The power of plasma

Look up on a clear night, and you will see blazing spheres of plasma powered by the fusion of atoms in their cores. That’s right, you’re looking at stars—a celestial demonstration of high-energy-density (HED) physics at work.

For Los Alamos National Laboratory scientist Sasikumar Palaniyappan, sometimes a typical workday involves making a star on Earth. Well, sort of. As an HED physicist, Palaniyappan’s work involves creating burning plasmas, concentrated balls of extremely hot, ionized gas generating light and heat—similar to the cores of those stars twinkling in the night sky. “It’s so cool. It’s an exciting physics platform,” Palaniyappan says.

Inside a burning plasma, high temperatures and pressures break apart atoms, causing their electrons and ions to move around chaotically. When the free-moving electrons and ions collide in these conditions, they bind together, or fuse. If the reactions produce enough heat to trigger ongoing reactions, the burning plasma becomes self-sustaining. That means it continues burning without any additional fuel. “A self-sustaining burning plasma is our goal,” Palaniyappan says. “For me, this is more than work; it’s fun.”

Creating burning plasmas is one of the keys to harnessing clean, affordable, and unlimited fusion energy. Researchers have been pursuing fusion energy for nearly 80 years. Recent breakthroughs suggest that commercial fusion energy is getting closer to becoming a reality, particularly due to successful experiments conducted by Lawrence Livermore and Los Alamos national laboratories.

Although Palaniyappan and his coworkers say they are ecstatic about the progress toward fusion energy, creating an alternative energy source is not the primary focus of their work. The conditions required to create fusion (and stars) also exist during the detonation of a thermonuclear weapon, in which a fission reaction triggers a fusion reaction to create kilotons or even megatons of explosive power.

“This extreme state occurs in astrophysical phenomena, such as stars, and during nuclear weapons detonations,” says Los Alamos HED physicist Forrest Doss. “It doesn’t exist anywhere else. Everything is hot and flowing; there are no solids; everything is glowing, and the energy balance is different than anything we experience during day-to-day life,” he says.

In the past, the United States conducted nuclear weapons tests, proving that scientists can create fusion. The data from those tests helped scientists build the seven types of thermonuclear weapons in the U.S. stockpile today. But, in 1992, the nation declared a moratorium on full-scale nuclear testing. The United States now relies on nonnuclear and subcritical experiments coupled with advanced computer modeling and simulations to evaluate the health and extend the lifetimes of America’s aging nuclear weapons, which are decades old. This approach is called stockpile stewardship, and HED physics is part of its success.

“There are serious questions about how nuclear weapons will perform in detonation conditions,” says Los Alamos physicist Joseph Smidt, the co-director of the Los Alamos Inertial Confinement Fusion (ICF) program. “If you don’t actually detonate a weapon, you have to ask yourself how you are going to obtain information on material properties, physics, and science at these extreme temperatures, densities, and pressures. That’s why creating similar conditions in the laboratory is essential for national security.”

Maintaining the truck

Smidt compares the weapons in the U.S. nuclear stockpile to an old truck parked in the garage—a truck that will need to run flawlessly if there is an emergency. “These weapons have not been tested in decades. It’s a little bit like saying, here’s this truck that you used to drive all the time, and you know it worked back then because you drove it all the time. We don’t want you to drive it again, but we want you to guarantee that 20 or 30 or 40 years later, if you ever had to drive it, it would start and run smoothly, safely, and correctly.”

Will the truck start? “Well, as you know, parts rust; things get old; the longer you go without driving it, the more uncertain you’ll start getting about whether this truck is going to start,” Smidt says. “How do you guarantee that it’s going to work?”

Smidt notes that while you can’t start the truck, you can conduct tests to see if individual parts are working correctly to help you understand its overall condition. HED physics experiments allow scientists to perform those sorts of tests on nuclear weapons.

In the past three years, Los Alamos scientists, working with their colleagues from throughout the nuclear enterprise, have made significant progress on HED research. These advances, combined with other recent breakthroughs, are opening windows into understanding weapons performance, aging, materials, and more.

