Nov. 6, 2024

LLNL's Scientific Expertise Highlighted at Plasma Physics Meeting

By Charlie Osolin

The breadth and depth of scientific research conducted at Lawrence Livermore National Laboratory (LLNL) was on full display at the 2024 meeting of the American Physical Society’s Division of Plasma Physics (APS DPP), held Oct. 7-11 in Atlanta.

Dozens of LLNL researchers were on hand to describe the Laboratory’s progress and goals, from performance advancements on the National Ignition Facility (NIF), to the Lab’s key role in laying the groundwork for inertial fusion energy (IFE) by achieving ignition on NIF, to the wide range of fundamental science explored in the NIF Discovery Science program.

The APS DPP meeting is an annual opportunity for researchers to meet, network, and compare notes about the latest developments in their disciplines.

“I first attended APS-DPP in 2009 when I was an LLNL postdoc” in the NIF & Photon Science Directorate, said Félicie Albert, director of the Lab’s Jupiter Laser Facility (JLF), who gave the meeting’s opening plenary presentation.

“Fifteen years later,” she said, “it was a huge honor to represent the LaserNetUS network, its facilities—which include JLF—and the amazing science our users have done.”

LaserNetUS is a collaborative network of cutting-edge laser facilities established in 2018 by the U.S. Department of Energy’s Office of Fusion Energy Sciences.

Felicie Albert Presents at APS DPP Meeting — Félicie Albert gives the opening plenary presentation at the 2024 American Physical Society Division of Plasma Physics conference. Albert was elected vice chair of the division at the meeting. Credit: Lan Gao, Princeton Plasma Physics Laboratory

Albert, who served as chair of LaserNetUS from 2020 to 2022, said the five-year-old community now has more than 1,300 users and “has established itself as a vital part of the scientific landscape, offering researchers access to world-class laser facilities and enabling discoveries that push the boundaries of plasma physics."

LaserNetUS “has been a wonderful story of amazing scientific discovery and community building,” she added. “LaserNetUS is not the work or initiative of a single person or institution; it does take a village to grow the network and the community.”

Boosting NIF’s Performance

LLNL has achieved fusion ignition at NIF five times since 2022, producing a record high yield of 5.2 megajoules (MJ) of fusion energy from 2.2 MJ of laser energy delivered to the target—a “target gain,” the ratio of fusion yield compared to laser energy, of about 2.4. These experiments support the National Nuclear Security Administration’s science-based Stockpile Stewardship Program, which helps ensure the safety and effectiveness of the nation’s nuclear deterrent.

The APS DPP conference featured invited talks by LLNL’s Matthias Hohenberger and Chris Young describing current efforts to further increase NIF’s target gain to factors of more than 10.

Hohenberger’s talk, “First demonstration of improved yield with increased compression in high-performance implosions on the National Ignition Facility,” focused on efforts to increase fusion yields by reducing the adiabat (resistance to compression) in NIF inertial confinement fusion (ICF) implosions. He described recent focused experiments to explore the impact of reduced adiabat and increased compression using an optimized shock timing via small changes to the laser pulse.

“Compared to the baseline experiment, the first NIF shot to reach more than one MJ of yield, these experiments demonstrated about 10 percent increased compression and about 80 percent increased yield,” Hohenberger said.

“This design is the only platform to have achieved ignition and gain greater than one with a driver energy of less than two MJ. "Notably, this is the first time a design has responded with increased performance to a reduced adiabat at the NIF.”

Young’s presentation, “Improving target gain in inertial confinement fusion implosions on NIF with increased energy coupling and areal density,” outlined the research that led to NIF’s record 5.2 MJ shot and described ongoing studies aimed at increasing the confinement (areal density) in an ICF implosion to improve burn efficiency of the deuterium-tritium fuel and further increase fusion performance.

“One key method for exploiting higher energy coupling to boost areal density,” Young said, “is to increase the stagnated mass by starting with a thicker ablator (target capsule). Doing so requires a longer laser pulse to maintain optimized shock timing and renders implosion symmetry control more challenging.”

Young then described “new mitigations to control symmetry swings by carefully balancing power across the various NIF beams. Detuning the laser wavelengths to drive cross-beam energy transfer (also) remains an important technique for maintaining symmetry control,” he said.

