MIT tests unique approach to fusion power
Posted on Saturday, March 29, 2008 @ 20:45:24 UTC by vlad
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 An MIT and Columbia University team has successfully tested a novel
reactor that could chart a new path toward nuclear fusion, which could
become a safe, reliable and nearly limitless source of energy.
Begun in 1998, the Levitated
Dipole Experiment, or LDX, uses a unique configuration where its main
magnet is suspended, or levitated, by another magnet above. The system
began testing in 2004 in a "supported mode" of operation, where the
magnet was held in place by a support structure, which causes
significant losses to the plasma--a hot, electrically charged gas where
the fusion takes place.
LDX achieved fully levitated
operation for the first time last November. A second test run was
performed on March 21-22 of this year, in which it had an improved
measurement capability and included experiments that clarified and
illuminated the earlier results. These experiments demonstrate a
substantial improvement in plasma confinement--significant progress
toward the goal of producing a fusion reaction-- and a journal article
on the results is planned.
Fusion--the process that provides the sun's energy--occurs when two
types of atoms fuse, creating a different element (typically helium)
and releasing energy. The reactions can only occur at extremely high
temperatures and pressures. Because the material is too hot to be
contained by any material, fusion reactors work by holding the
electrically charged gas, called plasma, in place with strong magnetic
fields that keep it from ever touching the walls of the device.
The LDX reactor reproduces the conditions necessary for fusion by
imitating the kind of magnetic field that surrounds Earth and Jupiter.
A joint project by MIT and Columbia University, it consists of a
supercooled, superconducting magnet about the size and shape of a large
truck tire. When the reactor is in operation, this half-ton magnet is
levitated inside a huge vacuum chamber, using another powerful magnet
above the chamber to hold it aloft.
The advantage of the levitating system is that it requires no
internal supporting structure, which would interfere with the magnetic
field lines surrounding the donut-shaped magnet, explains Jay Kesner of
MIT's Plasma Science and Fusion Center, joint director of LDX with
Michael Mauel of Columbia. That allows the plasma inside the reactor to
flow along those magnetic field lines without bumping into any
obstacles that would disrupt it (and the fusion process).
To produce a sustained fusion reaction the right kinds of materials
must be confined under enormous, pressure, temperature and density. The
"fuel" is typically a mix of deuterium and tritium (known as a D-T
cycle), which are two isotopes of hydrogen, the simplest atom. A normal
hydrogen atom contains just one proton and one electron, but deuterium
adds one neutron, and tritium has two neutrons. So far, numerous
experimental reactors using different methods have managed to produce
some fusion reactions, but none has yet achieved the elusive goal of
"breakeven," in which a reactor produces as much energy as it consumes.
To be a practical power source, of course, will require it to put out
more than it consumes.
If that can be achieved, many people think it could provide an
abundant source of energy with no carbon emissions. The deuterium fuel
can be obtained from seawater and there is a virtually limitless
supply.
Most fusion experiments have
been conducted inside donut-shaped (toroidal) chambers surrounded by
magnets, a design that originated in the Soviet Uni0n and is called by
the Russian name Tokamak. MIT also operates the most powerful Tokamak
reactor in the United States, the Alcator C-mod, which is located in
the same building as the new LDX reactor. Tokamaks require a large
number of magnets around the wall of the torus, and all of them must be
working properly to keep the plasma confined and make fusion possible.
The new approach to fusion being tested in the LDX is the first to
use the simplest kind of magnet, a dipole--one that has just two
magnetic poles, known as north and south, just like the magnetic fields
of Earth and Jupiter. Tokamaks and other fusion reactor designs use
much more complex, multi-poled magnetic fields to confine the hot
plasma.
Unlike the Tokamak design, in which the magnetic field must be
narrowed to squeeze the hot plasma to greater density, in a dipole
field the plasma naturally gets condensed, Kesner explains. Vibrations
actually increase the density, whereas in a Tokamak any turbulence
tends to spread out the hot plasma.
The renowned physicist Richard Feynman once compared confining a
plasma inside the magnetic field in a Tokamak to "trying to hold Jell-O
with rubber bands," says LDX chief experimentalist Darren Garnier of
Columbia. "It's the difference between pulling and pushing." Whereas
the Tokamak's magnetic field tries to push the plasma in from the
outside, "we have the field lines on the inside, pulling on the
plasma," which is inherently more stable, he says.
Another potential advantage of the LDX approach is that it could
use a more advanced fuel cycle, known as D-D, with only deuterium.
Although it's easier to get a self-sustaining reaction with D-T,
tritium doesn't exist naturally and must be manufactured, and the
reaction produces energetic neutrons that damage the structure. The D-D
approach would avoid these problems.
The LDX magnet has coils made of superconducting niobium-tin alloy,
which loses all electrical resistance when cooled below about 15
degrees Kelvin; in the device, it is cooled to 4 degrees Kelvin--4
degrees above absolute zero, or minus 269 degrees Celsius, a
temperature that can only be achieved by surrounding the coils with
liquid helium. This is the only superconducting magnet currently used
in any U.S. fusion reactor.
When in full operation, the frigid magnet, contained in a
double-walled vessel that is essentially a large thermos bottle, is
surrounded by plasma heated to millions of degrees Celsius. Garnier
says that in full operation, the system is quite literally a snowball
in hell.
Besides providing data that might someday lead to a practical
fusion reactor, the experimental device could provide important lessons
about how planetary magnetic fields work, which is still poorly
understood. So the experiment is of great interest to planetary
physicists as well as to energy researchers.
Keeping the huge magnet levitated to just the right height requires
a feedback system that constantly monitors its position, using eight
laser beams, and then adjusts the power of the lifting magnet
accordingly. "That was tricky to develop," Garnier says, and in early
experiments "we did drop it a couple of times." Fortunately, they had
designed the structure with a spring-mounted lifter under the magnet,
used to lift it into its starting position, which could absorb the
falling weight without damage.
Source: MIT
Via: http://www.physorg.com/news125929881.html
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