“The research answers questions related to weapons physics and how well our computer codes and design efforts work.  We can look at various material properties, processes that take place during a detonation, and energy and material interactions,” Smidt says. “These are all things that are very important.” In other words, they are things the scientists must study to ensure that if they need that truck to start up, the engine will purr, and the truck will operate safely and reliably.

Creating the conditions

Los Alamos scientists primarily create the conditions needed for this research at three facilities: Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) in Livermore, California; Sandia National Laboratories’ Z Machine in Albuquerque, New Mexico; and the University of Rochester’s Omega Laser Facility in Rochester, New York. These facilities achieve fusion conditions by rapidly compressing and heating a small quantity of fusion fuel. The process, called inertial confinement fusion, or ICF, gets its name from the word ‘inertia,’ because the goal is to compress fuel particles, so they are held together by their own inertia. NIF and Omega do this using lasers; Z uses pulsed power and magnetic fields.

Los Alamos physicists frequently travel to NIF, Z, and Omega to carry out experiments. Scientists also run diagnostics on other labs’ experiments and collaborate with researchers from other institutions. “There’s no way to study fusion and these extreme conditions outside of these HED facilities,” Doss says. “Fusion only happens when things are so hot and so dense.”

Whether he is designing an experiment to conduct at NIF, Z, or Omega, Doss says he always starts by identifying “a bit of physics we want to test, such as a term in the equations that make up the computer codes, or a particular interaction, effect, reaction, or process.”

Double shells at NIF

At NIF, 192 laser beams focus their energy into a single pulse fired at a capsule of nuclear fuel the size of a peppercorn. In 2022, scientists first used NIF to achieve a condition called fusion ignition. This means they created a sustained fusion reaction that generated more energy than the lasers initially put in.

Achieving ignition represents a significant breakthrough for HED physics experiments and is crucial for studying certain aspects of weapons performance.  “Ignition really opens the door to a whole new realm,” says Ann Satsangi, the other co-director of the Los Alamos ICF program. “For many years we were trying to get to ignition. Finally, we are here.”

In addition to collaborating with their Livermore colleagues on traditional NIF ignition experiments, Los Alamos scientists are focused on a series of experiments called the double-shell campaign. In these experiments, the NIF lasers shoot energy into a complex assembly called a double-shell target.

Double-shell targets, which are designed and made at Los Alamos, consist of two foam hemispheres and an aluminum outer shell surrounding an inner metal capsule. The tiny capsule, a couple hundred microns in radius, is filled with deuterium and tritium—the fusion fuel. Technicians at Livermore place the target inside what is called a hohlraum, a cylindrical container designed to capture the laser energy and convert it into x-rays that compress and heat the fuel capsule inside the double shells.

“It’s an exciting physics platform because of the small scale and short time frame,” Palaniyappan says. “We are dealing with a 30-micron target and process that lasts for 150 picoseconds.” To put that into perspective, a human hair is typically 70–100 microns in diameter and a picosecond is to 1 second what 1 second is to 31,700 years.

Palaniyappan and physicist Eric Loomis say they are focused on changing one variable at a time, such as using new materials, like tungsten or molybdenum, for inner shells and using low-density foams made at Los Alamos to hold the inner shells in place. Unlike the single-shell targets used for ignition experiments, the double-shell targets allow scientists to study how new materials interact with burning plasmas. These experiments also reveal  how incredibly small capsule-design details can impact the success of the implosion. Scientists are employing advanced diagnostics using high-speed imaging, neutron detectors, and spectroscopic techniques to analyze implosion dynamics.

“There’s a variety of things we have to think about to come up with solutions for complex problems,” Loomis says. “We have to find unique solutions using simulations, theory, and diagnostics. We have to develop unique ways to build a target. We have to develop novel ideas to overcome the challenges. Doing the science to see what will work, that’s what inspires me every day.”

Satsangi says that Los Alamos HED scientists have become more focused on the double-shell experiments in the past year. “We had three successful tests in 2024, and we’ve been able to get a tenfold increase in energy and repeat our results,” she says. “We are creating a solid platform where we can do repeatable, measurable science, and we are pushing forward the diagnostics we need for future stockpile work.”

Smidt hopes to increase the cadence and types of experiments at NIF, which will result in more data. “We are already discussing updates to NIF that will provide higher energies and higher yields so we can explore questions about how nuclear weapons can survive hostile environments,” he says.

