A levitated dipole is a proposed nuclear fusion reactor technology using a solid superconducting torus, magnetically levitated in the reactor chamber. The superconductor forms an axisymmetric magnetic field of a nature similar to Earth's or Jupiter's magnetospheres, and it is believed that such an apparatus could contain plasma more efficiently than other fusion reactor designs.
On Friday, August 13, 2004 at 12:53 PM, the Levitated Dipole Experiment, a collaboration between Columbia University and MIT, successfully energized a superconducting torus with RF and momentarily created plasma within the magnetic field of the dipole. The LDX team has since successfully conducted its first levitation tests, including a 40-minute suspension of the superconducting coil on February 9, 2007. Scientific results, including the observation of an inward turbulent pinch, were reported in Nature Physics.
The Levitated Dipole Experiment was funded by the United States Department of Energy's Office of Fusion Energy, but funding for the LDX and other alternative fusion projects was ended in November 2011 to concentrate funding on tokamak approaches. MIT and Columbia are attempting to find other funding sources.
Unlike other types of magnetically confined fusion, the Levitated Dipole is designed to be robust to external fluctuations in electric/magnetic fields. In most laboratory plasmas, small fluctuations can cause significant energy loss; however in a dipolar magnetic field, fluctations tend to actually compress the plasma without energy loss. This compression effect was first noticed by Akira Hasegawa (of the Hasegawa-Mima equation) after participating in the Voyager 2 encounter with Uranus.
A power source based on the LDX device would utilize an advanced fuel cycle known as the tritium suppressed DD reaction (deuterium-deuterium (D-D) reaction). In this fuel cycle the secondary tritium is removed from the plasma, a unique capability of a dipole. This has advantages relative to the more conventional deuterium-tritium (D-T) reaction of other confinement and compression devices such as the tokamak or inertial confinement fusion (ICF) devices that amplify and focus multiple high-energy beams of lasers, electrons or ions onto tiny pellets of D-T fuel.
Although heat and pressure requirements for D-D fusion are more difficult than D-T fusion, the payoff of the D-D reaction is the relative absence of "fast neutrons" produced by the D-T reaction that contaminate the containment vessel and require massive shielding. In addition, deuterium is readily available in sea water, unlike tritium, which must be bred in reactors before it can be used as fuel in the D-T reaction.
- "MIT tests unique approach to fusion power". MIT News, David Chandler, MIT News Office, March 19, 2008. Accessed March 2008
- "LDX begins first plasma experiments". Levitated Dipole Experiment. Retrieved April 3, 2007.
- "LDX levitation tests successful". Levitated Dipole Experiment. Retrieved February 9, 2007.
- "Turbulent inward pinch of plasma confined by a levitated dipole magnet". Nature Physics. Bibcode:2010NatPh...6..207B. doi:10.1038/nphys1510. Retrieved January 24, 2010.
- "LDX funding canceled". Retrieved June 27, 2012.