Fuel gain exceeding unity in an inertially confined fusion implosion

Fusion fuel gains greater than unity — which are crucial to the generation of fusion energy — are achieved on the US National Ignition Facility using the ‘high-foot’ implosion method, which reduces instability in the implosion of the fuel. Fusion shows a return Efforts to develop fusion as a viable alternative energy source continue but progress has been slow. In the context of inertial confinement fusion, in which a fuel target is compressed and heated to initiate nuclear fusion, a key experimental goal is to reach a stage where the amount of energy deposited into the fuel during the compression/heating process is exceeded by the amount of energy generated by the induced fusion reactions. This threshold — the attainment of a 'fuel gain' that is greater than one — has now been reached at the National Ignition Facility in Livermore, California. They used 192 laser beams to heat and compress a fuel pellet to the point at which nuclear fusion reactions take place and obtained a yield 10 times greater than previously achieved. Further advances will be required, however, before the fusion energy yield exceeds the total energy required to compress the fuel pellet. Ignition is needed to make fusion energy a viable alternative energy source, but has yet to be achieved 1 . A key step on the way to ignition is to have the energy generated through fusion reactions in an inertially confined fusion plasma exceed the amount of energy deposited into the deuterium–tritium fusion fuel and hotspot during the implosion process, resulting in a fuel gain greater than unity. Here we report the achievement of fusion fuel gains exceeding unity on the US National Ignition Facility using a ‘high-foot’ implosion method 2 , 3 , which is a manipulation of the laser pulse shape in a way that reduces instability in the implosion. These experiments show an order-of-magnitude improvement in yield performance over past deuterium–tritium implosion experiments. We also see a significant contribution to the yield from α-particle self-heating and evidence for the ‘bootstrapping’ required to accelerate the deuterium–tritium fusion burn to eventually ‘run away’ and ignite.