Laser Fusion Reactor Approaches ‘Hot Plasma’ Milestone | Science

By Daniel CleryNovember 23, 2020 at 10:45 a.m.

In October 2010, in a building the size of three American football fields, researchers from the Lawrence Livermore National Laboratory powered 192 laser beams, focused their energy on a pulse with the punch of a high-speed truck and l ‘fired at a nuclear pellet. fuel the size of a peppercorn. So began a campaign by the National Ignition Facility (NIF) to achieve the goal for which it was named: to trigger a fusion reaction that produces more energy than the laser puts out.

A decade and nearly 3,000 shots later, the NIF still generates more sparkle than bang, hampered by the complex and poorly understood behavior of laser targets as they vaporize and implode. But with new target designs and new forms of laser pulses, along with better tools for monitoring miniature explosions, NIF researchers believe they are near an important intermediate milestone known as “plasma. burning ”: a fusion burn sustained by the heat of the reaction itself. as the input of laser energy.

Self-heating is the key to burning off all the fuel and getting an uncontrollable energy gain. Once the NIF hits the threshold, simulations suggest it will have an easier path to ignition, says Mark Herrmann, who oversees Livermore’s fusion program. “We’re doing everything we can,” he says. “You can feel the acceleration of our understanding.” Foreigners are also impressed. “It feels 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 designs and trying new things.”

However, the NIF may not have the luxury of saving time. The proportion of NIF shots devoted to the ignition effort has been reduced from nearly 60% in 2012 to less than 30% today to reserve more shots for inventory management – experiments that simulate nuclear explosions for help verify the reliability of warheads. Presidential budget demands in recent years have repeatedly sought to curtail research into inertial confinement fusion at the NIF and elsewhere, only for Congress to 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 the pressure to modernize the nuclear arsenal, the agency could decide on a new evolution towards the management of stocks. “Will the ignition schedule be deleted?” 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, powered by readily available isotopes of hydrogen, and producing no long-lived radioactive waste. But that remains a distant dream, even for slow-burning magnetic donut-shaped ovens like the ITER project in France, which aims to achieve energy savings after 2035.

The NIF and other inertial fusion devices would look less like a furnace and more like an internal combustion engine, producing energy through rapid-fire explosions of tiny fuel pellets. While some fusion lasers aim their beams directly at the pellets, the shots of NIF are indirect: the beams heat a gold box the size of a pencil eraser called a hohlraum, which emits an x-ray pulse intended to trigger melting by heating the fuel capsule. at its center to tens of millions of degrees and compressing it to billions of atmospheres.

But the shots during the first 3 years of the ignition campaign yielded only 1 kilojoule (kJ) of energy each, short of the 21 kJ pumped into the capsule by the x-ray pulse and far from the 1.8 megajoules (MJ) in the original laser pulse. Siegfried Glenzer, who spearheaded the initial campaign, says the team was “too ambitious” to reach the ignition. “We were too dependent on simulations,” says Glenzer, now at SLAC’s National Accelerator Lab.

After the failure of the ignition campaign, NIF researchers strengthened their diagnostic tools. They added more neutron detectors to give them a 3D view of where the fusion reactions were occurring. They also adapted four of their laser beams to produce ultra-short high-power pulses moments after implosion to vaporize thin wires near the target. The wires act like an X-ray bulb, able to probe the fuel as it compresses. “It’s like a CT scan,” says planetary specialist Raymond Jeanloz of the University of California at Berkeley, who uses the NIF to replicate the pressures inside giant planets such as Jupiter. (About 10% of NIF plans are devoted to basic science.)

Using their sharper vision, the researchers spotted energy leaks from the imploding fuel pellets. One got to the point where a tiny tube injected fuel into the capsule before firing. To stop the leak, the team made the tube even thinner. Further leaks were blamed on the capsule’s plastic shell, so the researchers revamped the manufacturing to smooth out imperfections within a millionth of a meter. The improved diagnostics “really help scientists understand what improvements are needed,” says Mingsheng Wei of the University of Rochester Laser Energy Lab.

Fire by trial

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

In the current campaign, launched in 2017, researchers are raising temperatures by enlarging the hohlraum and capsule by up to 20%, thereby increasing the x-ray energy that 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 fuel more efficiently.

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

Even at maximum compression, NIF researchers believe that only the very center of the fuel is hot enough to fuse. But in an encouraging discovery, they see evidence that the hot spot receives a heating boost from the frenziedly moving helium nuclei, or alpha particles, created by the fusion reactions. If the NIF can pump a little more energy, it should trigger a wave that will escape from the hot spot, burning fuel as it goes.

Herrmann says the team still have a few tricks to try, each of which can bring temperatures and pressures to levels high enough to support plasma burning and ignition. They are testing different forms of hohlraum to better focus energy on the capsule. They are experimenting with double-walled capsules that could trap and transfer x-ray energy more efficiently. And by soaking the fuel in a foam inside the capsule, rather than freezing it as ice on the walls of the capsule, they hope to form a better central hot spot.

Will it be enough to reach ignition? If these steps are not enough, increasing the laser energy would be the next option. NIF researchers tested upgrades on four of the beamlines and were able to achieve an energy boost that, if the upgrades were applied to all beams, would bring the full installation to nearly 3MJ .

These upgrades would of course take time and money that NIF might not get. Fusion scientists at NIF and elsewhere eagerly await the findings of the journal NNSA. “How far can we go?” Herrmann asks. “I’m optimistic. We will push NIF as far as possible. “