Planning for Fusion Energy

The meeting also featured a one-day mini-conference organized by LLNL’s Veronika Kruse titled, “Progress in Making IFE-based Concepts a Reality.” The mini-conference “sought to bring together the members of the inertial fusion energy community to provide updates on the progress towards addressing science and technology questions relevant to the eventual commercialization of IFE,” Kruse said.

“The two sessions featured DOE representatives, national lab scientists, university partners, and private-sector employees, and drew dozens of attendees interested in the progress since the historic ignition results almost two years ago at NIF.

“With the recent demonstration of fusion ignition in the laboratory, a path to an IFE power plant has been laid,” she said. “While IFE research continues, this path to commercialization requires overcoming many scientific, engineering, and economic hurdles.”

Those hurdles include “laser-plasma instabilities in broadband laser technologies, developing a workforce skilled in laser-plasma science, and continued investment to resolve R&D gaps, support the domestic supply chain, and develop next-generation facilities.”

The mini-conference included talks by Tammy Ma, lead for the LLNL inertial fusion energy institutional initiative, on “Accelerating the path to realizing Inertial Fusion Energy via an integrated national plan,” and by Lab computational scientist Bogdan Kustowski, on “Optimizing the IFE design for a fusion pilot plant.”

Ma noted that IFE was a major topic of discussion at the conference.

“It was exciting to see the increased emphasis on IFE science and technology at this year’s APS DPP,” she said. “There were several mini-conferences on fusion energy topics that were very well attended and highlighted advances in that space over the past year.”

Partnerships Are Key

In her talk, Ma underscored the importance of partnerships between national laboratories, universities, the private sector, and multiple government agencies in “building the IFE ecosystem and accelerating IFE science and technology. Under the IFE-Science and Technology Accelerated Research (STAR) Hubs,” she said, “we are developing a common national IFE strategic plan that supports all major IFE approaches.”

Ma said this integrated national plan will lay out the guideposts, milestones, and nominal timeline for the key IFE technologies that pave the way toward a first IFE pilot plant on a timeframe consistent with the U.S. government’s Bold Decadal Vision for Commercial Fusion Energy.

“This will help industrial partners, university researchers, and national laboratory scientists understand the common directions within IFE and most effectively use their resources to make rapid progress,” she said.

To investigate designs for a fusion pilot plant, Kustowski said, researchers plan to use radiation-hydrodynamic simulations to “find a design that not only produces high yield, but also does so robustly, with little sensitivity to the uncontrolled variability in the laser delivery and target quality.”

He said the researchers “will leverage an iterative optimization workflow developed by LLNL’s ICECap team, which involves running integrated laser-hohlraum-capsule simulations in batches, building a machine-learning surrogate model of these simulations, and in-line proposing new samples of the design inputs automatically.”

According to the simulations, Kustowski said, the optimal pilot-plant design using ICF would exceed 50 MJ of output energy. Assuming a target gain of 10, a 10-Hertz (shot repetition) rate, and 10 percent efficiency, this would produce 50 megawatts of electricity, the output recommended for a pilot plant in a 2021 report, Bringing Fusion to the U.S. Grid, by the National Academies of Sciences, Engineering, and Medicine.

Other speakers at the IFE conference included leaders from IFE-STAR, including the Laboratory for Laser Energetics at the University of Rochester and Colorado State University.

New APS Fellows Honored

New APS Fellows Receive Award Certificates — LLNL’s **Dan Casey** (third from left) and **Dan Clark** (far right) received award certificates designating them as American Physical Society Fellows at the APS DPP awards banquet. Not pictured is **Raymond Smith**, who was also named an APS Fellow.

Frontier Science on NIF

A talk by Bruce Remington, director of the NIF Discovery Science Program, presented a selection of results of the program to date, including nuclear reactions relevant to stellar nucleosynthesis and equations of state at very high pressures relevant to white dwarf envelopes, planetary cores, and brown dwarf interiors.

“With the NIF Discovery Science program, we have studied laboratory astrophysics over a wide range of extreme conditions, including the properties and dynamics of a wide range of astrophysical objects and phenomena,” Remington said.

Users compete for experimental time on NIF through the Discovery Science program, which allots about eight percent of the available experimental time to study such phenomena as the Tycho supernova, supernova shock waves, and exoplanet interiors.

