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鈥檚 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.
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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鈥攕ome as weak as the
Earth鈥檚 magnetic field鈥攁nd then corrected them with arrays of magnets in
the cavity controlled via feedback loops. 鈥淚t takes very little power because
the errors are about one part in a thousand,鈥� 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鈥檚 tokamak until it was closed in 1997,
says he is very excited by the result. 鈥淭his is a very deep insight into the
behaviour of stable plasmas.鈥�
DIII-D is only about one-eighth the size you鈥檇 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. 鈥淲e鈥檙e doing this research with the belief that
the physics will transfer,鈥� he says.
Researchers in Europe, Japan, Russia and Canada are now lobbying governments
to fund a prototype called ITER that would produce power
(快猫短视频, 14 October 2000, p 4).
Michael Watkins of JET says that the work done at
Culham, combined with the DIII-D team鈥檚 method, should have real benefits for
the ITER project. 鈥淭okamak research is in a very strong position now,鈥� he says.