Zipping over to Z

Unlike NIF or Omega, which use lasers, the Z Machine at Sandia National Laboratories creates HED conditions by using a strong electric current to generate intense magnetic fields. The magnetic fields “pinch” a plasma to create extremely high temperature and pressure, resulting in fusion conditions.

“The lasers at NIF and Omega are much more delicate and precise; Z works differently to achieve the same conditions, so it allows us to double-check our results in a completely complementary way,” Doss explains. “Z throws out an enormous amount of energy. If you are in one of the adjacent buildings, it’s like a small earthquake when it goes off. Kind of feels like the building went over a pothole.”

New code capabilities

Because scientists rely heavily on computational simulations, data from HED experiments provide critical benchmarks for validating and refining simulations to make accurate predictions. To model and simulate data, HED physicists use computer codes—essentially instructions that tell a supercomputer what to do. The Laboratory’s state-of-the-art radiation hydrodynamics computer code used to simulate HED physics experiments is called xRAGE (Radiation Adaptive Grid Eulerian), and in 2023, xRAGE was enhanced to allow scientists to make more accurate 3D predictions of what happens when intense x-rays cause a fusion fuel capsule to compress and implode.

“This gives us a unique tool that can be used to study HED physics broadly and can push the ICF capabilities,” says physicist Brian Haines, the team leader for the effort. “It gives us the ability to study multiphysics problems (meaning many physical processes can be modeled simultaneously with the code) and see how tiny features and perturbations impact implosions.”

Haines says these new code capabilities have revealed the importance of capsule quality in obtaining a symmetrical (and therefore effective) implosion for creating a burning plasma. Los Alamos scientists are applying the code to both traditional NIF ignition experiments and double-shell tests.

“There are mysteries in implosions, and now we have a state-of-the-art tool to investigate those mysteries,” Haines says. “What really makes xRAGE unique is that we can directly model tiny details in the capsules that are prohibitively difficult to model in other codes. This is due to how we generate and adapt the mesh, which is the computational representation of the experiment. The new xRAGE capabilities are allowing us to answer questions we couldn’t answer before.”

Smidt says that because the ICF experiments involve numerous 3D features, the 3D code capabilities are invaluable. “We put a lot of effort into getting the 3D features right,” Smidt says. “Many of the recent advances in capsule design involved using 3D visualizations to figure out where the major perturbations on the capsules were and eliminating them,” Smidt says. “These advances led to ignition.”

Adding in AI

Like many scientists around the world, Los Alamos physicists are leveraging artificial intelligence (AI) to increase efficiency and decrease computational costs.

“We’re scoping out some new and exciting ideas to give physicists the ability to use AI tools to search for new HED experiment designs,” says physicist Michael Grosskopf. “By running lots of AI simulations, we can find optimal performance and robustness in an ICF target design.” Grosskopf adds that AI can help physicists automate tasks and use resources more efficiently while still maintaining confidence in their results.

Physicist Marc Klasky notes that AI allows scientists to generate accurate 3D simulations much faster than they could previously. “We have come up with a way to piece together 1D and 2D simulations to get a 3D result,” he says. “Before we developed this method, the computational demands and data sets were prohibitively time-consuming and expensive to generate.”

Klasky and Grosskopf say AI represents a significant tool for physicists. “AI can allow HED physicists to use their knowledge more effectively,” Grosskopf says. “It gives them a way to cast a wider net and scan possibilities in a way we have not been able to in the past.”

Reaching for the stars

HED experiments provide data for understanding how materials behave under conditions of extreme pressure and temperature, which is crucial for understanding how the aging of materials could impact performance over time.

“From conception to realization, these experiments are valuable for studying weapons science,” Palaniyappan says. “These are not miniature weapons, but in many ways, they behave similarly to a detonating weapon on a significantly different scale.”

Satsangi agrees. “This work is impacting stockpile science by answering key questions that allow us to avoid conducting full-scale nuclear testing,” she says. “As we advance, we develop new diagnostic and assessment methods.”

Smidt says the Los Alamos scientists’ success in these areas is a testament to their scientific and technological capabilities. “It shows that the people who are sustaining the stockpile have a level of competency in this science that no other nation has achieved.” ★

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