Laser fusion reactor approaches 'burning plasma' milestone - Science Magazine

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

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.

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.

“Will the ignition program be squeezed out?” asks Mike Dunne, who directed Livermore’s fusion energy efforts from 2010 to 2014.

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.

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.

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.

The National Ignition Facility has closed in on fusion ignition—getting more energy out than goes in—by altering its laser pulses and targets.

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.

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?

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.

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.