One of the last remaining milestones in fusion research before attaining ignition and self-sustaining energy production is creating a burning plasma, where the fusion reactions themselves are the primary source of heating in the plasma. A paper published in the journal Nature on January 26 describes recent experiments at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) that have achieved a burning plasma state.
“In these experiments, 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,” said LLNL physicist Annie Kritcher, who, with Chris Young, served as one of the lead authors of the Nature paper, “Design of inertial fusion implosions reaching the burning plasma regime.”
The work described in the paper was also the basis for the August 2021 NIF experiment that achieved 1.35 megajoules in the laboratory for the first time. According to LLNL, this accomplishment also validates the work done decades ago to establish the power and energy specifications for NIF.
Scaling up: The work outlined in the paper presents the inertial confinement fusion designs that enabled the achievement of laboratory burning plasma by developing more efficient ways to drive larger-scale fusion targets to the same extreme conditions required for significant fusion to occur, and 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 the 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 (HYDRA), 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 (i.e., 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.”
Future work: “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 said that this new platform is now the “basecamp” for a significant fraction of ongoing programmatic work, focusing on understanding the sensitivity of this new regime, improving the robustness of the platform, and further increasing the energy and pressure of the fusion hot spot. “This will be explored through a variety of ideas to increase fuel compression and energy coupling,” she said.
Nature also featured a paper titled “Burning plasma achieved in inertial fusion,” with LLNL physicists Alex Zylstra and Omar Hurricane serving as the lead authors.