Physicists working at a laser-fusion facility in the US have announced a world first – the generation of more energy from a controlled nuclear fusion reaction than was needed to power the reaction. They achieved this using the $3.5bn National Ignition Facility (NIF) – a football-stadium sized system of lasers based at the at the Lawrence Livermore National Laboratory (LLNL) in California. The laser shot, performed on 5 December, released 3.15 million joules (MJ) of energy from a tiny pellet containing two hydrogen isotopes – compared to the 2.05 MJ that those lasers delivered to the target.

Speaking yesterday at a press conference in Washington DC organized by the Department of Energy to announce the achievement, Mark Herrmann, head of weapons physics and design at LLNL, noted that the breakthrough has a dual importance. While more immediately it should improve the US’s ability to monitor its stockpile of nuclear weapons without testing – NIF’s primary objective – it could, in the longer term, lead to a new clean, sustainable form of energy. The result, he said, had left his colleagues “really pumped”.

For Michael Campbell at the University of Rochester in the US, the surpassing of “energy breakeven” – a goal of scientists for decades – constitutes a “Wright brothers moment” for fusion research. Steven Rose of Imperial College London argues that the result “shows conclusively that inertial fusion works at the megajoule scale”.

‘Something big’

NIF triggers fusion reactions by aiming nearly 200 high-powered laser beams at the inside of a 1 cm-long hollow metal cylinder. The intense X-rays generated in the process converge on a 2 mm-diameter spherical capsule placed in the middle of the cylinder that contains deuterium and tritium. As the outer portion of the capsule is blasted off, the deuterium and tritium are forced inwards and for a brief moment experience enormous pressures and temperatures – high enough that the nuclei overcome their mutual repulsion and fuse, yielding heat, helium nuclei and neutrons.

Having switched NIF on in 2009, researchers originally envisaged achieving breakeven (or “ignition”, as the milestone is often referred to) three years later. But problems caused by instabilities in the plasma generated during fusion and asymmetries in the capsule implosions limited the facility’s fusion output.

It’s been a 10-year slog of problem solving in-steps to get to this point

Omar Hurricane

It took until early 2021 for scientists to understand the implosions sufficiently that they could create a “burning plasma” and generate more heat from the helium nuclei than was supplied by the laser. Then later that year they finally obtained a self-sustaining fusion reaction in which the generated heat outflanked losses due to cooling – achieving an energy yield of 1.37 MJ.

LLNL physicist Annie Kritcher says that the latest result was achieved by slightly increasing the laser energy – some 8% up compared to the 1.92 MJ employed last year – while making the capsules a bit thicker, and so slightly more resilient to defects. In addition, they improved implosion symmetry by transferring energy between laser beams during the fusion process.

Kritcher’s colleague Alex Zylstra noted that the record-breaking shot was made at just after 1am local time on 5 December. The shot generated copious amounts of neutrons, suggesting that “something big had happened”, as lab director Kim Budil put it. Nevertheless, adds Budil, plenty of other measurements were carried out to confirm the unprecedented haul, with a team of independent experts being brought in to peer-review the results before they were announced yesterday.

Decade long ‘slog’

According to Omar Hurricane, chief scientist of Livermore’s fusion programme, there was no doubt that breakeven would be achieved given the observation of a burning plasma a couple of years ago. The only question for him was exactly when the landmark would occur. “It’s been a 10-year slog of problem solving in-steps to get to this point,” he told Physics World. “Ten years feels long but in reality I think it’s a relatively short time for such a hard scientific challenge.”

National Ignition Facility’s laser-fusion milestone ignites debate

As to where the latest result leaves inertial fusion compared to a rival scheme that relies on magnets to contain plasma for relatively long periods of time (as will be exploited at ITER in France), Livermore’s Tammy Ma says that both approaches have their “pros and cons”. While magnetic confinement has yet to achieve breakeven, she says it is more advanced when it comes to technology development. Indeed, she points out that NIF was not designed to demonstrate practical fusion energy – consuming as it does some 300 MJ of electricity for each 2 MJ laser shot.

Both Ma and Campbell believe there is plenty of scope for improvement. Whereas NIF’s 1990s-era technology is only 0.5% efficient, Campbell says that modern lasers can get as high as 20%. When combined with further improvements to the energy gain on the target, he maintains that inertial fusion could become a commercial reality. But he reckons that point is still likely decades away with “many challenges” first being needed to be overcome.

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• Source: https://physicsworld.com/a/national-ignition-facility-demonstrates-net-fusion-energy-gain-in-world-first/