The power of the sun has edged a little closer to Earth. Under x-ray assault, the rapid implosion of a plastic shell onto icy isotopes of hydrogen has produced fusion and, for the first time, 170 micrograms of this superheated fusion fuel released more energy than it absorbed. Experimental shots of the 192 lasers at the National Ignition Facility at Lawrence Livermore National Laboratory in California have reproduced such fusion at least four times since September 2013. The advance offers hope that someday in the far future scientists might reliably replicate the power source of the sun and stars.
"This is closer than anyone's gotten before, and it's really unique to get out of the fuel as much energy as put in," says Livermore physicist Omar Hurricane, lead author of the paper presenting the results published in Nature. "We got more fusion energy out of the DT fuel than we put in to the DT fuel." (Scientific American is part of Nature Publishing Group.)
DT fuel stands for deuterium and tritium, the isotopes of hydrogen that encompass one proton and one neutron or one proton and two neutrons, respectively. One shot at Livermore’s National Ignition Facility (NIF) on November 19, 2013, that lasted less than 2 X 10^–8 seconds—less time than the blink of an eye—produced nearly twice as much energy as was applied, according to the new paper. Changing the timing of how the lasers put energy into the hohlraum, a tiny can that holds the fusion fuel pellet, proved key. The scientists concentrated more energy earlier in the shot to make conditions hotter earlier in the process, which seems to help hold the fuel pellet together longer as it implodes.
The fuel pellet itself is a perfectly spherical capsule of plastic, roughly two millimeters in diameter and precisely shaped (at a cost of roughly $1 million per pellet) to ensure the best performance. The deuterium and tritium are added as a gas to the hollow pellet. Then the sphere is cooled to 18.6 kelvins, or –254.55 degrees Celsius. That cooling causes the deuterium and tritium to form a layer of ice on the inside of the sphere roughly 70 micrometers thick—thinner than the width of a human hair. Roughly 500 megajoules of electricity feed lasers that then pump out 1.9 megajoules worth of energy. Those lasers take a long, power-boosting trip through amplifying optics and shoot into the hohlraum, which is made of gold and measures 5.75 millimeters in diameter and 9.425 millimeters long. "It's a soup can but very small [and] made out of gold with two holes on the end where the lasers go in," explains Livermore physicist Debbie Callahan, a member of the fusion team.
Employing 1.9 megajoules in slightly more than a nanosecond, the lasers deliver 500 terawatts of power inside the hohlraum (a terawatt is a trillion watts). A cloud of helium gas holds back the gold plasma that would otherwise intrude as the laser power is translated into x-rays by the hohlraum. These x-rays hit the plastic shell of the capsule, which absorbs roughly one tenth of the energy put into the lasers to begin with. That's enough energy to obliterate the outside shell and drive the fuel together "like a rocket," in the words of Hurricane, collapsing the sphere of fuel until it is one thirty-fifth its original size in almost no time at all, the equivalent of going from a sphere the size of a basketball to one the size of a pea almost instantly. The fuel absorbs roughly one tenth of the energy delivered by the x-rays onto the plastic capsule. That energy and implosion create a high pressure (150 gigabars) region of fusion that is even smaller than the layer of fuel itself—a hotspot that is 60 microns in diameter and shaped, depending on the qualities of the shot, like a doughnut without a hole, or an apple. "The conditions are quite ferocious," Hurricane says, noting the key challenge of maintaining a roughly spherical shape. "Mother Nature doesn't like putting a lot of energy in small volumes so she fights you on it."
It is here in the hotspot that the fuel reaches more than 50 million kelvins (about 50 million degrees C) and experiences 150 billion Earth-atmospheres worth of pressure. The fusion of deuterium and tritium that results under those conditions produces helium and a spare neutron, and releases some 17,000 joules of energy in the process.
In other words, these ferocious conditions almost three times denser than the center of the sun release the same amount of energy embodied by a downhill skier going 58 kilometers per hour (by Hurricane's calculations). All told, only about 1 percent of the energy from the lasers ends up in the fuel, which then pumps out 17,000 joules’ worth of energy, or less than the energy needed to make the DT fuel in the first place. All of it lasts for 150 picoseconds, or 150 trillionths of a second, before the fusion zone blows itself apart. "We focus all this energy into a very small volume to get the pressure high to make these reactions happen," Hurricane explains. "It's a trade-off. We're eventually working toward making that [energy] back."
Scientists remain a long way from what's known as ignition: the point at which fusion of any kind releases more energy than was consumed to start it. And the method used to produce this result is unlikely to create the conditions needed to reach that goal. "By lowering the compressibility, they have lowered the pressure that can be reached," explains physicist Mark Herrmann, director of the Pulsed Power Sciences Center at Sandia National Laboratories, who wrote a commentary accompanying the research paper in Nature.
