient conditions. Large-scale molecular dynamics simulations reproduce this atypical strength anisotropy and attribute it to microstructural evolution during the phase transition and subsequent ε-phase plasticity. Ripple growth analysis further constrains viscosities of 100-170 Pa·s under the driven conditions. Our results provide experimental benchmarks for Fe rheology at inner-core conditions, with implications for seismic anisotropy and the geodynamo.

Iron (Fe) is a primary constituent of terrestrial planetary cores, yet its rheological properties under extreme conditions remain uncertain. Here we present direct measurements of Fe strength at 310-430 GPa pressures and 3700-5800 K temperatures, obtained using Rayleigh-Taylor (RT) instability experiments at the National Ignition Facility. Single-crystal α-Fe samples with [001] and [111] orientations are shock-ramp compressed past the α-ε transition along paths approaching Earth's inner core conditions. We find that ε-Fe derived from [001] α-Fe is consistently stronger (11-20 GPa), than that from [111] α-Fe (8-18 GPa), contrary to the trend at ambient conditions. Large-scale molecular dynamics simulations reproduce these values and attribute this anisotropy to microstructural evolution during the phase transition and subsequent ε-phase plasticity. Ripple growth analysis further constrains viscosities of 100-170 Pa·s under the driven conditions. Our results provide new experimental benchmarks for Fe rheology at inner-core conditions, with implications for seismic anisotropy and the geodynamo.

Shock-compressing material samples that are precompressed with diamond-anvil cells (DACs) enables benchmarking equation of state measurements over a wide range of high-pressure conditions, in particular, warm dense matter relevant to planetary science, astrophysics, and inertial confinement fusion. These targets provide optical access to the shocked sample, so optical velocity interferometry and pyrometry are typical diagnostics. We present a scaling for the coupling of the laser drive to the shock pressure in DACs based on over 150 prior experiments that can be used to design future experiments. For initial static pressures ≤ 5 GPa, using a sapphire window mitigates the effect of photoionization compared to diamond windows and allows the measurement of shock pressures up to 1 TPa with optical diagnostics.

The high-pressure strength of solid platinum is studied at the National Ignition Facility via Rayleigh-Taylor ripple growth under ramp compression. Laser-driven experiments reached pressures up to 430 GPa, collecting velocimetry and radiography data. Despite no phase transformation, the observed ripple growth aligns with hydrodynamic simulations using an amplified Steinberg-Guinan model, resulting in a 200-fold increase in strength. Molecular dynamics simulations of platinum ramp compression yield similar von Mises stress at comparable high pressure and high strain rate conditions. These findings represent the highest-pressure strength measurements ever achieved for platinum and offer critical insight into its significant strengthening behavior under high pressure and strain rate conditions.