物理学：核融合炉にとって火急の問題

Self-heating of matter in a plasma state through nuclear fusion, a milestone towards fusion energy potentially becoming a viable energy source, is reported in Nature this week. An accompanying paper in Nature Physics describes the optimization of the experimental design that allowed such an achievement.

Nuclear fusion, which involves combining atomic nuclei to release energy, has the potential to be a sustainable energy source. It is a physical process that powers stars, but recreating this process in the laboratory has proven challenging, and uses far more energy than it produces. A critical step towards fusion that is a net generator of energy is a burning plasma, in which nuclear fusion is the primary source of heat for sustaining the fuel in a plasma state that is hot enough to allow further fusion reactions. Such a regime is reported by Alex Zylstra and colleagues in inertial-confinement fusion experiments, in which fusion reactions are initiated by compressing and heating capsules filled with thermonuclear fuel.

Experiments at the National Ignition Facility achieved the burning of plasma using 192 laser beams to quickly heat and implode a capsule containing 200-µg of deuterium–tritium fuel, reaching temperatures and pressures high enough to trigger self-heating fusion reactions. Previous attempts have been limited by challenges in controlling the plasma shape to avoid disrupting how the laser beams deposit energy in the plasma, but Zylstra and colleagues’ improved experimental design allowed the use of capsules that can hold more fuel and absorb more energy while containing the plasma. The performance generated by these experiments — up to a maximum yield of 170 kilojoules of energy — triples the yields obtained in previous experiments.