Gigabars of Pressure

NIF’s gigabar platform uses an intense x-ray drive incident on a solid sphere of plastic or beryllium to drive the material to exceedingly high pressures (about one billion Earth atmospheres) and densities. It enables precise measurements of the characteristics of warm dense matter (WDM), a state of matter exhibiting properties of both solids and plasmas.WDM is present in the interior of planets, exoplanets, and brown dwarfs and in some ICF experiments.

Sophisticated x-ray diagnostics are available to measure a material sample’s density, electron temperature, and ionization state in a single experiment, enabling high-precision equation-of-state measurements.

Experiments on NIF, Remington said, can probe matter at regimes relevant to the interior of the sun and the red dwarf star Proxima Centauri, the white dwarf envelope of Sirius B, the interior of the brown dwarf Gliese 229b, and the deep interior of giant planets such as Jupiter. He said upcoming experimental campaigns hope to use the gigabar platform to achieve even higher pressures—greater than 50 Gbar—as predicted by radiation hydrodynamics simulations. These could test the dense plasma properties relevant to astrophysical bodies such as brown dwarfs, where peak pressures in the deep interior are predicted to reach 40 Gbar and higher.

Collisionless Astrophysical Shocks

In a “vastly different regime,” Remington said, remnants from a supernova explosion expand into interstellar space, where the densities are in the range of one to 100 atoms per cubic centimeter. One example is the remnant from the Tycho supernova, SN-1572.

“At such low densities in interstellar space,” he said, “one would normally assume that shock waves could not exist, but in fact they do, as is clearly evident at the leading edge of the remnant from SN-1572. These are called collisionless shocks.”

Experiment Studies Collisionless Shocks — (Left) The Tycho supernova exploded in the year 1572 and has been evolving as a supernova remnant (SNR) ever since. A distinct feature of this SNR is the collisionless shock at its perimeter. Credit: MPIA/NASA/Calar Alto Observatory

(Right) Artist’s rendition of the NIF experimental setup, showing how researchers simulated the shock waves in a supernova remnant by firing NIF’s lasers at two carbon targets set 25 millimeters apart, sending two plasma flows into each other (see “Electron acceleration in laboratory-produced turbulent collisionless shocks”). The shock density structure, shown in blue, was obtained from numerical simulation. Credit: Greg Stewart/SLAC National Accelerator Laboratory

To better understand these astrophysical shocks, a collisionless shock experimental platform was developed on NIF through the Discovery Science program.

“The Tycho supernova remnant is about 8,000 to 9,800 light years from Earth, and its diameter is 15 to 30 light years across,” Remington said. “The image suggests that the Tycho remnant appears quite turbulent in its interior.  The source of this turbulence is still a topic of debate: was it seeded by the explosion itself, or did it arise from the subsequent expansion and evolution?”

Invited Talks

Along with Hohenberger and Young, LLNL researchers presenting invited talks were:

• Dan Casey, “Physics and DT implosion results from the high-Trad hohlraum, thick-ablator, (‘HiT’) experimental effort”

• Joshua Ludwig, “A Laser-Based 100 GeV Electron Plasma Accelerator to Enable Compact Muon Sources”

• Tom Chapman, “Past, current, and potential future measurements of backscatter from hohlraum targets at the National Ignition Facility”

• Hui Chen, “Key advancements towards eliminating the 'drive-deficit' in ICF hohlraum simulations”

• Isabella Pagano from the University of Texas at Austin, who is doing her LLNL-funded Ph.D. research in residence at the Laboratory, “Tomographic reconstruction and x-ray phase contrast imaging of inertial confinement fusion capsules with betatron x-ray radiation from a laser wakefield accelerator”

• Seth Davidovits, “Assessing explanations for unexpected fuel-ablator mixing measurements in HDC implosions at the NIF”

• Drew Higginson, “Evidence of multi-species ion effects in National Ignition Facility hohlraums using a DT gas-filled platform”

• Clément Goyon, “Improved current scaling and neutron yield for MJ-Class dense plasma focus (DPF) via simulation-guided design”

• Min Sang Cho, “Surprises in the Ionization Dynamics of Intense Laser-Produced Plasmas Revealed by NLTE Modeling”

• Ivan Novikau, “Quantum algorithms for simulating dissipative linear and nonlinear dynamics of plasmas”