<<返回上一页

Nuclear fusion gets quadruple boost

发布时间:2019-03-06 14:18:01来源:未知点击:

By Eugenie Samuel, Boston The decades-long effort to build a nuclear fusion reactor has received a major boost. In experiments at the US National Fusion Facility in San Diego, researchers have quadrupled the rate of fusion in superhot deuterium gas. Fusion reactors aim to reproduce the Sun’s power source, but the problem is containing the hot plasma. The San Diego team achieved more stable containment and higher pressure by carefully manipulating the magnetic fields that control the spinning plasma. This brings us a step closer to a commercial reactor that could provide enormous amounts of energy with hardly any pollution or waste. The team, which includes researchers from Columbia and Princeton universities, as well as General Atomics of San Diego and others, is using DIII-D, a tokamak reactor whose heart is a doughnut-shaped cavity 4.5 metres in diameter. Inside the cavity a plasma of deuterium is heated to 100 million kelvin and held in place with powerful magnetic fields. Deuterium is a heavy isotope of hydrogen, and when its nuclei collide under this intense pressure, some of them fuse to form helium, releasing large amounts of energy. The goal of fusion research is a reactor that produces much more energy than the large amounts needed to run it. The experimental tokamaks that exist around the world, such as the Joint European Torus (JET) reactor at Culham near Oxford, have to date not progressed far beyond the break-even point. Early theoretical and experimental results suggested that there is a limit to how pressurised the plasma can be before it begins to bulge unpredictably. But then other work in the early 1990s suggested that you could solve the problem by spinning the plasma around the cavity as if it were a racetrack. Experiments at Columbia and DIII-D had shown it was easy to set the plasma spinning, but it tended to slow down and become unstable again. Now the DIII-D team has found out why: the plasma was magnifying tiny imperfections in the magnetic field that contained it. So they fitted sensors to detect the imperfections – some as weak as the Earth’s magnetic field – and then corrected them with arrays of magnets in the cavity controlled via feedback loops. “It takes very little power because the errors are about one part in a thousand,” says Ronald Stambaugh of General Atomics. The team found that the spinning plasma did not slow down and they could ramp up the pressure to twice the previous limit, quadrupling the rate of fusion. Rob Goldston, who worked on Princeton’s tokamak until it was closed in 1997, says he is very excited by the result. “This is a very deep insight into the behaviour of stable plasmas.” DIII-D is only about one-eighth the size you’d need for a commercial reactor, and such a reactor would have to run on a mixture of deuterium and its heavier sibling tritium. Despite these differences, Stambaugh believes the principle will work in a commercial model. “We’re doing this research with the belief that the physics will transfer,” he says. Researchers in Europe, Japan, Russia and Canada are now lobbying governments to fund a prototype called ITER that would produce power (New Scientist, 14 October 2000, p 4). Michael Watkins of JET says that the work done at Culham, combined with the DIII-D team’s method, should have real benefits for the ITER project. “Tokamak research is in a very strong position now,