Scientists at the National Ignition Facility, a science institute in Livermore, California, believe they are on the verge of achieving a longstanding goal in nuclear fusion research, namely, to create a fusion reaction that generates more energy than is required to maintain it.
The experiment at the NIG uses a powerful laser to heat and compress hydrogen fuel, initiating fusion. In a process called inertial confinement fusion, 192 beams from NIF's laser – the highest-energy example in the world – are directed towards a peppercorn-sized capsule containing deuterium and tritium.
This compresses the fuel to 100 times the density of lead and heats it to 100 million degrees Celsius – hotter than the centre of the Sun. These conditions help kickstart thermonuclear fusion.
An experiment carried out on 8 August yielded 1.35 MJ of energy, around 70% of the 1.9 MJ laser energy delivered to the fuel capsule. That is a yield eight times greater than NIF's previous record, established in spring 2021, and 25 times the yield from experiments carried out in 2018.
"The pace of improvement in energy output has been rapid, suggesting we may soon reach more energy milestones, such as exceeding the energy input from the lasers used to kick-start the process," commented professor Jeremy Chittenden, co-director of the Centre for Inertial Fusion Studies at Imperial College London.
NIF scientists also believe they have now achieved something called "burning plasma", where the fusion reactions themselves provide the heat for more fusion, making the process self-sustaining.
"Self-sustaining burn is essential to getting high yield," said Dr Debbie Callahan, a physicist at the Lawrence Livermore National Laboratory, which hosts NIF. "The burn wave has to propagate into the high density fuel in order to get a lot of fusion energy out. We believe this experiment is in this regime, although we are still doing analysis and simulations to be sure that we understand the result."
Dr Callahan said that as a next step the experiments would be repeated. "We need to understand how reproducible and how sensitive the results are to small changes," she said. “After that, we do have ideas for how to improve on this design and we will start working on those next year."
Prof Chittenden commented: "The mega-joule of energy released in the experiment is indeed impressive in fusion terms, but in practice this is equivalent to the energy required to boil a kettle. Far higher fusion energies can be achieved through ignition if we can work out how to hold the fuel together for longer, to allow more of it to burn. This will be the next horizon for inertial confinement fusion."
