High-Energy-Density Physics

High-energy-density (HED) physics studies matter under extreme conditions of temperature, pressure, and density. These conditions exist in stellar interiors, during nuclear detonations, and inside inertial confinement fusion capsules. Over the past several decades, laboratory facilities have made it possible to recreate and study HED states of matter in controlled experiments.

What is HED

A system is considered to be in the high-energy-density regime when its energy density exceeds approximately 10¹¹ joules per cubic meter, which corresponds to a pressure above about 1 megabar (100 GPa, or roughly one million times atmospheric pressure).

For context, 1 Mbar is the pressure at Earth's core. At these conditions, matter behaves very differently from everyday experience:

• Materials compress to multiples of their normal solid density.
• Atoms ionize even in dense solids, creating warm dense matter—a state intermediate between a solid and a classical plasma.
• Radiation transport becomes significant, as the material emits, absorbs, and re-emits X-rays that carry substantial energy.
• Hydrodynamic instabilities (Rayleigh-Taylor, Richtmyer-Meshkov) grow rapidly at interfaces between materials of different density.

The physics at these conditions is inherently multi-physics: fluid dynamics, radiation transport, atomic physics, and nuclear physics are all coupled together.

How to Get There

Creating HED conditions in the laboratory requires delivering large amounts of energy to small targets in very short times. Two main approaches are used today:

High-Power Lasers

Large laser facilities focus intense beams onto millimeter-scale targets, depositing megajoules of energy in nanoseconds. Key facilities include:

• National Ignition Facility (NIF) — Located at Lawrence Livermore National Laboratory in California. NIF's 192 laser beams deliver up to 2.05 megajoules of ultraviolet light to a target. In December 2022 NIF achieved fusion ignition, producing more fusion energy than the laser energy delivered to the target.
• OMEGA Laser Facility — Located at the University of Rochester's Laboratory for Laser Energetics. OMEGA's 60 beams deliver up to 30 kilojoules and are used extensively for direct-drive fusion research and HED experiments.

Short-pulse (petawatt) lasers achieve even higher intensities over femtosecond to picosecond durations, enabling studies of relativistic plasma physics and advanced radiography.

Pulsed Power

Pulsed-power machines store electrical energy in capacitor banks and then discharge it through a load in microseconds, generating enormous magnetic fields and pressures.

• Z Machine — Located at Sandia National Laboratories. The Z Machine drives roughly 26 million amperes through a cylindrical array of fine tungsten wires (a “wire array z-pinch”) or a metal liner, producing powerful X-ray sources and achieving pressures in excess of 5 Mbar. It is the most powerful laboratory X-ray source in the world.

Pulsed-power approaches are particularly well suited for studying material properties under quasi-isentropic (shockless) compression, which preserves the sample in a more uniform thermodynamic state than shock-driven experiments.

What You Can Measure

HED experiments aim to characterize the fundamental properties of matter under extreme conditions. Key measurements include:

• Equation of state (EOS) — The relationship between pressure, density, and temperature. Shock-wave experiments measure the Hugoniot (the locus of states reachable by a single shock), while ramp-compression experiments probe states along the isentrope. Accurate EOS data are essential for simulating planetary interiors, inertial confinement fusion (ICF) capsule implosions, and astrophysical phenomena.
• Opacity — How strongly a material absorbs and emits radiation as a function of photon energy. Opacity measurements require heating a sample to a known temperature and density, then measuring its transmission spectrum. These data feed directly into radiation-hydrodynamic simulations.
• Transport properties — Thermal and electrical conductivity under extreme conditions, which govern energy flow in compressed matter. These are among the most difficult quantities to measure and model accurately in the warm dense matter regime.

Diagnostics for HED experiments include X-ray spectrometers (grating, crystal, and filtered diode arrays), X-ray imaging systems, velocity interferometry (VISAR), and nuclear diagnostics for fusion reactions.

Applications

HED physics underpins several areas of national and scientific importance:

• Inertial confinement fusion (ICF) — ICF uses lasers or X-rays to compress a small capsule of deuterium-tritium fuel to extreme densities and temperatures, initiating thermonuclear burn. NIF's 2022 ignition result was a landmark achievement, and ongoing research aims to increase the fusion yield toward eventual energy applications.
• Magneto-inertial fusion (MIF) — MIF combines magnetic fields with pulsed compression to reduce the required implosion velocity and driver energy. Approaches using pulsed power (e.g., magnetized liner inertial fusion on the Z Machine) are being actively explored.
• Laboratory astrophysics — HED experiments can recreate conditions found in supernova remnants, giant planet interiors, and accretion disks, allowing scientists to test astrophysical models with controlled measurements.
• National security science — HED experiments provide data needed to validate computational models used in national security and defense applications.