The challenge
The mechanism that can cause a rapidly expanding plasma — the superhot state of matter harnessed in fusion energy systems — to spontaneously generate its own magnetic fields was identified through a new set of simulations. This improves our understanding of naturally occurring plasmas in our universe and advances the development of fusion systems based on an approach called direct-drive inertial fusion.
In a direct-drive inertial fusion system, powerful lasers compress a small, fuel-filled capsule, heating it until fusion reactions occur. Unexpected magnetic fields can change how heat moves through the plasma in ways that existing simulation tools can miss. Accurate simulations are critical to designing fusion systems that will behave as expected and deliver net energy on a long-term basis.
In laboratory experiments, researchers found that high-powered lasers can vaporize a solid target in an instant, turning it into plasma that rapidly expands. Experiments have repeatedly detected very strong magnetic structures emerging from this expanding plasma, but the precise origin of these fields has long been a matter of debate. This new research, published in Physical Review Letters, sheds light on how those fields form and could change plasma simulations for fusion experiments.
The science
Using computer simulations, the research team tracked plasma behavior as a high-powered laser struck an aluminum target. When the laser intensity exceeded a particular threshold, the expanding plasma self-magnetized within a billionth of a second and generated magnetic fields as strong as 40 tesla. That's roughly one million times stronger than the Earth's magnetic field. Below that intensity threshold, however, the plasma remains largely unmagnetized.
The cause is two competing processes. As laser-heated plasma expands outward, it cools faster along the direction of expansion than in the perpendicular directions, creating a temperature imbalance. That imbalance acts as the engine for a phenomenon, known as the Weibel instability, that generates the magnetic fields. However, collisions between particles nudge the plasma back toward a balanced state. At higher laser intensities, the temperature imbalance is large enough to trigger the magnetic fields.
"The uniqueness of our work is that we show that even if the laser drive is very uniform, just by virtue of expansion, plasma can still generate magnetic fields," said Kirill Lezhnin, an associate research physicist at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) and the study's lead author. "These fields could change the behavior of the system."
Once the magnetic fields emerge, they fundamentally change the evolution of the plasma: The fields trap electrons in spinning orbits, ultimately suppressing heat flow away from the region where the laser strikes the target. Simulations demonstrated that the magnetic effects are large enough to influence the overall behavior and temperature of the plasma.
To make the finding immediately useful to other researchers, the team derived a simple threshold criterion that can be used to predict plasma magnetization for given laser and target parameters.
"The threshold turns out to be somewhat smaller than I would have expected," Lezhnin said. "It falls right around the typical intensity for common inertial fusion experiments, which makes these magnetic field effects very relevant to that research."
The team
The research team included Lezhnin and Ahmed Diallo of PPPL; Samuel Totorica, Jesse Griff-McMahon and Huws Landsberger of Princeton University; Mikhail Medvedev of the University of Kansas and the Massachusetts Institute of Technology; and Will Fox of the University of Maryland and PPPL.
The funding
This work was supported by DOE under contract number DE-AC02-09CH11466 and by the Laboratory Directed Research and Development Program of PPPL. Griff-McMahon received support from the National Science Foundation under grant number 2039656. Medvedev received support from the National Science Foundation under grant PHY-2409249. Simulations were performed on computational resources managed and supported by Princeton Research Computing at Princeton University.
About PPPL
PPPL is mastering the art of using plasma — the fourth state of matter — to solve some of the world's toughest science and technology challenges. Nestled on Princeton University's Forrestal Campus in Plainsboro, New Jersey, our research ignites innovation in a range of applications, including fusion energy, nanoscale fabrication, quantum materials and devices, and sustainability science. The University manages the Laboratory for the U.S. Department of Energy's Office of Science, which is the nation's single largest supporter of basic research in the physical sciences. Feel the heat at https://energy.gov/science and https://www.pppl.gov.
