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A cutaway view of
the magnets and internal components inside the ITER vacuum vessel.
Whyte’s method for soft-landing plasmas may alleviate costly
damage to interior reactor walls and could “clean”
residual tritium without disrupting operation.
(View
larger image)
(© 2005 www.kennisinbeeld.nl)
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GOOD HOUSEKEEPING:
New method calms unruly plasmas, cleans reactors
new
method developed at UW-Madison could, in a tokamak reactor, alleviate
the damaging effects of a plasma gone awry—while at the same time,
“clean” the buildup of radioactive tritium off the reactor’s
interior walls.The method could save engineers
who operate large experiments like ITER (formerly, the International
Thermonuclear Experimental Reactor) both time and money.
In a tokamak, which uses a toroidal, or doughnut-shaped,
magnetic field to confine the super-hot ionized gas called a plasma,
sometimes the plasma builds up so much pressure that it breaks the “magnetic
bottle” and crashes into the reactor wall. While it’s completely
safe outside the reactor, the plasma hits with such vigor that it can
damage the reactor’s internal components. When that happens, engineers
must stop operations, open the reactor, and repair the materials.
This plasma crash, says Assistant Professor Dennis
Whyte, is typical in an experimental reactor. “You’ve
pushed the parameters too far, some instability has arisen, and it’s
coming,” he says, of the impending crash. “So do you just
let it happen, or do you something to help alleviate it?”
In Whyte’s case, you turn the culprit—the
plasma itself—into the solution. Anticipating the crash, he shoots
a neutral, radiation-emitting gas like neon at the plasma. Within a
few thousandths of a second, the number of particles inside the reactor
multiply on the order of a factor of 100. “It completely overwhelms
the plasma and essentially turns the plasma into an enormous source
of light,” says Whyte. “The energy of the plasma gets converted
into light energy.”
Rather than colliding into one area of the reactor’s
interior wall, the newly created light energy coats the whole wall uniformly.
“So what you do is you spread the pain, and if you do it efficiently
enough, you stop any damage to the materials that are inside,”
says Whyte.
Working first at the DIII-D National Fusion Facility
at General Atomics in San Diego, and more recently at the Alcator C-MOD
tokamak at the Massachusetts Institute of Technology, Whyte and his
colleagues showed that after this “soft” termination of
the experiment, researchers could resume additional experiments immediately.
Next year, they will further their solution in Oxford, England, when
they have research time scheduled on JET, the world’s largest
tokamak. Particularly important, he says, is that the group is learning
how to design such a system for ITER.
ITER has about 1,000 square meters of interior material
surface, but in a natural disruption—something like a balloon
bursting—the plasma slams into an area of only 10 or 20 square
meters. “We realized for a long time that disruptions are going
to be a real concern for a device like ITER,” says Whyte.
Another issue in large experimental reactors like
ITER, he says, is tritium retention. A man-made radioactive hydrogen
isotope with only a 12 1/2-year half life, tritium is one of two fusion
reactor fuels. As a reactor operates, the unused fuel must be efficiently
recirculated through the system, but the tritium can become trapped
in the reactor walls. “So what happens is you have this repository
of tritium inside of the vacuum vessel,” says Whyte.
As a result, occasionally engineers need to stop operations,
open the reactor, and clean the tritium out—a process that eats
up valuable experimental time. “It turns out, though, that there’s
a very well-known way to get tritium out of materials,” he says.
“You heat the materials.”
Plasmas create very thin film layers on the reactor’s
interior materials, and the residual tritium resides within those layers.
So by exploiting the soft termination of a plasma, which uniformly distributes
energy inside the reactor, Whyte hit on a tritium-removal process that’s
much like a self-cleaning oven. “The plasma’s energy forces
the hydrogen molecules inside of the film layers to recombine and forces
them out by diffusion,” he says. “It basically cleans out
all of the hydrogen, including the tritium.”
Whyte proposed the cleaning method a year ago at the
Plasma Surface Interactions in Controlled Fusion Devices meeting in
Portland, Maine. Since then, he and his MIT colleagues conducted experiments
on the MIT reactor and showed the method cleans tritium better than
other techniques they tried. “We used the disruptions in a controlled
way to force the fuel out of the wall, and then we did a very careful
particle accounting,” he says. “We showed that, in fact,
what we were doing was forcing more hydrogen out of the wall than we
were leaving behind.”
They also showed that you don’t have to wait
for a plasma disruption to come along. Rather, reactor operators could
use the technique as they’re ending their experiments. “The
plasma’s shutting down anyway—you just shut it down a little
bit earlier than you normally would, and it’s very soft, and you
get all the tritium that you just lost in the last discharge,”
says Whyte. “It takes no external intervention.”
