Laser fusion reactor approaches “plasma in flames” goal | Science

By Daniel Clery23 November 2020, 10:45

In October 2010, in a building the size of three football fields, researchers at Lawrence Livermore National Laboratory ignited 192 laser beams, focused their energy in a pulse with the punch of a moving truck, and fired it. against a nuclear ball. food the size of a peppercorn. Thus began a campaign by the National Ignition Facility (NIF) to achieve the goal for which it is named: triggering a fusion reaction that produces more energy than is emitted by the laser.

A decade and nearly 3,000 shots later, NIF is still generating more fizz than bang, hampered by the complex and little-known behavior of laser targets when they vaporize and implode. But with new target designs and laser pulse shapes, coupled with better tools to monitor miniature explosions, NIF researchers believe they are close to an important intermediate milestone known as “burning plasma”: a fusion burn sustained rather by the heat of the reaction itself. with respect to the input of laser energy.

Self-heating is the key to burning all fuel and achieving uncontrolled 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 are pushing as hard as we can,” he says. “You can feel the acceleration in our understanding.” The exteriors are also impressed. “You feel like there is steady progress and less guesswork,” says Steven Rose, co-director of the Center for Inertial Fusion Studies at Imperial College London. “They are moving away from traditionally held projects and trying new things.”

However, NIF may not allow itself the luxury of time. The proportion of NIF shots devoted to the ignition effort has been reduced from a high of nearly 60% in 2012 to less than 30% today to reserve more shots for inventory management – experiments that simulate nuclear detonations to help verify l reliability of the warheads. Presidential budget calls in recent years have repeatedly tried to cut research into inertial confinement fusion at the NIF and elsewhere, just to get Congress to preserve it. The NIF’s financier, 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 may decide on a further shift towards inventory management. “Will the ignition program be deactivated?” asks Mike Dunne, who led Livermore’s fusion energy efforts from 2010 to 2014. “The jury is out.”

Fusion has long been considered a carbon-free energy source, fueled by readily available hydrogen isotopes and producing no long-lived radioactive waste. But it remains a distant dream, even for donut-shaped slow-burning magnetic ovens such as the ITER project in France, which aims to achieve energy gain 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 tiny fuel pellets. While some fusion lasers aim their beams directly at the pellets, the NIF hits are indirect: the beams heat a pencil eraser-sized gold can called a hohlraum, which emits an X-ray pulse intended to ignite the fusion by heating the fuel capsule at its center to tens of millions of degrees and compressing it to billions of atmospheres.

But the shots in the first 3 years of the firing campaign produced only about 1 kilojoule (kJ) of energy each, less than the 21 kJ pumped into the capsule by the X-ray pulse and well below 1.8 megajoules (MJ) in the original laser pulse. Siegfried Glenzer, who led the initial campaign, says the team was “overly ambitious” in achieving ignition. “We relied heavily on simulations,” says Glenzer, now at the SLAC National Accelerator Laboratory.

After the failed ignition campaign, NIF researchers have upgraded their diagnostic tools. They added more neutron detectors to give them a 3D view of where the fusion reactions were taking place. They also adapted four of their laser beams to produce ultra-short, high-power pulses moments after the implosion in order to vaporize thin strands near the target. The wires act like an X-ray light bulb, capable of probing fuel as it compresses. “It’s like a CT scan,” says planetary scientist Raymond Jeanloz of the University of California, Berkeley, who uses NIF to replicate pressures at the center of giant planets like Jupiter. (About 10% of NIF shots are devoted to basic science.)

With their sharpest vision, the researchers tracked down the energy losses from the imploding fuel pellet. One came to the spot 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 to the capsule’s plastic shell, so the researchers revamped production to smooth out imperfections of just one millionth of a meter. The improved diagnostics “really help scientists understand what improvements are needed,” says Mingsheng Wei of the University of Rochester’s Laboratory for Laser Energetics.

Fire for test

The team also played with the shape of the 20-nanosecond laser pulses. The first few hits slowly increased in power, to avoid heating the fuel too quickly and making it more difficult to compress. Subsequent pulses increased more aggressively so that the plastic capsule had less time to mix with the fuel during compression, a tactic that increased yields slightly.

In the current campaign, which began in 2017, researchers are raising temperatures by enlarging the hohlraum and capsule by up to 20%, increasing the X-ray energy the capsule can absorb. To increase the pressure, they extend the duration of the pulse and switch from plastic capsules to denser diamond capsules to compress the fuel more efficiently.

NIF has repeatedly achieved yields close to 60 kJ. But Herrmann says a recent snap, discussed at the American Physical Society’s Plasma Physics Division meeting earlier this month, surpassed him. Repeated hits are planned to gauge how far they’ve gotten to a burning plasma, which is expected to occur at around 100 kJ. “It’s pretty exciting,” he says.

Even at maximum compression, the NIF researchers believe that only the center of the fuel is hot enough to melt. But in an encouraging discovery, they see evidence that the hot spot is receiving a warming boost from the franticly moving helium nuclei, or alpha particles, created by fusion reactions. If NIF can pump just a little more energy, it should set off a wave that will rush out of the hot spot, burning fuel as it goes.

Herrmann says the team still has a few more tricks to try, all of which could bring temperatures and pressures to levels high enough to sustain plasma combustion and ignition. They are testing different forms of hohlraum to better focus the energy on the capsule. They are experimenting with double-walled capsules that could trap and transfer X-ray energy more efficiently. And by dipping the fuel into a foam inside the capsule, instead of freezing it like ice on the capsule walls, they hope to form a better central hot spot.

Will it be enough to reach ignition? If these steps aren’t enough, increasing the laser energy would be the next option. The NIF researchers tested the updates on four of the light lines and managed to achieve an energy boost that, if the updates were applied to all beams, would bring the entire structure close to 3 MJ.

Such updates, of course, would take time and money that NIF may not get. Fusion scientists at NIF and elsewhere are eagerly awaiting the conclusions of the NNSA review. “How far can we get?” Herrmann asks. “I am optimistic. We will push the NIF as far as possible. “