New NIF Diagnostic Tool Will Make ‘Movies’ of Phase Changes in Materials

In a significant advance in high-pressure physics, Lawrence Livermore National Laboratory (LLNL) researchers have developed a tool to capture real-time sequential images of phase changes in materials. This new diagnostic instrument—dubbed FIDDLE—is shedding light on the nanosecond dynamics of matter under extreme conditions.

Materials scientists are keen to learn how changes in pressure and temperature can alter the atomic structure, or phase, of solids and liquids. This knowledge can lead to innovations in fields ranging from medical technology to industrial materials, astrophysics, geology, and national security.

One of the best ways to secure that information is through laser-driven dynamic compression experiments at LLNL’s National Ignition Facility (NIF), the world’s highest-energy laser system. FIDDLE, short for flexible imaging diffraction diagnostic for laser experiments, has been added to NIF’s suite of more than 100 diagnostic instruments that gather data from the hundreds of NIF experiments that take place each year. FIDDLE is helping researchers track how fast those phase changes occur during a few nanoseconds (billionths of a second).

“We now know that over a few nanoseconds or so, we see these changes,” said LLNL experimental physicist Cara Vennari, the responsible scientist for Livermore’s Time Resolved X-Ray Diffraction (XRDt) campaign. “But we don’t know how fast it’s happening. It’s a fundamental question of the kinetics of phase transitions.

“The really exciting thing about FIDDLE,” she said, “is that we’ll get four snapshots of the phase changes per shot, and we can understand how fast it actually is happening and thus how to predict a material’s behavior in different scenarios.”

X-ray diffraction works by analyzing how x rays interact with a material’s atomic arrangement, providing detailed information about its crystal structure, phase composition, density, and other structural properties. Materials researchers use XRD to analyze a wide range of materials, from powders to solids, thin films, and nanomaterials.

Why FIDDLE Matters

FIDDLE can record multiple XRD measurements of phase changes under pressures of more than one million Earth atmospheres, providing important information to help determine the material’s strength, compressibility, thermal conductivity, and equation of state. By providing multiple diffraction measurements over the course of a single shot, XRDt substantially increases the amount of data that can be recorded compared to its predecessor, the TARDIS (target diffraction in situ) platform.

“Our goal is to make ‘movies’ of phase transitions in real time as they occur in dynamic compression experiments,” the XRDt researchers said in a 2024 Review of Scientific Instruments paper describing the FIDDLE diagnostic.

“The capability we are developing to capture multiple measurements during a single compression experiment is transformative,” they said. “It provides a huge increase in data return for each shot, allowing us to map out phase transitions with fewer shots. It also reduces uncertainties that arise from shot-to-shot variability of the target and laser pulse.”

FIDDLE will enable new insights into studies ranging from NIF Discovery Science investigations of the dynamics of star formation, to high energy density (HED) materials research to monitor shock-induced phase transitions, to the ability of NIF inertial confinement fusion (ICF) experiments to support the National Nuclear Security Administration’s science-based Stockpile Stewardship Program. By supplying new data about the properties of critical nuclear weapon components under dynamic compression on a rapid time scale, XRDt experiments will inform advanced computer simulations used to certify the safety and effectiveness of the nation’s nuclear arsenal without the need for underground testing.

“This is another example where Discovery Science and the advancements of platforms necessary to support fundamental science missions connect directly into enabling new stockpile stewardship capabilities,” said NIF Director Gordon Brunton. “That shows how the two programs are symbiotic in the advancement of our missions.”

A Successful Experiment

FIDDLE works by placing five ultrafast Icarus2 hybrid-CMOS (complementary metal-oxide semiconductor) multi-frame sensors, with logic gates (switches) as short as about 2 nanoseconds, within a few centimeters of a laser-driven target. The sensors, which were developed at Sandia National Laboratories and are widely used in NIF ICF experiments, must be protected from the extremely harsh environment in the NIF Target Chamber, including debris, electromagnetic pulses (EMP), and stray laser light. Another key challenge is reducing the x-ray background relative to the faint diffraction signal.

After several years of development and a half-dozen FIDDLE tests with mixed results, a Nov. 26, 2024, experiment designed to trace the phase changes in a sample of lead under a peak pressure of about 690,000 Earth atmospheres obtained the first images showing clear diffraction lines on all four frames of five Icarus2 detectors.

