HEDS fellow Patricia Cho probes cosmic mysteries

The High Energy Density Science (HEDS) Center fellowship at Lawrence Livermore National Laboratory (LLNL) encourages postdoctoral scientists to expand their horizons and pursue new research possibilities related to the study of matter and energy under extreme conditions.

For HEDS Center fellow and experimental physicist Patricia Cho, the fellowship has allowed her to branch out from her Ph.D. work, exploring new areas of laboratory astrophysics and introducing new experimental approaches into her collection of work.

Iron in the cosmos

While earning her Ph.D. in astronomy from the University of Texas at Austin, Cho sought to answer the research question: Why do high amounts of iron appear in the accretion disks that swirl around black holes?

An accretion disk is a flat, spinning disk of gas, dust and other material that forms around a massive object, like a black hole. When modeling black hole accretion disks, in order to replicate experimental observations, more iron has to be put into the model than expected.

“One thing that’s so weird about this,” Cho explained, “is that we’re talking about seeing an overabundance of iron in two massively different black hole populations: stellar mass and supermassive black holes.”

Not only are stellar mass black holes extremely small compared to supermassive black holes, but their formation mechanisms are vastly different.

“A stellar mass black hole is thought to form from the direct collapse of a star’s core, while supermassive black holes are believed to grow over time through the mergers of smaller black holes and other massive objects,” said Cho.

So why do researchers see a systematic trend of iron overabundance in both types of systems? To investigate this question, Cho conducted experiments at Sandia National Laboratories’ Z machine to determine whether these black hole accretion disks actually contain a lot of iron by reproducing in a laboratory setting what she was seeing in the model.

This introduction into the world of laboratory astrophysics at Sandia eventually led her to Livermore in 2024, where she would continue her cosmic-related research under the HEDS fellowship — but this time tackling a different fundamental question about the universe.

Finding the solar convection zone

“My research is now focused on questions about opacity; basically, how transparent or opaque plasmas are in astrophysical contexts, and how that influences the way radiation interacts with matter,” said Cho. “Understanding opacity is crucial because it helps us build accurate models of how stars and galaxies form and evolve. The answers we find can influence our understanding of the age and development of the universe, and ultimately, our place within it.”

Cho’s opacity research is driven by a disagreement between two different methods (helioseismic measurements and stellar structure models) used to identify where the boundary of the solar convection zone is; that is, the boundary inside the Sun that initiates a change in the way energy moves.

Below this boundary, energy is transported mainly by radiation, so photons are bouncing around and gradually making their way out from the core. Within this region, the material is relatively stable and doesn’t move much. But above this boundary, the material becomes unstable to convection, so instead of just photons carrying energy, the hot plasma itself starts to move and churn.

The problem is that helioseismology, which uses the vibrations of the Sun to pinpoint this boundary, says the base of the convection zone is at one radius, while the stellar structure models, which rely heavily on opacity calculations, predict a different radius.

Currently, Cho is conducting experiments at LLNL’s National Ignition Facility (NIF) to help theorists confirm the accuracy of the opacity measurements and reconcile the differences between these two approaches.

While pressing forward with her opacity experiments at NIF, she has also begun working on experiments with more direct, terrestrial applications — work that she says could have a real impact on humanity.

Opening new doors

Cho’s latest research effort started earlier this year when she became curious about what others were working on at the Laboratory. After knocking on a few doors and connecting with other researchers, she discovered that one of her colleagues was researching electron fast ignition (EFI), an alternative fusion energy concept, and was connected to an experimental campaign in Paris at the APOLLON laser facility.

While working on this campaign, Cho helped field five core diagnostics used to collect data. One of the diagnostics she worked on was called a titanium K alpha imager.

“It is used to look at the back portion of the target to visualize the localization of the hot spot and tell us how well the electrons are being collimated [focused] onto the back surface of the target,” Cho explained. “Because that's the whole idea: to collimate the electron beam and produce hotter conditions that could hopefully lead to ignition.”

Back at Livermore, Cho is continuing to work with colleagues on similar EFI experiments at LLNL’s Jupiter Laser Facility (JLF), where she is using JLF’s Titan laser to refine their experimental platform.

“I feel very fortunate that I don't feel like there are many boundaries to the things that I can pursue if I'm interested in them; like this EFI experiment, I've been having so much fun with it,” said Cho. “The underlying physics is so new to me and super exciting; and it's a combination of the fact that I have the HEDS fellowship and the fact that Livermore is so supportive of providing postdocs with career development opportunities that I am able to explore these new things.”

–Shelby Conn