achieving ‘ignition’ over and over

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In December 2022, after more than a decade of effort and frustration, scientists at the US National Ignition Facility (NIF) announced that they had set a world record by producing a fusion reaction that released more energy than it consumed — a phenomenon known as ignition. They have now proved that the feat was no accident by replicating it again and again, and the administration of US President Joe Biden is looking to build on this success by establishing a trio of US research centres to help advance the science.

The stadium-sized laser facility, housed at the Lawrence Livermore National Laboratory (LLNL) in California, has unequivocally achieved its goal of ignition in four out of its last six attempts, creating a reaction that generates pressures and temperatures greater than those that occur inside the Sun.

“I’m feeling pretty good,” says Richard Town, a physicist who heads the lab’s inertial-confinement fusion science programme at the LLNL. “I think we should all be proud of the achievement.”

The NIF was designed not as a power plant, but as a facility to recreate and study the reactions that occur during thermonuclear detonations after the United States halted underground weapons testing in 1992. The higher fusion yields are already being used to advance nuclear-weapons research, and have also fuelled enthusiasm about fusion as a limitless source of clean energy. US special presidential envoy for climate John Kerry called for new international partnerships to advance fusion energy at the COP28 climate summit in Dubai last week, and the US Department of Energy (DOE), which oversees the NIF, followed up by announcing the new research hubs, to be led by the LLNL, the University of Rochester in New York and Colorado State University in Fort Collins.

Building the NIF was “a leap of faith” for many, and its success has had a real impact on the fusion community, as well as on public perception, says Saskia Mordijck, a physicist at William & Mary, a university in Willamsburg, Virginia. “In that sense, what is important is that scientists said they could do something, and then they actually did do something.”

Hot shots

The NIF works by firing 192 laser beams at a frozen pellet of the hydrogen isotopes deuterium and tritium that is housed in a diamond capsule suspended inside a gold cylinder. The resulting implosion causes the isotopes to fuse, creating helium and copious quantities of energy. On 5 December 2022, those fusion reactions for the first time generated more energy — roughly 54% more — than the laser beams delivered to the target.

The facility set a new record on 30 July when its beams delivered the same amount of energy to the target — 2.05 megajoules — but, this time, the implosion generated 3.88 megajoules of fusion energy, an 89% increase over the input energy. Scientists at the laboratory achieved ignition during two further attempts in October (see ‘A year of progress’). And the laboratory’s calculations suggest that two others in June and September generated slightly more energy than the lasers provided, but not enough to confirm ignition.

A year of progress: Timeline of 'ignition' experiments conducted by the US National Ignition Facility since December 2022.

Source: Lawrence Livermore National Laboratory

For many scientists, the results confirm that the laboratory is now operating in a new regime: researchers can repeatedly hit a goal they’ve been chasing for more than a decade. Tiny variations in the laser pulses or minor defects in the diamond capsule can still allow energy to escape, making for an imperfect implosion, but the scientists now better understand the main variables at play and how to manipulate them.

“Even when we have these issues, we can still get more than a megajoule of fusion energy, which is good,” says Annie Kritcher, the NIF’s lead designer on this series of experiments.

New hubs

It’s a long way from there to providing fusion energy to the power grid, however, and the NIF, although currently home to the world’s largest laser, is not well-suited for that task. The facility’s laser system is enormously inefficient, and more than 99% of the energy that goes into a single ignition attempt is lost before it can reach the target.

Developing more efficient laser systems is one goal of the DOE’s new inertial-fusion-energy research programme. This month, the agency announced US$42 million over four years to establish three new research centres — each involving a mix of national laboratories, university researchers and industry partners — that will work towards this and other advances.

This investment is the first coordinated effort to develop not just the technologies, but also the workforce for a future laser-fusion industry, says Carmen Menoni, a physicist who is heading up the hub at Colorado State University.

So far, most government investments in fusion-energy research have gone towards devices known as tokamaks, which use magnetic fields inside a doughnut-shaped ‘torus’ to confine fusion reactions. This is the approach under development at ITER, an international partnership to build the world’s largest fusion facility near Saint-Paul-lez-Durance, France. Tokamaks have also been the focus of many fusion investments in the private sector, but dozens of companies are pursuing other approaches, such as laser fusion.

The timing for a dedicated laser-fusion programme is right, says Menoni, and the decision to pursue it wouldn’t have happened without the NIF’s recent success. “We now know it will work,” she says. “What will take time is to develop the technology to a level where we can build a power plant.”

Back at the NIF, Kritcher’s latest series of experiments features a 7% boost in laser energy, which should, in theory, lead to even larger yields. The first experiment in this series was one of the successful ignitions, on 30 October. Although it didn’t break the record, an input of 2.2 megajoules of laser energy yielded an output of 3.4 megajoules of fusion energy.

Kritcher chalks up the fact that it didn’t break the record for energy yield to growing pains with the new laser configuration, which is designed to squeeze more energy into the same gold cylinder. Before moving to a larger cylinder, Kritcher says her team is going to focus on changes to the laser pulse that could produce a more symmetrical implosion. “We’ve got four experiments next year,” she says. “Let’s see.”

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