Plasma pursuits: HEDS Center fellows illuminate the fourth state of matter

In 2019, the High Energy Density Science (HEDS) Center at Lawrence Livermore National Laboratory (LLNL) launched its postdoctoral fellowship program, welcoming one new scientist annually to come and conduct research for a two-year term. Supported by LLNL’s Weapons Physics and Design program, HEDS fellows are encouraged to pursue their own research agenda as it relates to the study of matter and energy under extreme conditions.

The most recent postdoctoral fellows, physicist Elizabeth “Liz” Grace (2022 fellow) and plasma physicist Graeme Sutcliffe (2023 fellow), are using high-intensity lasers and advanced diagnostics to observe the behaviors of plasma. A plasma, known as the “fourth state of matter,” is a superheated, ionized gas that makes up the majority of visible matter in the universe, like stars and nebulae. Replicating these conditions is a key step to achieving robust igniting inertial fusion designs for energy resilience.

Seeing is believing

Ever since Grace was in the fifth and sixth grade, she has been inspired by astrophysics — black holes, dark matter and star formation.

“In high school, I began to learn about special relativity in my AP physics class, which further cemented my interest,” Grace said. After taking a summer course in astrophysics at Cornell University and getting excited about all the new concepts she was learning, it was then that Grace ultimately knew she was meant to study physics in college.

While pursuing her Ph.D. in physics at the Georgia Institute of Technology, Grace became captivated by the field of laser science, specifically the study of optical rogue or “freak” waves. This rare phenomenon occurs when the generation of supercontinuum optical light — also known as “white light,” spanning the near ultraviolet to the infrared spectrum — produces high-intensity spikes that seemingly appear out of nowhere, similar to the unexpected and dangerous rogue waves in the ocean.

At LLNL, Grace studies high-intensity laser–plasma physics, an extension of her Ph.D. research, focusing on the development of new optical diagnostics to understand the interactions between laser light and matter. “To see something for the first time, you have to first build a tool that is capable of seeing it,” she said. “My work centers around developing diagnostic tools capable of uncovering new phenomena.”

Her research seeks to understand three key questions: 1) how do small, local fluctuations in laser energy and intensity produce drastically different outcomes, 2) how can we observe microscopic changes on an ultrafast timescale and 3) can we use special relativity to improve the efficiency of laser-driven inertial fusion energy? 

To answer these questions, Grace conducts experiments at LLNL’s Jupiter Laser Facility, where she develops and tests single-shot, time-resolved probe diagnostics that can produce a movie of every shot’s laser–matter interaction, helping to quantify the shot-to-shot fluctuations in plasma dynamics. Typically, this would require a series of identical shots to form a sequence, which can be difficult to produce due to the unstable nature of high-energy systems. However, single-shot diagnostics, like the one Grace is developing, can provide a more complete picture of each interaction, giving unprecedented insight into how various small-scale changes can drastically affect outcomes. 

“My current research allows me to push the frontiers of my own knowledge in new directions every day,” Grace said. “Due to the broad scope of my assignment, I am able to easily investigate new ideas and avenues for research.”

Harnessing the force

Before joining LLNL, Sutcliffe studied at the University of British Columbia and the Massachusetts Institute of Technology (MIT), earning a bachelor’s in applied science in engineering and a Ph.D. in physics, respectively. At MIT, Sutcliffe was a part of the High Energy Density Physics group, where he was “excited by the breadth of problems that could be investigated using large laser facilities,” such as the University of Rochester’s OMEGA Laser Facility and LLNL’s National Ignition Facility.

Carrying over his Ph.D. research to LLNL, Sutcliffe studies how magnetic fields are generated in plasmas, which is important for understanding both astrophysical-scale plasma systems as well as laboratory scale ones, e.g., inertial fusion designs.

“Specifically, I am studying how the Weibel instability [a common kinetic instability in laboratory plasmas] generates magnetic fields, and how these fields may coalesce to form larger structures.” This research can lend insights into the origin of large astrophysical magnetic fields that permeate galaxies, as well as to help improve the efficiency of laser energy coupling, thereby informing better designs for inertial fusion targets.

Sutcliffe primarily conducts his experiments at the OMEGA Laser Facility, using the laser’s 30 kilojoules of energy to create astrophysically relevant plasma conditions, allowing him to then study how the magnetic fields are generated.

To measure the plasma conditions, Sutcliffe employs Thomson scattering, which reveals the plasma characteristics that are encoded in a scattered light spectrum. For measuring the magnetic fields, he uses a novel technique called proton deflectometry (also called proton radiography) that he helped refine at MIT. In proton deflectometry, a beam of protons is produced and sent through a plasma; when the electromagnetic fields deflect this beam, the fields can be inferred from the proton positions on the detector. To extend the capabilities of this technique, Sutcliffe also developed a tri-particle backlighter that helps the diagnostic to distinguish between magnetic and electric fields more effectively.

Following an experiment, Sutcliffe analyzes the experimental data using techniques capable of decoding the magnetic field from the distribution of proton particles measured on the detector. He then compares this data to his pre-experimental simulations, refining the simulations to match the experimental plasma conditions and working to fill in any gaps between the two measurements.

“I enjoy the problem-solving aspects of designing an experiment,” Sutcliffe said. “The HEDS fellowship offers me the freedom, resources and tools to pursue the physics questions that I think are important and interesting.”

–Shelby Conn