Il 24 Agosto potrebbe essere un giorno importante: si è avuto notizia di un passo avanti molto significativo verso il controllo della fusione nucleare, il processo che darebbe energia in quantità illimitate, a basso costo, senza emissioni di gas nocivi, senza i pericoli degli attuali impianti nucleari, e senza lasciare scorie che restano radioattive per millenni. La stessa energia che alimenta il sole.

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Articolo collegato di Daniel Clery

In a suburban industrial park south of Los Angeles, researchers have taken a significant step toward mastering nuclear fusion—a process that could provide abundant, cheap, and clean energy. A privately funded company called Tri Alpha Energy has built a machine that forms a ball of superheated gas—at about 10 million degrees Celsius—and holds it steady for 5 milliseconds without decaying away. That may seem a mere blink of an eye, but it is far longer than other efforts with the technique and shows for the first time that it is possible to hold the gas in a steady state—the researchers stopped only when their machine ran out of juice.
“They’ve succeeded finally in achieving a lifetime limited only by the power available to the system,” says particle physicist Burton Richter of Stanford University in Palo Alto, California, who sits on a board of advisers to Tri Alpha. If the company’s scientists can scale the technique up to longer times and higher temperatures, they will reach a stage at which atomic nuclei in the gas collide forcefully enough to fuse together, releasing energy.
“Until you learn to control and tame [the hot gas], it’s never going to work. In that regard, it’s a big deal. They seem to have found a way to tame it,” says Jaeyong Park, head of the rival fusion startup Energy/Matter Conversion Corporation in San Diego. “The next question is how well can you confine [heat in the gas]. I give them the benefit of the doubt. I want to watch them for the next 2 or 3 years.”
Although other startup companies are also trying to achieve fusion using similar methods, the main efforts in this field are huge government-funded projects such as the $20 billion International Thermonuclear Experimental Reactor (ITER), under construction in France by an international collaboration, and the U.S. Department of Energy’s $4 billion National Ignition Facility (NIF) in Livermore, California. But the burgeoning cost and complexity of such projects are causing many to doubt they will ever produce plants that can generate energy at an affordable cost.
Tri Alpha’s and similar efforts take a different approach, which promises simpler, cheaper machines that can be developed more quickly. Importantly, the Tri Alpha machine may be able to operate with a different fuel than most other fusion reactors. This fuel—a mix of hydrogen and boron—is harder to react, but Tri Alpha researchers say it avoids many of the problems likely to confront conventional fusion power plants. “They are where they are because people are able to believe they can get a [hydrogen-boron] reactor to work,” says plasma physicist David Hammer of Cornell University, also a Tri Alpha adviser.
But burning hydrogen-boron fuel requires truly enormous temperatures, more than 3 billion degrees Celsius, and that will be “very challenging,” says plasma physicist Jon Menard of the Princeton Plasma Physics Laboratory in New Jersey, who is not involved in the project. He says it’s very hard to predict how the gas will behave at higher temperatures. “I’m a little concerned that their [simulations] lag behind their experience,” he says, but the approach “is worth further investigation.”
Like other fusion techniques, Tri Alpha’s device aims to confine a gas so hot that its atoms are stripped of electrons, producing a roiling mixture of electrons and ions known as plasma. If the ions collide with enough force, they fuse, converting some of their mass into energy, but this requires temperatures of at least 100 million degrees Celsius with conventional fuel, hot enough to melt any container. So the first challenge for reactor designers is how to confine the plasma without touching it. Facilities like the NIF rapidly implode the plasma, relying on its inward inertia to hold it long enough for a burst of fusion reactions. The ITER, in contrast, holds the plasma steady with powerful magnetic fields inside a doughnut-shaped chamber known as a tokamak. Some of the field is provided by a complex network of superconducting magnets, the rest by the plasma itself flowing around the ring like an electric current.
