A New Road to Fusion?
May 18, 2010 - David Chandler - technologyreview.com
After a decade of work conceiving, designing, building, and testing a new type of fusion reactor, a team of MIT and Columbia scientists published their first successful experimental results this year in Nature Physics.
The idea for the novel reactor was inspired by studies of planets--specifically, the magnetic fields of Earth and Jupiter, which are basic dipoles. Recent results from the Levitated Dipole Experiment (LDX) show that it holds promise in the quest to produce power from controlled nuclear fusion--the process that generates the prodigious energy output of the sun.
Practical fusion power has been a cherished goal of physicists and energy researchers for more than 50 years. It offers the tantalizing possibility of producing a nearly endless supply of energy with no carbon emissions. But developing a fusion reactor that produces more energy than is required to run it has proved more challenging than initially thought.
The idea is to get light atoms, usually forms of hydrogen, to fuse together into heavier elements, releasing huge amounts of energy in the process. That requires overcoming the atoms' natural electrical repulsion by getting the gases extremely hot and compressing them dramatically. Most experimental fusion reactors have been either tokamaks, which use a complex doughnut-shaped magnetic field to confine a very hot ionized gas called a plasma, or inertial systems that use powerful lasers to cause a fuel pellet to implode. But the LDX reactor, a joint project of MIT and Columbia University that is housed in MIT's Plasma Science and Fusion Center, uses the simplest kind of magnetic field: a dipole like Earth's. This is produced by a half-ton doughnut-shaped magnet about the size and shape of a large truck tire, made of superconducting wire coiled inside a stainless-steel vessel.
Plasmas are naturally turbulent, and in reactors such as tokamaks, that turbulence tends to make the atoms spread out so that they are less inclined to fuse. But observations of the way plasmas in space interact with planetary magnetic fields have revealed that under those conditions, turbulence actually pulls the plasma more closely together. This counterintuitive phenomenon, which had never previously been re-created in the laboratory, was confirmed by results from the early LDX experiments.
One key to achieving "turbulent pinching" in the reactor is that its magnet is suspended by an electromagnet overhead rather than being held in place by any physical structure. Any such objects could disturb the magnetic field used to pinch the plasma. In the experimental runs, the researchers created the same temperature and pressure conditions with and without a support system in place. They confirmed that the levitated mode produced dramatically greater pinching.
LDX might be a boon for basic science as well as for energy production. Jay Kesner, MIT's physics research group leader for the project, says it could reveal aspects of the behavior of planetary magnetic fields "[with] a lot more subtle detail than you can get by launching satellites, and more cheaply."