Researchers at the National Ignition Facility (NIF) have taken another step toward their long-overdue goal of using lasers to attain a self-sustaining nuclear fusion reaction. On 14 June in Physical Review Letters, researchers described experiments that doubled previous records both for neutron yield (now at 1.9 x 1016) and fusion energy output (now at 54 kilojoules) generated from capsules containing cryogenic deuterium–tritium fusion fuel. The progress was due largely to changes to both the capsules and the hohlraums—cylinders in which the capsules are suspended—that helped maintain the symmetry of the fuel’s implosion.
“This is a critical step forward in the quest for achieving laboratory ignition,” says Richard Petrasso, a senior scientist with MIT’s Plasma Science and Fusion Center who was not involved in the research. Ignition—the point at which the heating provided by nuclei in the plasma exceeds the heat lost from the system—has been pursued primarily for its relevance to nuclear weapons physics, since it would mimic on a laboratory scale the dynamics in a weapon’s fusion stages. Laser fusion is also considered a possible future energy source.
When NIF was completed at Lawrence Livermore National Laboratory in 2009 at a cost of $3.5 billion, researchers were confident they would attain ignition by 2012. But hydrodynamic instabilities and laser–plasma interactions that weren’t foreseen in computer simulations have kept experimenters from achieving the uniform implosions that are required for the reaction. Since that time, slow but steady improvements have been made by adjusting various parameters in hohlraums and capsules and by fine-tuning the timing of laser pulses. Hohlraums convert the laser’s UV light to x rays that drive capsule implosion.
In the latest round of experiments, the capsule shells consisted of diamond doped with a thin layer of tungsten, and the hohlraums were made of depleted uranium. Earlier experiments had used plastic shells and gold hohlraums. Sebastien Le Pape, lead author of the paper, says the uranium hohlraum boosted the peak energy deposited on the capsule by 25 terawatts, for a total of about 450 TW.
The D–T fusion reaction produces neutrons and alpha particles, or helium nuclei. Though the neutrons can’t be contained, trapping enough alpha particles in a compressed plasma will add heat and thereby boost the fusion yield.
Omar Hurricane, the chief scientist for inertial confinement fusion at Livermore, says the experiments put NIF “at the threshold of achieving a burning plasma state,” in which alpha-particle deposition surpasses compression to become the main source of plasma heating. The latest shots have achieved 360 gigabars of pressure—exceeding that at the center of the Sun—which is around 70% of what’s needed for ignition, says E. Michael Campbell, the director of the University of Rochester’s Laboratory for Laser Energetics who was not involved in the NIF experiments. He says about 10 times as much alpha heating will be needed.
“Not too many more doublings” will be required for ignition, Hurricane says, although he and Le Pape caution that scaling from the results isn’t linear. Ignition won’t be indicated simply by having surpassed some number of neutrons or other single benchmark. Researchers use the Lawson criterion, which defines the minimum values for plasma temperature, confinement time, and density in determining the conditions at which ignition will occur.
Hurricane says the ignition program continues its quest to improve both the quality and the scale of implosions. Scaling up didn’t make sense until now because implosion symmetry couldn’t be controlled. Over the past two years, scientists were able to discern a “relatively simple” underlying pattern in symmetry control that should allow larger implosions. Further advances will unfold over the next year or two, he says. “Things don’t go quickly in this business.”
Whether NIF’s 1.8 megajoule peak energy capacity is sufficient to achieve ignition remains unknown.
The new results were achieved before the Trump administration proposed cutting funding for NIF by $57 million, to $287 million in fiscal year 2019. That would have forced a 30% reduction in the number of experimental shots of the 192-beam laser. Lawmakers instead added to NIF’s funding next year: The final appropriation will likely end up between the $330 million included in a House-passed bill and $344 million included in a Senate measure.
