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Laser fusion reactor approaches ‘burning plasma’ milestone

In the target chamber of the National Ignition Facility, 192 laser beams are focused on pellets of fusion fuel the size of peppercorns.

Lawrence Livermore National Laboratory

In October 2010, in a building the size of three U.S. football fields, researchers at the Lawrence Livermore National Laboratory powered up 192 laser beams, focused their energy into a pulse with the punch of a speeding truck, and fired it at a pellet of nuclear fuel the size of a peppercorn. So began a campaign by the National Ignition Facility (NIF) to achieve the goal it is named for: igniting a fusion reaction that produces more energy than the laser puts in.

A decade and nearly 3000 shots later, NIF is still generating more fizz than bang, hampered by the complex, poorly understood behavior of the laser targets when they vaporize and implode. But with new target designs and laser pulse shapes, along with better tools to monitor the miniature explosions, NIF researchers believe they are close to an important intermediate milestone known as “burning plasma”: a fusion burn sustained by the heat of the reaction itself rather than the input of laser energy.

Self-heating is key to burning up all the fuel and getting runaway energy gain. Once NIF reaches the threshold, simulations suggest it will have an easier path to ignition, says Mark Herrmann, who oversees Livermore’s fusion program. “We’re pushing as hard as we can,” he says. “You can feel the acceleration in our understanding.” Outsiders are impressed, too. “You kind of feel there’s steady progress and less guesswork,” says Steven Rose, co-director of the Centre for Inertial Fusion Studies at Imperial College London. “They’re moving away from designs traditionally held and trying new things.”

NIF may not have the luxury of time, however. The proportion of NIF shots devoted to the ignition effort has been cut from a high of nearly 60% in 2012 to less than 30% today to reserve more shots for stockpile stewardship—experiments that simulate nuclear detonations to help verify the reliability of warheads. Presidential budget requests in recent years have repeatedly sought to slash research into inertial confinement fusion at NIF and elsewhere, only to have Congress preserve it. NIF’s funder, the National Nuclear Security Administration (NNSA), is reviewing the machine’s progress for the first time in 5 years. Under pressure to modernize the nuclear arsenal, the agency could decide on a further shift toward stockpile stewardship. “Will the ignition program be squeezed out?” asks Mike Dunne, who directed Livermore’s fusion energy efforts from 2010 to 2014. “The jury’s out.”

Fusion has long been held up as a carbon-free source of energy, fueled by readily available isotopes of hydrogen and producing no long-lived radioactive waste. But it remains a distant dream, even for the slow-burning, doughnut-shaped magnetic furnaces like the ITER project in France, which aims to achieve energy gain sometime after 2035.

NIF and other inertial fusion devices would be less like a furnace and more like an internal combustion engine, producing energy through rapid-fire explosions of the diminutive fuel pellets. Whereas some fusion lasers aim their beams straight at the pellets, NIF’s shots are indirect: The beams heat a gold can the size of a pencil eraser called a hohlraum, which emits a pulse of x-rays meant to ignite fusion by heating the fuel capsule at its center to tens of millions of degrees and compressing it to billions of atmospheres.

But shots in the first 3 years of the ignition campaign only yielded about 1 kilojoule (kJ) of energy each, short of the 21 kJ pumped into the capsule by the x-ray pulse and far short of the 1.8 megajoules (MJ) in the original laser pulse. Siegfried Glenzer, who led the initial campaign, says the team was “overly ambitious” about reaching ignition. “We were overly reliant on simulations,” says Glenzer, now at the SLAC National Accelerator Laboratory.

After the failed ignition campaign, NIF researchers beefed up their diagnostic instruments. They added more neutron detectors to give them a 3D view of where the fusion reactions were happening. They also adapted four of their laser beams to produce high-power, ultrashort pulses moments after the implosion in order to vaporize thin wires close to the target. The wires act as an x-ray flashbulb, able to probe the fuel as it compresses. “It’s like a CAT scan,” says planetary scientist Raymond Jeanloz of the University of California, Berkeley, who uses NIF to replicate the pressures at the core of giant planets such as Jupiter. (About 10% of NIF shots are devoted to basic science.)

With their sharper vision, researchers have tracked down energy leaks from the imploding fuel pellet. One came at the point where a tiny tube injected fuel into the capsule before the shot. To plug the leak, the team made the tube even thinner. Other leaks were traced back to the capsule’s plastic shell, so researchers revamped manufacturing to smooth out imperfections of just a millionth of a meter. The improved diagnostics “really helps the scientists to understand what improvements are required,” says Mingsheng Wei of the University of Rochester’s Laboratory for Laser Energetics.

Fire by trial

The National Ignition Facility has closed in on fusion ignition—getting more energy out than goes in—by altering its laser pulses and targets. It is even closer to the temperatures and pressures needed for an intermediate goal: a self-heating “burning plasma.”
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The team has also played with the shape of the 20-nanosecond laser pulses. Early shots ramped up in power slowly, to avoid heating the fuel too quickly and making it harder to compress. Later pulses ramped up more aggressively so that the plastic capsule had less time to mix with the fuel during compression, a tactic that boosted yields somewhat.

In the current campaign, begun in 2017, researchers are boosting temperatures by enlarging the hohlraum and the capsule by up to 20%, increasing the x-ray energy the capsule can absorb. To up the pressure, they’re extending the duration of the pulse and switching from plastic capsules to denser diamond ones to compress the fuel more efficiently.

NIF has repeatedly achieved yields approaching 60 kJ. But Herrmann says a recent shot, discussed at the American Physical Society’s Division of Plasma Physics meeting earlier this month, has exceeded that. Repeat shots are planned to gauge how close they got to a burning plasma, which is predicted to occur around 100 kJ. “It’s pretty exciting,” he says.

Even at maximum compression, the NIF researchers believe only the very center of the fuel is hot enough to fuse. But in an encouraging finding, they see evidence that the hot spot is getting a heating boost from frenetically moving helium nuclei, or alpha particles, created by the fusion reactions. If NIF can pump in just a bit more energy, it should spark a wave that will race out from the hot spot, burning fuel as it goes.

Herrmann says the team still has a few more tricks to try out—each of which could drive temperatures and pressures to levels high enough to sustain burning plasma and ignition. They are testing different hohlraum shapes to better focus energy onto the capsule. They’re experimenting with double-walled capsules that could trap and transfer x-ray energy more efficiently. And by soaking the fuel into a foam within the capsule, rather than freezing it as ice to the capsule walls, they hope to form a better central hot spot.

Will that be enough to reach ignition? If these steps don’t suffice, boosting the laser energy would be the next option. NIF researchers have tested upgrades on four of the beamlines and managed to get an energy boost that, if the upgrades were applied to all the beams, would bring the full facility close to 3 MJ.

Those upgrades would, of course, take time and money NIF may not end up getting. Fusion scientists at NIF and elsewhere are anxiously awaiting the conclusions of the NNSA review. “How far can we get?” Herrmann asks. “I’m an optimist. We’ll push NIF as far as we possibly can.”

Source: Science Mag