Levitating magnet brings space physics to fusion
Tests on an experimental machine that mimics a planet’s magnetic field show that it may offer an ‘alternative path’ to taming nuclear fusion for power generation.
January 25, 2010
A new experiment that reproduces the magnetic fields of the Earth and other planets has yielded its first significant results. The findings confirm that its unique approach has some potential to be developed as a new way of creating a power-producing plant based on nuclear fusion — the process that generates the sun’s prodigious output of energy.
The new results come from an experimental device on the MIT campus, inspired by observations from space made by satellites. Called the Levitated Dipole Experiment, or LDX, a joint project of MIT and Columbia University, it uses a half-ton donut-shaped magnet about the size and shape of a large truck tire, made of superconducting wire coiled inside a stainless steel vessel. This magnet is suspended by a powerful electromagnetic field, and is used to control the motion of the 10-million-degree-hot electrically charged gas, or plasma, contained within its 16-foot-diameter outer chamber.
The results, published this week in the journal Nature Physics, confirm the counter-intuitive prediction that inside the device’s magnetic chamber, random turbulence causes the plasma to become more densely concentrated — a crucial step to getting atoms to fuse together — instead of becoming more spread out, as usually happens with turbulence. This “turbulent pinching” of the plasma has been observed in the way plasmas in space interact with the Earth’s and Jupiter’s magnetic fields, but has never before been recreated in the laboratory.
When operating, the huge LDX magnet is supported by the magnetic field from an electromagnet overhead, which is controlled continuously by a computer based on precision monitoring of its position using eight laser beams and detectors. The position of the half-ton magnet, which carries a current of one million amperes (compared to a typical home’s total capacity of 200 amperes) can be maintained this way to within half a millimeter. A cone-shaped support with springs is positioned under the magnet to catch it safely if anything goes wrong with the control system.
Levitation is crucial because the magnetic field used to confine the plasma would be disturbed by any objects in its way, such as any supports used to hold the magnet in place. In the experimental runs, they recreated the same conditions with and without the support system in place, and confirmed that the confinement of the plasma was dramatically increased in the levitated mode, with the supports removed. With the magnet levitated, the central peak of plasma density developed within a few hundredths of a second, and closely resembled those observed in planetary magnetospheres (such as the magnetic fields surrounding Earth and Jupiter).
Summarizing the difference between the two approaches, Kesner explains that in a tokamak, the hot plasma is confined inside a huge magnet, but in the LDX the magnet is inside the plasma. The whole concept, he says, was inspired by observations of planetary magnetospheres made by interplanetary spacecraft. In turn, he says, for planetary research the experiments in LDX can yield “a lot more subtle detail than you can get by launching satellites, and more cheaply.”