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By creating plasmas—mixtures of ions and free electrons—under extreme conditions, the National Ignition Facility (NIF) becomes a terrestrial test bed for extraterrestrial conditions. In these environments, NIF provides an Earth-based laboratory for understanding the cosmos, providing insight into questions about how stars and planets begin and end their lives. Researchers in the Discovery Science Program at Lawrence Livermore are particularly interested in understanding the mechanism by which atoms gain or lose electrons (ionization) at high pressures and densities.
The extent of ionization in stars and giant planets determines their material properties, such as their thermodynamic behavior and electrical conductivity, and whether they have a surrounding magnetic field. Understanding the ionization process is, therefore, crucial to accurately modeling these objects. Ionization is conventionally driven by high temperatures such as those found in burning stars. Cooler astrophysical objects, however, can be ionized by a different mechanism: pressure-driven ionization, in which the object’s outer layers compress the matter in its interior. This effect is felt even at the atomic level, where extreme pressure squeezes atoms together so intensely that the atoms’ nuclei approach their neighbors’ K-shell, the innermost and most tightly bound shell of electrons around the nucleus. This proximity leads nearby particles to interact and modify one another’s electron structure, causing the atoms to become ionized. As the ions are forced closer together under pressure, electrons are released into the surrounding plasma, further ionizing it. However, despite its importance, many parameters in this process remain unknown.
Coming Out of the K-Shell
Since electrons in the K-shell are the most strongly bound in an atom, they require the most energy to unbind. As a result, these K-shell electrons are the dominant factor in determining the radiation transport—the process of energy transfer through radiation. “Radiation transport is very important in stars, for example, because it determines their structure, how energy flows from the center of a star to the outside, and vice versa,” explains NIF experimentalist Tilo Doeppner. Understanding how K-shell electrons behave and survive at the extreme temperatures and densities in stars is a significant step toward improving understanding of radiation transport as a whole.

In a yearslong collaborative campaign, Doeppner and his research team used NIF to generate the extreme conditions necessary to achieve pressure-driven ionization and study its effects on K-shell electrons. NIF’s 192 lasers heated up the inside of a hollow cavity, called a hohlraum, and a beryllium shell placed at its center. Upon heating the hohlraum with lasers, the outside of the shell rapidly expands while its inner surface accelerates toward the center at pressures exceeding 3 billion atmospheres and temperatures of around 2 million Kelvin. For just a few nanoseconds, this process creates a tiny piece of matter analogous to a dwarf star. “Right now, NIF is the only experimental facility where we can create and study these states under controlled conditions,” says Doeppner.
Using a technique called x-ray Thomson scattering (XRTS), Doeppner and his collaborators worked to determine how many electrons remain in a bound state during pressure-driven ionization—that is, remaining part of an ionized atom—versus how many would release into the surrounding plasma. A major benefit of XRTS is its simplicity; the technique depends purely on the analytical relationships among quantities. Lawrence Fellow Max Boehme says physicists can define and solve these relationships without complex computational techniques. “There’s a sophisticated quantum mechanical theory behind XRTS, and it gives us a lot of information,” he says. “This is one of the most promising methods to directly probe the microphysics of these dense plasmas.”
To carry out an XRTS measurement experimentally, the researchers shine x-rays into a plasma. The x-ray photons scatter off electrons in the plasma, which can be either free, loosely bound, or tightly bound to the ionic cores of atoms. XRTS measures the intensity and spectral distribution of these photon scattering events under a certain deflection angle. In each scattering scenario, photons emerge with an identifiable energy spectrum that reflects the type of interactions that occurred with the electrons in the plasma. “I describe this as a relativistic billiard ball problem,” says Alison Saunders, a Livermore experimental physicist working at NIF. “If someone were to roll one ball—representing the x-ray photon—on a billiards table so that it impacts a second ball—representing an electron—the first ball would bounce differently off the second one depending on whether the second ball was stationary or already in motion. The mechanics of interpreting XRTS is conceptually similar. If we know the momentum of the ball going in and the momentum of the ball going out, we can say something about what it must have scattered off of.”
If an x-ray intercepts a free electron, energy transfers from the photon to the electron, and the photon emerges with slightly less energy than when it entered. Intercepting a loosely bound electron also reduces a photon’s energy, but the amount of energy transferred is distinct from the first case, resulting in a unique energy spectrum that allows researchers to differentiate between the scattering events. Finally, a photon scattering off a tightly bound electron, such as the ones in the K-shell, rebounds as if off a mirror—known as elastic scattering—and retains its initial energy. “Scattering a lot of photons off the plasma, we can begin to understand how many electrons were free, loosely bound, or tightly bound from the distributions in the resulting scattering spectra, thereby providing a constraint on the plasma’s overall ionization state,” says Saunders.
Testing the Models

