Nature Physics Paper Describes Burning Plasma Target and Laser Designs

One of the last remaining milestones in fusion research before attaining ignition and self-sustaining energy production is creating a burning plasma. In this state, the fusion reactions themselves become the dominant source of heating in the plasma, but do not yet overcome all mechanisms of energy loss.

In a paper featured in the Jan. 26 issue of Nature Physics, LLNL researchers and their collaborators described the experimental designs that achieved the first burning plasma state on NIF, the world’s highest-energy laser. The inertial confinement fusion (ICF) experiments, in November 2020 and February 2021, led directly to NIF’s record-breaking 1.3-megajoule (MJ) shot on Aug. 8, 2021.

“In these experiments,” said LLNL physicist Annie Kritcher, lead author of the Nature Physics paper along with Chris Young, “we achieved, for the first time in any fusion research facility, a burning plasma state where more fusion energy is emitted from the fuel than was required to initiate the fusion reactions, or the amount of work done on the fuel.”

This was achieved through an energy feedback process called self-heating, in which the plasma created by a high-velocity implosion of a NIF target capsule heats itself (see “How NIF Targets Work”). When the energy from self-heating exceeds the energy that was injected to initiate the fusion reactions, the plasma enters a burning plasma state.

Creating this new energy regime in a controlled laboratory setting is of great scientific importance for fundamental fusion research as well as for the National Nuclear Security Administration’s science-based Stockpile Stewardship Program.

The work was also the basis for NIF’s milestone achievement of reaching 1.35 MJ of fusion energy, and it validates the work done decades ago to establish the power and energy specifications for NIF.

The ICF designs enabled the achievement of burning plasma by developing more efficient ways to drive larger-scale fusion targets to the same extreme conditions required for significant fusion to occur within the current experimental confines of NIF. By increasing the scale while maintaining high levels of plasma pressure, the team was able to ultimately deliver more of the initial laser energy directly to the fusion plasma and jump-start the burn process.

In doing so, the team found novel ways to control implosion symmetry by transferring energy between laser beams in a new way and by changing the target geometry. The designs were generated and optimized using a combination of theory, computational modeling by LLNL’s HYDRA multi-physics radiation-hydrodynamics code, and semi-empirical models informed by experimental data.

“We learned where we could and could not trust the modeling and when to rely on semi-empirical models,” Kritcher said. “We also found that keeping the driver pressure up longer (with a longer laser pulse) relative to the time it takes the target to implode was important for maintaining a high plasma pressure.

“Without this pressure, and enough energy coupled to the hot dense plasma, we would not reach the extreme conditions required for significant fusion.”

Seeking Solutions

The design work supporting these ICF platforms began several years ago. The teams first recognized technical issues limiting performance, explored many different options to solve these technical problems, presented choices to the broader team, and down-selected.

Designs were updated continuously with data from tuning experiments and ultimately put to the test in high-performance experiments with fusion fuel.

These designs are typically generated by a lead designer with input from the broader interdisciplinary team including designers, experimentalists, theorists, and facility experts (laser and target). Each new iteration builds on decades of previous work, solving an existing problem or incorporating a new idea that advances fusion performance to a new level.

Through the design changes outlined in the paper, researchers were able to increase the size of the target and still maintain the same levels of extreme plasma pressure that were achieved at smaller scale, all while operating at the maximum available NIF energy.

“There is much work yet to be done and this is a very exciting time for fusion research,” Kritcher said. “Following this work, the team further improved hohlraum efficiency in both platforms, increasing hot spot pressure which resulted in higher performance and the record 1.35-MJ Hybrid-E experiment.”

Kritcher explained that this new platform is now the “base camp” for a significant fraction of ongoing programmatic work, focusing on understanding the sensitivity of this new regime, improving robustness of the platform, and further increasing the energy and pressure of the fusion hot spot (see “Climbing the Mountain of Fusion Ignition”).

“This will be explored through a variety of ideas to increase fuel compression and energy coupling,” she said.

A companion paper by lead authors Alex Zylstra and Omar Hurricane was published in Nature on Jan. 26. The paper, “Burning plasma achieved in inertial fusion,” chronicles the research that produced a burning plasma regime for the first time in a laboratory experiment. An analysis of the achievement, “Self-heating plasmas offer hope for energy from fusion,” appears in the same issue of Nature.

More Information;

“Achieving record hot spot energies with large HDC implosions on NIF in HYBRID-E,” Physics of Plasmas, July 21, 2021

“Record Energetics for an Inertial Fusion Implosion at NIF,” Physical Review Letters, January 11, 2021

“Threshold of Ignition,” NIF & Photon Science News

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