Scientists Are Trying to Make Nuclear Fusion With Frickin’ Lasers

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BIG SCIENCE IS really small. In Central Europe, a 17-mile loop looks for subatomic particles. In Washington and Louisiana, massive L-shaped detectors sniff for invisible gravitational perturbations. And a national lab in California’s lumpy hill country is home to a 10-story building where scientists are using laser beams to try and figure out nuclear fusion.

Ah, fusion: energy of the future. In principle, if you get a bunch of atoms hot enough, and squeeze them together hard enough, their nuclei will smush together, release highly energetic particles, and kickstart a chain reaction that creates more and more energy. Sounds easy, is hard. Hence the high-rise at Lawrence Livermore National Lab filled with gigantic lasers. And hence a report released in May (recently resurfaced by Physics Today) that questioned whether the so-called National Ignition Facility would ever meet its goal.

“Ignition” is these physicists’ modest name for a successful bout of nuclear fusion. “It’s a tremendously ambitious goal, something we always knew would be hard to achieve,” says Mark Herrmann, director of the NIF.

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Here’s how hard. It starts with a bunch of energy—the electrical kind, same stuff that toasted your bagel this morning, except way more. “We need to pull energy off the grid to fire this experiment,” says John Edwards, associate director at the NIF. The facility pumps the stuff into its capacitor banks (capacitors are basically short-term batteries) before discharging it into its flash banks, which convert the electricity into light.

That light gets split, amplified, split again, and injected into 192 gigantic laser amplifiers, each of which is about three football fields long. These purify and amplify the light, which then gets routed into a target chamber about 30 feet across. The target itself is a tiny cylinder, one centimeter tall, half as much wide, called a hohlraum—a German word meaning cavity.

The laser beams pass through openings in the hohlraum’s top and bottom and strike its inside walls. The lasers are so intensely focused that their beams heat the hohlraum’s inner surface to about 50 million degrees Kelvin—hotter than the sun’s core. This releases a bunch of x-rays, which compress a tiny, frozen capsule of nuclear fuel suspended right in the middle of the cavity. This all takes about 20 billionths of a second. But in that time the fuel capsule implodes. Deuterium and tritium molecules get smushed together so tightly they shed things called alpha particles.

Those alpha particles add more heat, more pressure. Enough of both sets off a chain reaction: more heat, more pressure, more alpha particles, more, more, more, until ignition. Congratulations, you have just solved one of the most vexing energy problems of all time.

Except You Have Not
The NIF still falling short of fusion. The problem isn’t temperature; it’s pressure. “What happens is if the pressure on the capsule isn’t uniform it does not converge into a nice spherical plasma that converts kinetic energy into thermal energy,” says Craig Sangster, experimental division director at the Laboratory for Laser Energetics at the University of Rochester in New York.

Say again? “Pretend you have a water balloon,” Sangster says, “and as you are squeezing it the balloon starts bulging out between your fingers.” OK, go on. “The pressure from the imploding fuel capsule needs to be nice and uniform so the energy doesn’t get lumpy like that balloon you’re squeezing.”
If the lump of energy being released by the imploding fuel capsule is not perfectly spherical, it won’t be dense enough for fusion. Right now, the NIF lasers are only getting the fuel capsules to about 50 grams per cubic centimeter. (For reference, water in a glass has pressure around 1 gram per cubic centimeter.) It needs to be at least double that.

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