"On the first two shots, no signal was recorded on the sensors; they were disrupted by the EMP” and by the effects of high-energy electrons flowing from the plasma created when the laser irradiates a germanium foil to generate the diffraction x rays, Vennari said. In subsequent experiments, “we were only able to get diffraction data on three time-steps of one and maybe two sensors, because the x-ray background was so high.

“The plasma lets off a lot of hot electrons,” she said, “and those hot electrons fly everywhere, and they come back and hit the sides of the target. That interaction creates secondary x rays that shine in every direction” and create a shadow of the target on the sensors’ image plates.

Modifying the Target

To solve that problem, the Target Fabrication team designed and built a new target aimed at reducing the background “noise” on the sensors by geometrically limiting the x rays generated by the hot electrons.

“In the past,” said LLNL materials scientist Neal Bhandarkar, who led the target development team, “we’ve had (external) EMP shielding, and fluorescent shielding, and hot electron shielding, and bremsstrahlung radiation (x-ray) shielding—many different things that can cause background on the sensors.”

The new target, with a parylene-coated aluminum cap shield, “is designed to be self-shielding,” he said. “It may seem like a small thing, but it’s the most recent success on the march up the mountain.”

Bhandarkar stressed that the recent breakthrough was not due to “just one shot. This was the most recent in a series of about 20 shots over the past five years that have been improving the target little by little. Each shot gets progressively better as we add or remove features and tweak the existing design.

“These are complex designs that require several reviews to verify they pass safety and operations concerns, provide adequate shielding, and are feasible to actually machine and even model,” he said.

The Next Steps

The XRDt researchers plan to conduct more FIDDLE experiments in the 2024-2025 shot cycle using different cap shield materials and other target and experimental design modifications in an effort to further reduce the background noise.

For the longer term, the campaign is developing a next-generation diagnostic called FIDDLE-B, which will replace the current five Icarus sensors with eight even faster Dedalus2 sensors with higher temporal resolution and dynamic range.​ FIDDLE-B will also add an integrated streak camera, have better EMP shielding, and will place the sensors closer to the x-ray beam for better coverage.

With FIDDLE now delivering on its promise, researchers can shift their focus from troubleshooting to discovery. “We want to move on from lead to different materials,” Vennari said, “to understand what happens with the kinetics of other phase transitions” relevant to astrophysics, geology, and other fields.

“Using NIF and having such high pressures is really helpful in studying exoplanets, so we can look at materials that would be good for that,” she said. “And understanding the kinetics of what happens during impact; the moon-forming impact is the classic example for geology. There are a lot of possibilities.

“I feel like we’ve been in the weeds for so long,” Vennari said, “trying to get the diagnostic to work and lowering the background on the sensors. But now we’re finally at a place where we can think about science, which is really exciting.”

Joining Vennari and Bhandarkar on the XRDt campaign are LLNL researchers Nathan Palmer, Peter Nyholm, Saransh Soderlind, Ian Ocampo, Peter Celliers, Amy Coleman, Robert Petre, Arthur Carpenter, Brandon Morioka, Sabrina Nagel, Jon Eggert, David Bradley, Andy MacKinnon, Yuan Ping, Andrew Sharp, Mike Hardy, Nick Durst, Rashid Abdul-Rahman, Chris Cashin, Jeff Baron, Tim Cunningham, and former Lab researcher Robin Benedetti.

LLNL and General Atomics Target Fabrication team members who worked on the design, production and assembly, and modeling, safety, and mechanical engineering for the target were Chuck Heinbockel, Scott Vonhof, Nicholas Hash, Joshuah Ponce, Darrold Ponce, Jean Jensen, Rosita Cheung, Nathan Masters, Gunner Scott, and Justin Buscho.

More Information:

Developing time-resolved x-ray diffraction diagnostics at the National Ignition Facility, Review of Scientific Instruments, September 17, 2024

Electrical design of the flexible imaging diffraction diagnostic for laser experiments (FIDDLE) at the National Ignition Facility (NIF)—Requirements, design, and performance, Review of Scientific Instruments, July 23, 2024

Lab team uses giant lasers to compress iron oxide, revealing the secret interior of rocky exoplanets, LLNL News Release, February 11, 2021

NIF’s TARDIS Featured in Review of Scientific Instruments, NIF & Photon Science News, June 3, 2020

NIF’s TARDIS Aims to Conquer Time and Space, NIF & Photon Science News, December 10, 2014

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