NIF's 2021 Fusion Shot: What the 1.3 Megajoule Result Meant

Originally published 2021-12-06.

In 2021, scientists at Lawrence Livermore National Laboratory (LLNL) in California reported that a single laser shot at the National Ignition Facility (NIF) had produced more than 1.3 megajoules of fusion energy, more than the energy absorbed by the tiny fuel capsule itself. Headlines called it fusion's first "net energy" result, but that needs care. The shot released about 70% of the laser energy fired at the target, so it was not yet a gain overall. True scientific breakeven came a year later, on 5 December 2022. Here is what happened, why the energy comparison matters, and how far the technology still is from powering homes.

The August 2021 shot in numbers

The experiment took place on 8 August 2021, and LLNL announced initial results ten days later. Researchers then presented the findings at the 63rd Annual Meeting of the American Physical Society's Division of Plasma Physics that autumn. The key figures were:

• Yield: more than 1.3 MJ of fusion energy, about eight times the best results from spring 2021 and 25 times NIF's 2018 record.
• Laser input: roughly 1.9 MJ of laser energy delivered into the target.
• Power: more than 10 quadrillion watts of fusion power, lasting about 100 trillionths of a second.
• Hot spot: a burning core of fuel roughly the width of a human hair.

LLNL director Kim Budil called it "a historic step forward for inertial confinement fusion research". Peer-reviewed papers published in August 2022 in Physical Review Letters and Physical Review E confirmed that the shot had exceeded the Lawson criterion for ignition, the threshold at which a fusing plasma heats itself faster than it loses energy.

Which energy is being compared?

Much of the confusion around fusion "breakthroughs" comes from different starting points for the energy count. There are at least three:

1. Energy absorbed by the fuel. Only a small fraction of the laser energy ends up in the fuel itself. In 2021, the fusion yield beat this figure, which is why some reports said the reaction produced more energy than the fuel absorbed.
2. Laser energy delivered to the target. This is the usual definition of scientific breakeven, or target gain. In 2021, 1.3 MJ out versus about 1.9 MJ in meant a ratio of about 0.7, so this milestone had not been reached.
3. Electricity drawn by the whole facility (the "wall plug"). Powering NIF's lasers takes hundreds of megajoules from the grid for each shot. By that measure, every experiment so far has used far more energy than it produced.

So the 2021 result was a genuine scientific landmark, the first ignited plasma in a laboratory, but it was not net energy in any sense that would matter for a power station.

How NIF makes fusion happen

NIF uses an approach called inertial confinement fusion. Its 192 laser beams, housed in a building the size of three football fields, fire into a small gold cylinder called a hohlraum. The hohlraum converts the laser light into a bath of X-rays. Those X-rays hit a peppercorn-sized capsule containing the hydrogen isotopes deuterium and tritium, blasting away its outer layer so the rest implodes inward at enormous speed.

For a split second, the fuel at the centre is squeezed and heated to conditions hotter and denser than the core of the Sun. The nuclei fuse into helium, releasing a neutron and energy. The helium nuclei, known as alpha particles, deposit their energy back into the fuel. LLNL chief scientist Omar Hurricane described the goal as a regime where "alpha-particle self-heating outstrips all the cooling mechanisms". In August 2021, that happened for the first time, if only for about a hundred trillionths of a second.

This method of driving the capsule with X-rays rather than shining the lasers straight on it is called indirect drive. It is precise and well suited to research, but it wastes much of the laser energy along the way.

Ignition a year later, and the records since

On 5 December 2022, NIF delivered 2.05 MJ of laser energy to a target and got 3.15 MJ of fusion energy back, a gain of about 1.5. It was the first time any laboratory fusion experiment released more energy than the laser energy that drove it. The US Department of Energy announced the result at a live-streamed event a week later.

The facility has since repeated and improved on that result:

• July 2023: 3.88 MJ, a second breakeven.
• February 2024: 5.2 MJ from 2.2 MJ of laser energy.
• 2025: shots reaching around 8.6 MJ.

Each step shows that ignition can be repeated rather than being a lucky one-off, and that yields rise as targets and laser pulses are refined.

Why NIF exists, and why it is not a power plant

NIF was built mainly for the US Stockpile Stewardship programme. Since the United States stopped explosive nuclear testing in 1992, laboratories have used experiments like these to study matter under conditions similar to those inside a nuclear weapon, and to check the computer models used to certify the stockpile. Energy research is an important but secondary goal.

Turning laser fusion into electricity would require several leaps:

• Much higher gain. A power plant would need target gains many times larger than today's, to cover the lasers' electricity use and the losses in converting heat into power.
• Efficient lasers. NIF's lasers date from 1990s technology and are very inefficient. Newer laser designs convert electricity into light far better.
• Speed. NIF fires a full-energy shot only a handful of times a day. A power plant would need several shots per second, with cheap, mass-produced targets.
• Materials. Reactor walls would have to survive constant bombardment by high-energy neutrons.

Magnetic confinement machines such as tokamaks, including the international ITER project in France, pursue the same goal by a different route, holding hot plasma with powerful magnetic fields for long periods instead of crushing tiny capsules.

Fusion vs fission: common myths

"Fusion is a hypothetical energy source." Not quite. Fusion itself is well understood and powers every star; it has been achieved on Earth for decades. What remains unproven is a fusion power plant that delivers more electricity than it uses, reliably and affordably.

"Fusion produces no radioactive material." It produces far less long-lived waste than today's fission reactors, and there is no risk of a runaway chain reaction. However, tritium fuel is radioactive, and the neutrons released make reactor materials radioactive over time, so some waste handling is still needed.

"Fusion would end greenhouse emissions." A working fusion plant would produce no carbon dioxide while running, which is a major attraction. But commercial plants are unlikely to be widespread for decades, so cutting emissions today still depends on renewables, storage, efficiency and existing low-carbon sources.

The 2021 shot did not solve fusion energy. What it did was prove, for the first time, that a laboratory plasma could ignite, which opened the door to the 2022 breakeven and to the steadily larger yields that have followed.

Sources: LLNL: NIF experiment puts researchers at threshold of fusion ignition; LLNL: Three peer-reviewed papers highlight NIF record yield shot; American Nuclear Society: Breakeven breakthrough at NIF; Wikipedia: National Ignition Facility; original report via ScienceAlert.