Doeppner, Saunders, Boehme, and their team used XRTS measurements to observe the onset of pressure-driven ionization in beryllium. The researchers found at least three of beryllium’s four electrons were removed during compression. At the highest levels of compression, they found a reduction in elastic scattering, which is related to tightly bound electrons, thus confirming the tendency of the single remaining K-shell electron to delocalize.
This information about pressure-induced ionization can be used to predict the extreme temperature and pressure conditions needed to accurately model the inner workings of stars, planets, and even inertial confinement fusion (ICF) experiments. Doeppner says, “A lot of similarities exist between these astrophysical questions and the ICF experiments that we do at Lawrence Livermore.” However, because the reduction in elastic scattering that the team observed with XRTS is often overlooked, most of the widely used ionization models for ICF might underestimate the degree of ionization and the effects it can have on experiments.
Incorrect ionization models have long-ranging effects as they determine how easily x-rays can pass through a plasma and, hence, determine the radiational energy transport in stars and in ICF experiments. With XRTS studies, scientists can now confirm the validity of their ICF models for the first time—and NIF is the only place where that can be done. “We find that electrons can be in a weird state between bound and free, which is challenging for simulations to describe,” says Doeppner.
Highlighting the importance of new experiments that create conditions matching those at cores of planets, exoplanets, and some stars, Boehme adds, “NIF has the ability to reach conditions at which today’s models haven’t been tested before—a unique situation.” The ability to field experiments that test such extreme conditions is central to NIF’s core function of supporting stockpile stewardship.
Other NIF Discovery Science Projects
NIF’s Discovery Science Program offers opportunities for external facility users and Lawrence Livermore scientists to perform research in high-energy-density science. As part of the Discovery Science Program, researchers have:
- Recreated the conditions inside Jupiter, Saturn, and other planets by compressing a diamond sample to a record 50 million megabars—50 million times the pressure of Earth’s atmosphere.
- Brought new insights into fluid hydrogen transformations, providing key information about how giant planets and solar systems form.
- Recreated conditions similar to the inside of stars.
- Used collisionless shocks to study exotic astrophysical phenomena, including supernova remnants and cosmic magnetic fields.
Exemplar for Discovery Science

A portion of NIF’s approximately 400 annual experiments are set aside for basic science campaigns through NIF’s Discovery Science Program, with the bulk supporting NIF’s primary application in support of the National Nuclear Security Administration’s science-based Stockpile Stewardship Program. The Discovery Science Program provides opportunities for external users to take advantage of NIF’s unique facilities and capabilities and to perform laboratory experiments in high-energy-density regimes. As both a recruiting tool and a scientific success story, XRTS measurements are exemplary of the Discovery Science Program and its mission. To date, more than 100 such experiments have been conducted or are planned. (See box above.)
“Alison Saunders and Max Boehme were early career scientists when this project started. They exemplify how NIF’s Discovery Science Program provides opportunities for young scientists to participate in the work done at NIF and how that can serve as a hiring pipeline for the Laboratory,” says Doeppner. Now a staff scientist and group leader, Saunders began work on this project as a postdoctoral researcher. Boehme first came to the Laboratory as a summer intern analyzing XRTS spectra for this project. As a Lawrence Fellow, he runs quantum simulations bridging the gap between theory and experiment.
The cycle between theory and newly gathered data creates a test bed for advancing science with new, testable ideas. “I have a high-quality data set of conditions that have never been created in any experiment before. We’re literally the first ones to probe this data, and that’s exciting to me,” says Boehme.
The collaborative nature of Discovery Science projects attracts researchers from prestigious universities, national laboratories, and others from around the world, creating new research opportunities for the future. “Putting words to the level of exhilaration we feel is difficult,” says Doeppner. “Perhaps the best comparison is that of children exploring the world by playing in a sandbox. In our case, we get to explore the principles of the universe with the biggest laser in the world.”
—Anashe Bandari
For further information contact Tilo Doeppner (925) 422-2147 (doeppner1 [at] llnl.gov (doeppner1[at]llnl[dot]gov)).