From: Massachusetts Institute of Technology
Published December 4, 2008 08:58 AM
MIT: New insights on fusion power
CAMBRIDGE, Mass.--Research carried out at MIT's Alcator C-Mod fusion
reactor may have brought the promise of fusion as a future power source a
bit closer to reality, though scientists caution that a practical fusion
powerplant is still decades away.
Fusion, the reaction that produces the sun's energy, is thought to have
enormous potential for future power generation because fusion plant
operation produces no emissions, fuel sources are potentially abundant, and
it produces relatively little (and short-lived) radioactive waste. But it
still faces great hurdles.
"There's been a lot of progress," says physicist Earl Marmar, division
head of the Alcator Project at the MIT Plasma Science and Fusion Center (PSFC).
"We're learning a lot more about the details of how these things work."
The Alcator C-Mod reactor, in operation since 1993, has the highest magnetic
field and the highest plasma pressure of any fusion reactor in the world,
and is the largest fusion reactor operated by any university.
One of the most vexing issues facing those trying to construct a fusion
plant that produces more power than it consumes (something never achieved
yet experimentally) is how to propel the hot plasma (an electrically charged
gas) around inside the donut-shaped reactor chamber. This is necessary to
keep it from losing its heat of millions of degrees to the cooler vessel
walls. Now, the MIT scientists think they may have found a way.
Physicist Yijun Lin and principal research scientist John Rice have led
experiments that demonstrate a very efficient method for using
radio-frequency waves to push the plasma around inside the vessel, not only
keeping it from losing heat to the walls but also preventing internal
turbulence that can reduce the efficiency of fusion reactions.
"That's very important," Marmar says, because presently used techniques to
push the plasma will not work in future, higher-power reactors such as the
planned ITER (International Thermonuclear Experimental Reactor) now under
construction in France, and so new methods must be found. "People have been
trying to do this for decades," he says.
Lin says that "some of these results are surprising to theorists," and as
yet there is no satisfying theoretical foundation for why it works as it
does. But the experimental results so far show that the method works, which
could be crucial to the success of ITER and future power-generating fusion
reactors. Lack of a controllable mechanism for propelling the plasma around
the reactor "is potentially a showstopper," Rice says, and the ITER team is
"very concerned about this."
Rice adds that "we've been looking for this effect for many years," trying
different variations of fuel mixture, frequency of the radio waves, and
other parameters. "Finally, the conditions were just right." Given that the
ITER project, which will take 10 years to build, is already underway, "our
results are just in time for this," Lin says. These results are being
published in Physical Review Letters on Dec. 5. The work was sponsored by
the US Department of Energy.
A number of other recent findings from Alcator C-Mod research could also
play a significant role in making fusion practical, and several papers on
these new results were presented at the Plasma Physics Divisional meeting of
the American Physical Society held in November.
One of these is a method developed by Dennis Whyte and Robert Granetz for
preventing a kind of runaway effect that could cause severe damage to
reactor components. When a fusion reactor is in operation, any disruption of
the magnetic field that confines the super-hot plasma could cause a very
powerful beam of "runaway electrons," with enough energy to melt through
solid steel. This would not be dangerous to personnel because everything is
well-shielded, but it could cause hardware damage that would be expensive
and time-consuming to repair.
But Whyte and Granetz have developed a kind of high-powered fire
extinguisher for such runaway beams: A way of suddenly injecting a blast of
argon or neon gas into the reactor vessel that turns the plasma energy into
light, which is then harmlessly absorbed by the reactor walls, and
suppresses the beam by apparently making the magnetic fields more
disorganized.
For about a thousandth of a second, Whyte says, this brilliant flash of
light is the world's brightest light — the equivalent of a billion-watt bulb
— though it's in a place where nobody can directly see it.
Because the Alcator C-Mod's design is very closely matched to that of ITER,
"we are uniquely positioned to explore what happens when these disruptions
occur," Whyte says. ITER will be 10 times the diameter, with a thousand
times the energy, so if this quenching system is used there it would produce
a trillion-watt bulb — for a fleeting instant, nearly equivalent to the
total electricity output of the United States.
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