A New Fusion Project Fires Up
MIT and Columbia University are trying a new approach to nuclear fusion with the Levitated Dipole Experiment
By Mara E. Vatz
Deep in the bowels of Building NW-21, a former Nabisco facility on the west side of the MIT campus, is a three-story-tall, five-meter-wide metal structure that resembles a flying saucer. Inside it is a 560-kilogram, doughnut-shaped superconducting magnet that will float 1.5 meters off the ground when another magnet above it is charged. This is the playground of the Levitated Dipole Experiment (LDX), a collaboration between MIT and Columbia University dedicated to developing a new approach to nuclear fusion.
The LDX project, which began in 1998 with funding from the U.S. Department of Energy, is a newcomer to the field of fusion. Down the hall in the same building, Alcator C-Modthe third in a series of MIT fusion projects that began in 1971has a much larger staff and budget. But the LDX, unlike Alcator, isnt trying to create fusion conditions or a fusion deviceyet. For now, its researchers are focused on a fairly basic question that could have far-reaching results: how stable is magnetically pulled plasma?
Realizing the Fusion Dream
Fusion could someday provide an ideal alternative to fission, the type of nuclear reaction used in todays nuclear power plants. For one thing, fusion runs on hydrogen, a virtually limitless fuel, while fission requires uranium or plutonium, rare elements that carry the added risk of nuclear-weapons proliferation. And while a malfunction in a fission reactor can have devastating consequences, fusion reactions are by nature extremely difficult to sustain; they therefore pose little threat of propagating out of control and causing large-scale disasters.
But a practical method for producing fusion reactions has long eluded scientists. Ever since the hydrogen bomb was exploded in 1952, explains Jay Kesner, a leader of the LDX projects research team, people have been trying to learn how to confine plasma and make energy from hydrogen fusionand to do it in a controlled way so that you can make electricity. One of the key problems is that fusion fuelplasma consisting of hydrogen nuclei that have been stripped of their electronsis extremely difficult to confine and control.
When hydrogen plasma reaches sunlike temperatures, nuclei can collide and undergo nuclear fusion reactions. The nuclei of two hydrogen isotopes meld to produce a helium nucleus along with a highly energetic neutron, which can be captured and used to heat water. To ensure that the hydrogen nuclei collide, you have to hold the plasma together and make it very dense and very hot, says Kesner.
In the most successful fusion projects to date, including Alcator, magnetic coils on the outside of a doughnut-shaped chamber create a magnetic field that tends to steer the moving plasma particles along fixed trajectories. But that can get tricky. Plasma, like a gas, will expand into the space available. It also has the ability to take on collective motionthat is, to move in waveswhich can cause the particles to fly off the magnetic-field lines and lose energy by colliding with the chamber walls. To further complicate matters, a hot plasma creates its own magnetic fields that counteract the confinement fields applied by the external magnets. [Fusion] is one of these phenomena that, the closer you get, the harder it gets, Kesner says.
A Magnetic Star
The LDX may make possible a significant improvement in plasma confinement. Rather than using external magnets to push on the plasma, the LDX uses an internal dipole magnet to restrain it. The resulting plasma cloud will resemble a star, which is just a giant natural fusion device held together by gravity, explains Darren Garnier, PhD 96, a Columbia research scientist who heads experimental operations for the LDX project. But instead of using gravity to confine the plasma, the LDX will use a magnetic field. Basically, were trying to make a magnetic star, Garnier says.
The LDX is not the first fusion project to confine plasma by pulling on it, but it will be the first to use a floating magnet. In earlier, nonlevitated-dipole experiments, the magnets physical supports interfered with the magnetic-field lines and therefore with the confined plasma, causing a significant loss of energy. The LDXs floating dipole will have no such interference.
According to theory, a fusion reaction in which plasma is pulled into place should be more stable than one in which its pushed. The LDX groups initial experiments, which began last fall and still rely on a physically supported dipole, are beginning to test that theory. The real test will come this summer when the researchers levitate the dipole for the first time. Everyone is still waiting with bated breath for our levitation results, says Garnier. If all goes well, within a few months the scientists will know whether their design could help make fusion energy a reality.