Tri Alpha’s machine also produces a doughnut of plasma, but in it the flow of particles in the plasma produces all of the magnetic field holding the plasma together. This approach, known as a field-reversed configuration (FRC), has been known since the 1960s. But despite decades of work, researchers could get the blobs of plasma to last only about 0.3 milliseconds before they broke up or melted away. In 1997, the Canadian-born physicist Norman Rostoker of the University of California, Irvine, and colleagues proposed a new approach. The following year, they set up Tri Alpha, now based in an unremarkable—and unlabeled—industrial unit here. Building up from tabletop devices, by last year the company was employing 150 people and was working with C-2, a 23-meter-long tube ringed by magnets and bristling with control devices, diagnostic instruments, and particle beam generators. The machine forms two smoke rings of plasma, one near each end, by a proprietary process and fires them toward the middle at nearly a million kilometers per hour. At the center they merge into a bigger FRC, transforming their kinetic energy into heat.
Previous attempts to create long-lasting FRCs were plagued by the twin demons that torment all fusion reactor designers. The first is turbulence in the plasma that allows hot particles to reach the edge and so lets heat escape. Second is instability: the fact that hot plasma doesn’t like being confined and so wriggles and bulges in attempts to get free, eventually breaking up altogether. Rostoker, a theorist who had worked in many branches of physics including particle physics, believed the solution lay in firing high-speed particles tangentially into the edge of the plasma. The fast-moving incomers would follow much wider orbits in the plasma’s magnetic field than native particles do; those wide orbits would act as a protective shell, stiffening the plasma against both heat-leaking turbulence and instability.
To make it work, the Tri Alpha team needed to precisely control the magnetic conditions around the edge of the cigar-shaped FRC, which is as many as 3 meters long and 40 centimeters wide. They did it by penning the plasma in with magnetic fields generated by electrodes and magnets at each end of the long tube.
In experiments carried out last year, C-2 showed that Rostoker was on the right track byproducing FRCs that lasted 5 milliseconds, more than 10 times the duration previously achieved. “In 8 years they went from an empty room to an FRC lasting 5 milliseconds. That’s pretty good progress,” Hammer says. The FRCs, however, were still decaying during that time. The researchers needed to show they could replenish heat loss with the beams and create a stable FRC. So last autumn they dismantled C-2. In collaboration with Russia’s Budker Institute of Nuclear Physics in Akademgorodok, they upgraded the particle beam system, increasing its power from 2 megawatts to 10 megawatts and angling the beams to make better use of their power.
The upgraded C-2U was back in operation by March. At a symposium today in memory of Rostoker, who died in December, Tri Alpha’s chief technology officer Michl Binderbauer announced that by June the new machine was producing FRCs lasting 5 milliseconds with no sign of decay; they remained the same size throughout.
Binderbauer says that next year they will tear up C-2U again and build an almost entirely new machine, bigger and with even more powerful beams, dubbed C-2W. The aim is to achieve longer FRCs and, more crucially, higher temperature. A 10-fold increase in temperature would bring them into the realm of sparking reactions in conventional fusion fuel, a mixture of the hydrogen isotopes deuterium and tritium, known as D-T. But that is not their goal; instead, they’re working toward the much higher bar of hydrogen-boron fusion, which will require ion temperatures above 3 billion degrees Celsius.
Researchers have several reasons for wanting to go that extra mile. First, tritium doesn’t occur naturally on Earth, so it has to be made by bombarding lithium with neutrons. Physicists plan to do this in the fusion reactors that will one day consume the tritium, but no one has shown that such a process is practical. Because D-T reactions also produce large quantities of high-energy neutrons, the reactors need thick shielding. But the neutrons still degrade the structure of the reactor and make it radioactive. Researchers don’t yet know if it will be possible to find radiation-hard materials capable of surviving the onslaught. Many think these make D-T fusion impractical for a commercial reactor. “I wouldn’t have spent 10 years on [Tri Alpha’s advisory] committee if it was working on a D-T system,” Richter says.
Hydrogen-boron, at first, doesn’t look much more promising. “It takes 30 times as much energy to cook, and you get half as much energy out per particle,” Binderbauer says. But boron is abundant, and the reaction produces no neutrons, just three alpha particles (helium nuclei)—hence the company’s name. Hydrogen-boron fuel “makes conversion to electricity much easier and simpler,” Richter says.
Says one investor in the company, who asked not to be named, “for the first time since we started investing, with this breakthrough it feels like the stone is starting to roll downhill rather than being pushed up it.”