#laser-powered #fusion #ignition #NIF #physics
More than a decade ago, the world’s most energetic laser started to unleash its blasts on tiny capsules of hydrogen isotopes, with managers promising it would soon demonstrate a route to limitless fusion energy. Now, the National Ignition Facility (NIF) has taken a major leap toward that goal. Last week, a single laser shot sparked a fusion explosion from a peppercorn-size fuel capsule that produced eight times more energy than the facility had ever achieved: 1.35 megajoules (MJ)—roughly the kinetic energy of a car traveling at 160 kilometers per hour. That was also 70% of the energy of the laser pulse that triggered it, making it tantalizingly close to “ignition”: a fusion shot producing an excess of energy.
“After many years at 3% of ignition, this is superexciting,” says Mark Herrmann, head of the fusion program at Lawrence Livermore National Laboratory, which operates NIF.
NIF’s latest shot “proves that a small amount of energy, imploding a small amount of mass, can get fusion. It’s a wonderful result for the field,” says physicist Michael Campbell, director of the Laboratory for Laser Energetics (LLE) at the University of Rochester.
“It’s a remarkable achievement,” adds plasma physicist Steven Rose, co-director of the Centre for Inertial Fusion Studies at Imperial College London. “It’s made me feel very cheerful. … It feels like a breakthrough.”
And it is none too soon, as years of slow progress have raised questions about whether laser-powered fusion has a practical future. Now, according to LLE Chief Scientist Riccardo Betti, researchers need to ask: “What is the maximum fusion yield you can get out of NIF? That’s the real question.”
Fusion, which powers stars, forces small atomic nuclei to meld together into larger ones, releasing large amounts of energy. Extremely hard to achieve on Earth because of the heat and pressure required to join nuclei, fusion continues to attract scientific and commercial interest because it promises copious energy, with little environmental impact.
Yet among the many approaches being investigated, none has yet generated more energy than was needed to cause the reaction in the first place. Large doughnut-shaped reactors called tokamaks, which use magnetic fields to cage a superhot plasma for long enough to heat nuclei to fusion temperatures, have long been the front-runners to achieve a net energy gain. But the giant $25 billion ITER project in France is not expected to get there for more than another decade, although private fusion companies are promising faster progress.
NIF’s approach, known as inertial confinement fusion, uses a giant laser housed in a facility the size of several U.S. football fields to produce 192 beams that are focused on a target in a brief, powerful pulse—1.9 MJ over about 20 nanoseconds. The aim is to get as much of that energy as possible into the target capsule, a diminutive sphere filled with the hydrogen isotopes deuterium and tritium mounted inside a cylinder of gold the size of a pencil eraser. The gold vaporizes, producing a pulse of x-rays that implodes the capsule, driving the fusion fuel into a tiny ball hot and dense enough to ignite fusion. In theory, if such tiny fusion blasts could be triggered at a rate of about 10 per second, a power plant could harvest energy from the high-speed neutrons produced to generate electricity.
When NIF launched, computer models predicted quick success, but fusion shots in the early years only generated about 1 kilojoule (kJ) each. A long effort to better understand the physics of implosions followed and by last year shots were producing 100 kJ. Key improvements included smoothing out microscopic bumps and pits on the fuel capsule surface, reducing the size of the hole in the capsule used to inject fuel, shrinking the holes in the gold cylinder so less energy escapes, and extending the laser pulse to keep driving the fuel inward for longer. The progress was sorely needed, as NIF’s funder, the National Nuclear Security Administration, was reducing shots devoted to ignition in favor of using its lasers for other experiments simulating the workings of nuclear weapons.
Earlier this year, combining those improvements in various ways, the NIF team produced several shots exceeding 100 kJ, including one of 170 kJ. That result suggested NIF was finally creating a “burning plasma,” in which the fusion reactions themselves provide the heat for more fusion—a runaway reaction that is key to getting higher yields. Then, on 8 August, a shot generated the remarkable 1.35 MJ. “It was a surprise to everyone,” Herrmann says. “This is a whole new regime.”
Exactly which improvements had the greatest impact and what combination will lead to future gains will take a while to unravel, Herrmann says, because several were tweaked at once in the latest shot. “It’s a very nonlinear process. That’s why it’s called ignition: It’s a runaway thing,” he says. But, “This gives us a lot more encouragement that we can go significantly farther.”
Herrmann’s team is a long way from thinking about fusion power plants, however. “Getting fusion in a laboratory is really hard, getting economic fusion power is even harder,” Campbell says. “So, we all have to be patient.” NIF’s main task remains ensuring the United States’s nuclear weapons stockpile is safe and reliable; fusion energy is something of a sideline. But reaching ignition and being able to study and simulate the process will also “open a new window on stewardship,” Herrmann says, because uncontrolled fusion powers nuclear weapons.
Herrmann admits that, when he got a text last week from colleagues saying they’d gotten an “interesting” result from the latest shot, he was worried something might be wrong with the instruments. When that proved not to be the case, “I did open a bottle of champagne.”