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High-energy-density (HED) science is the study of matter under extreme pressure and temperature. Matter subject to these conditions exhibits a wide range of interesting and often unpredictable behavior that transforms atomic bonds and material structures, creating complex chemical reactions, highly ionized materials, and plasmas. HED science exemplifies the dual-purpose research we conduct at Lawrence Livermore—working to support both our core nuclear weapons research and the fundamental science explorations that underpin our mission-focused work.
Today, HED science is a growing research discipline at the Laboratory. It has proven essential to modeling nuclear weapons, advancing the pursuit of controlled fusion energy, and understanding the composition and dynamics of planets and stars. An increasingly multidisciplinary field, HED leverages Livermore strengths in high-performance computing (HPC), materials science, chemistry, physics, and engineering.
A major focus of HED experiments is determining a material’s equation of state (EOS), or the relationship between pressure, temperature, and density. As described in the feature article, Gently Compressing Materials to Record Levels, a useful technique for obtaining an EOS is ramp (or quasi-isentropic) compression. In this technique, refined over the past decade with important contributions from Livermore researchers, a material is pressurized “gradually,” over small fractions of a second. As a result, heating is limited to lower temperatures, maintaining a solid crystalline state at higher pressures. In contrast, a standard, nearly instantaneous shock raises temperatures significantly, melting or even ionizing a sample under investigation and limiting the study of its properties.
The 192-beam National Ignition Facility (NIF), the largest and most energetic laser in the world, is superbly equipped for conducting ramp compression experiments. NIF routinely creates temperatures and pressures similar to those that exist in the interiors of stars, the cores of planets inside and outside our solar system, and detonating nuclear weapons. The laser’s high energy and power, pulse shape control, and state-of-the-art diagnostics make NIF the premier facility for ramp compression at pressures measured in terapascals (10 million times Earth’s ambient air pressure).
Scientists adjust NIF’s pulse shape by varying the power of individual lasers over 31 billionths of a second to match the material under investigation and keep it relatively cool and highly compressed. Determination of the pulse shape is guided by high-resolution, predictive simulations performed on Lawrence Livermore’s world-class HPC resources. Over the decades, Livermore scientists have pioneered advanced scientific modeling codes on some of the world’s most powerful computers.
Ramp compression experiments draw upon our expertise in materials science, precision machining, and metrology (measurement science) to create submillimeter targets comprising intricate assemblies of extremely small components, including a stepped target with thicknesses ranging from 50 to 100 micrometers. Designing, machining, and assembling these parts requires an integrated team of highly skilled physicists, materials scientists, chemists, engineers, technicians, and machinists. The exquisitely manufactured and characterized targets are part of a larger Livermore effort to create advanced—and cost-effective—manufacturing processes that produce structurally and compositionally tailored materials with novel properties, shapes, and interior structures.
Precise crafting and metrology of targets’ dimensional characteristics have significantly reduced uncertainties in the experimental data. Experimental measurements are highly susceptible to manufacturing imperfections, so material samples are diamond turned to astonishing flatness and parallelism, equivalent to trimming a football field to within the thickness variation of a No. 2 pencil lead.
Ramp compression experiments on high-Z (high atomic number) materials support stockpile stewardship, the effort to assure the safety, security, and effectiveness of our aging nuclear weapons stockpile without relying on nuclear testing. At the same time, NIF operates as the premier national user facility for HED science. The Laboratory’s Discovery Science program enables a broad national user community to perform ramp compression experiments that in essence create a microphysics observatory for studying materials under the extreme conditions of astrophysical environments.
Matter at HED conditions is found throughout the universe, especially the interiors of planets and stars. Results from discovery science ramp compression experiments have been published in leading scientific journals. For example, experiments have provided insights into the possible interior structure composition of large rocky exoplanets known as “super Earths.” HED science has entered a particularly exciting era, and the Laboratory is proud to be playing a leading role.