But the discovery team has also seen for the first time the early stages of the kind of physical processes needed to create such fusion. Specifically, the fuel showed evidence of what fusion physicists like to call "bootstrapping." Essentially, the helium nuclei (otherwise known as alpha particles) thrown off by the fusing hydrogen isotopes left their energy behind, maintaining the conditions needed for yet more fusion. That helped more than double the superheating of the fusing fuel and suggests the team is halfway to the kinds of energies needed to achieve ignition. "As we pushed it in experiments, the bootstrapping kicks in more and more," Hurricane says. "Seeing that kick is quite exciting and does show there is progress."
Hurricane and other fusion scientists will need to roughly double the pressure experienced by the DT fuel—to more than 300 gigabars—to achieve ignition, according to theory and computer model simulations. That feat will require crushing the capsule even faster—at speeds above hundreds of kilometers per second—while maintaining a more perfect, spherical shape for the fusion hotspot. Such conditions might be achieved by trying materials other than plastic for the capsule's outer shell—like diamond or beryllium—or changing the hohlraum's shape. "It's not clear that a cylinder is optimal," Callahan says, offering the shape of a rugby (egg-shaped) ball as a possible alternative.
Increasing the laser power could also enable researchers to double the pressure on the DT fuel and achieve ignition. But whether NIF’s 192 lasers, which cost some $3.5 billion to build, are up to this task is unknown. "We don't know yet if ignition will be achievable on NIF," Herrmann remarks. "The easiest ways to increase the pressure, as needed to obtain ignition, are to make the implosion faster or increase the compressibility of the fusion fuel. Both of these approaches run the risk of increased instabilities, like those that reduced the fusion yield in earlier NIF experiments."
During the National Ignition Campaign that inaugurated NIF and ran from 2010 to 2012 the facility failed to achieve ignition despite pushing for the highest energy yield possible. In part, this latest series of experiments was designed to determine what went wrong. Part of the problem now appears to be allowing for too much compressibility in the DT fuel, which then made the fusion process itself too unstable to control. "The new implosion is less compressible but more stable," Hurricane explains. "We self-limited ourselves to gain this control, but this isn't one thing we're stuck with, it's a point of departure."
Hurricane compares the ongoing ignition quest with climbing a mountain of unknown height with a summit wreathed in clouds and therefore invisible. This step of getting more energy out of the fuel than is put in represents a base camp of sorts, farther up the mountain than any have ever tread before and from which new paths to reach the summit of ignition might be tried.
Laser-based compression of the fuel, an approach that is part of a suite of techniques known as inertial confinement, is not the only means of achieving fusion. Physicists have also employed magnetic fields to shape and contain the superhot plasma that allows fusion to occur. In 1997 the Joint European Torus (JET) in the U.K. released 16 megawatts of power (or 22 megajoules of energy for less than as second) from fusion, using 24 megawatts-worth of heat—the prior record. "From the burning plasma physics point of view, the NIF plasma has outperformed the JET plasma," explains physicist Riccardo Betti of the University of Rochester, because it created the self-heating bootstrapping process.
But a magnetic confinement designed based on JET, known as the International Thermonuclear Experimental Reactor, or ITER (pronounced "eater"), is under construction near the village of Cadarache in France. "ITER will produce 500 megawatts of fusion power according to predictions made based on JET and theory," says physicist Steven Cowley, director of the Culham Center for Fusion Energy where JET is located. "Input of energy to plasma will be less than 50 megawatts and electrical power to make this input will be about 200 to 250 megawatts. It's an experiment, so we don't know exactly."
Regardless of whether NIF achieves ignition or not, the facility will continue to create the kind of high-density fusion conditions that have also proved useful to those charged with ensuring that the U.S. nuclear arsenal remain in working order. Instead of occasionally setting off nuclear bombs, this weapons crew now relies on such tests that create conditions similar to those at the core of a thermonuclear weapon. The NIF shots also simulate conditions that are found at the center of gas-giant planets like Jupiter or brown dwarf stars. "It allows us to study nuclear synthesis processes that we've never had access to before," says Livermore experimental plasma physicist Tammy Ma, another member of the fusion team.
But even if scientists do achieve ignition one day, whether at NIF, ITER or some other, yet-undreamt-of facility, there will still be a long road to building an actual fusion power plant. For one thing, a fresh source of rare tritium would be required to sustain fusion. Current ideas focus on a so-called blanket of lithium that would be bombarded by the spare neutrons from fusion itself, producing yet more helium and tritium while still leaving some energy leftover to be harvested for electricity production, an idea that leaves little room for error. "We have waited 60 years to get close to controlled fusion—we are now close in both magnetic and inertial," Cowley says. "We must keep at it."
After all, E = mc^2, which means a very small amount of mass can produce a great amount of energy, given the speed of light. The prospect of near limitless, sustainable energy with just a whiff of helium as a by-product ensures continued interest, although NIF itself will come up for review in 2015. Just last year the National Research Council of the U.S. National Academy of Sciences endorsed continued investment in inertial confinement fusion, given the potential benefits, although it also called for backing of more approaches, such as particle beams or pulsed magnetic beams, not just the high-powered lasers employed at NIF. "Our theoretical understanding is that if we keep pushing in this direction we have a chance, but we can't really promise one way or the other," Hurricane notes. "We can't honestly tell you when we will get ignition. We are working like mad to go in that direction."