Peter M. Celliers

At the National Ignition Facility, the time-resolved x-ray diffraction campaign records four snapshot diffraction patterns—with a temporal resolution of 2 ns and separated by 2 ns—during a single compression experiment. Data are collected with the Flexible Imaging Diffraction Diagnostic for Laser Experiments (FIDDLE) using five Icarus hybrid complementary metal–oxide–semiconductor (hCMOS) sensors. This multi-frame capability enables measurements of phase-transition kinetics, including solid–solid and eventually solid–liquid transformations, and supports efficient mapping of phase space with four data points per experiment. Achieving these measurements requires a high signal-to-noise ratio with a backlighter (BL) x-ray source, but the background is complex, and generally increases over the course of a shot. This paper details improvements in continuous x-ray generation with a Ge BL foil for >15 ns. In addition, we investigate a background contribution originating in the BL plasma, which generates hot electrons that impact the target and produce secondary x-rays. In the prior target configuration, secondary x-rays generated on the target sidewall had a direct line of sight to the Icarus sensors, increasing the background on the sensor as time progressed. We redesigned the target to reduce this line of sight, thereby more effectively “shadowing” the sensors. The modified design improved signal-to-noise ratio dramatically, thereby enabling us to improve from observing diffraction on only two sensors at 1–2 time steps to observing diffraction on all five Icarus sensors at all time steps.

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.

This roadmap presents the state-of-the-art, current challenges and near future developments anticipated in the thriving field of warm dense matter (WDM) physics. Originating from strongly coupled plasma physics, high pressure physics and high energy density science, the WDM physics community has recently taken a giant leap forward. This is due to spectacular developments in laser technology, diagnostic capabilities, and computer simulation techniques. Only in the last decade has it become possible to perform accurate enough simulations & experiments to truly verify theoretical results as well as to reliably design experiments based on predictions. Consequently, this roadmap discusses recent developments of and contemporary challenges for theoretical methods and experimental techniques needed to describe, create and diagnose WDM. A large part of this roadmap is dedicated to specific WDM systems and applications in astrophysics, inertial confinement fusion and novel material synthesis.

This paper presents an enhanced analysis method for investigating material properties at high strain rates, extending the capability of established experimental techniques to gain more information. The ramp-release method has been applied to many experiments reported at high (≈105 − 106 s−1) strain-rates. More recent data gathered at the National Ignition Facility (NIF) has enabled higher (≈108 s−1) strain-rates to be studied. Here, we present an initial application of ramp-release analysis to NIF ramp-compression data, illustrating both the opportunities and the practical challenges of extending these methods to laser-driven platforms. The higher strain-rates accessed at the NIF mean that there is more strain-rate enhancement to strength, and the experimental configuration means that this enhancement is more readily seen in the data. This is enabled by the capability of avoiding peak-compression attenuation through the sample thickness with a designed hold period made possible by the pulse-shaping capability of NIF. We propose that this combination of experimental conditions and an enhanced analysis method enables the strain-rate enhancement to strength to be studied, and potentially for this to inform physics models at smaller scales than the continuum.

Successful demonstration of an implosion that meets the physics metrics for ignition on the National Ignition Facility (NIF) in August 2021 and demonstration of target energy gain G > 1 in December 2022 represented the culmination of more than a decade of research on the NIF. An in-depth analysis of the key ignition metrics achieved on the NIF over the past decade shows sustained progress toward identifying and mitigating degradation effects. Ignition implosions on the NIF have progressed from early experiments whose compressed fuel conditions were degraded by a factor of 10–20 below the ignition threshold with yields of only 1%–2% of the laser energy, to current experiments that significantly exceed the ignition threshold and in the best experiments achieve yields greater than twice the laser energy. It is very likely that NIF can get more than 20 MJ of yield and possibly 50–70 MJ yields when fully optimized at an ultraviolet laser energy of 2.6 MJ following the completion of the NIF Extended Yield Capability Project. Achieving these higher yields will also require improvements to capsules at larger size and possibly with alternate materials, as well as an improved understanding of hohlraum and capsule physics. Now that NIF has achieved ignition, the design of a Next Generation High Energy Density Facility that could achieve the National Nuclear Security Administration long-term goal of substantially higher yields than the NIF can proceed with a more confident physics base. We evaluate these opportunities including possible applications for inertial fusion energy and likely challenges to achieving this level of performance over the next decade.

Data for the shock equation of state of helium are obtained up to 360 G Pa , nearly doubling the pressure of previous experimental measurements. The helium samples are first precompressed to 0.35–0.42 g c m − 3 in a diamond anvil cell prior to laser-driven shock compression at the Omega Laser Facility. Time-resolved Doppler velocimetry and pyrometry reveal significant reflectivity and greater compressibility compared to the predictions of existing broad-range tabular equation of state models, which may be caused by the onset of ionization. These experimental observations, however, are largely captured with molecular-dynamics simulations based on density functional theory, affirming the ability of first-principles techniques to capture complex physics, while enabling critical insight for the behavior of warm dense helium in Jovian interiors and white dwarf atmospheres.

Advances in shock and ramp compression techniques now allow experimental access to unprecedented extreme conditions of pressure and temperature, providing a means to test theoretical models. Here, we describe a simple methodology to compute multi-phase shock Hugoniot and isentropes using multiphase equation of state tables. We treat explicitly the phase coexistence along the phase boundary to reveal the evolution of the sample as it undergoes the phase transformation in adiabatic conditions. We illustrate the method by calculating the predicted shock and shock-and-release behavior of diamond at conditions relevant for the initial stage of inertial confinement fusion implosions.

A set of experiments in the SQ-n campaign attempts to increase the compression in layered implosions by reducing the design adiabat from 3.0 down to ∼2.6. The experiments utilize nanocrystalline high density carbon ablators driven by a ramped laser pulse drive. Contrary to expectations, the lower design adiabat layered implosion reaches the same level of compression, within the measurement uncertainty, as the previous set of SQ-n layered implosions at the higher design adiabat. We examine potential explanations for this result, including incomplete melting or refreezing of the ablator after the first shock passes into the fuel layer, reproducibility, and dopant areal density. We also discuss additional potential paths to reaching lower adiabat including increased dopant areal density, ramped dopant profiles, and increased initial capsule size which are also based on experimental campaign results from the National Ignition Facility.

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.