A levitated dipole is a nuclear fusion experiment using a solid superconducting torus which is magnetically levitated inside the reactor chamber. It is believed that such an apparatus could contain plasma more efficiently than other fusion reactor designs. The superconductor forms an axisymmetric magnetic field of a nature similar to Earth's or Jupiter's magnetospheres. The machine was run in a collaboration between MIT and Columbia University.
Unlike other types of magnetically confined fusion, the Levitated Dipole is designed to be stable against "gentle" changes in the electric or magnetic field. 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.
The concept was first proposed by Dr. Jay Kesner (MIT) and Dr. Micheal Mauel (Columbia) in the mid to late nineties. The machine achieved first plasma on Friday, August 13, 2004 at 12:53 PM. This was done by successfully levitating the dipole and RF heating of the plasma created within the magnetic field of the dipole. The LDX team has since successfully conducted several levitation tests, including a 40-minute suspension of the superconducting coil on February 9, 2007. Shortly after, the coil was damaged in a control test in February 2007 and replaced ion May of 2007. The replacement coil was inferior, a copper wound electromagnet, that was also water cooled. Scientific results, including the observation of an inward turbulent pinch, were reported in Nature Physics.
This experiment needed a unique, free floating, electromagnet. The magnetic field was originally made of two counter wound ring of currents. Each ring contained a 19 strand rutherford cable (common in superconducting magnets). Each strand was Niobium-tin. These looped around inside a inconel donut magnet. The donut was charged inductively and once charged it generated a magnetic field for roughly an 8 hour period. Overall, the ring weighed 450 kilograms and levitated 1.6 meters above a superconducting ring. The ring produced roughly a 5 tesla field. This superconductor was encased inside a liquid helium, which kept the electromagnet below 10 degrees kelvin. This design is similar to the D20 dipole experiment at Berkeley and the RT-1 experiment at the University of Tokyo.
The dipole was suspended inside a mushroom-shaped vacuum chamber, which was about 5 meters in diameter and ~3 meters high. At its base was a charging coil. The dipole would charge here. Next, the dipole was raised into the center of the chamber, which could be done with supports. Around the outside of this chamber were Helmholtz coils, which were used to produce a uniform surrounding magnetic field. This external field would interact with the dipole field, suspending the dipole. It was in this surrounding field that plasma moved. The plasma is formed inside a vacuum around the dipole. It is heated using a radio frequency field, essentially microwaving the plasma in a 17-kilowatt field.
The machine was monitored using diagnostics fairly standard to all of fusion. These included:
- A flux loop. A thin loop of wire inside the path of the magnetic field. As the field varied inside the loop, it generated a current. This was measured and from the signal the magnetic flux was measured.
- An X-ray detector. This diagnostic measured the X-rays emitted. From this, the plasmas' temperature was found. There were four of these inside the machine, each measuring along a line. This detector was good for measuring electrons, typically around 100 electron-volts. All plasma loses energy by emitting light. This covers the whole spectrum: visible, IR, UV, and X-rays. This occurs anytime a particle changes speed, for any reason. If the reason is deflection by a magnetic field, the radiation is Cyclotron radiation at low speeds and Synchrotron radiation at high speeds. If the reason is deflection by another particle, plasma radiates X-rays, known as Bremsstrahlung radiation.
- An X-ray Camera. This can read lower energy X-rays.
- A Conventional Video Camera 
- An emissive Langmuir probe. A Langmuir probe is a wire, stuck into a plasma, which absorbs the surrounding charged particles. You can vary the voltage on this wire. As the voltage changes, the charged particles absorbed change, making an IV curve. This can be read and used to measure the density and temperature of the nearby plasma.
- A triple Langmuir probe
- A dozen Langmuir probes grouped together
Single particles corkscrew along the field lines, flowing around the dipole electromagnet. This leads to a giant encapsulation of the electromagnet. As material passes through the center, the density spikes. This is because lots of plasma is trying to squeeze through a limited area. This is where most of the fusion reactions occur. This behavior has been called a turbulent pinch.
In large amounts, the plasma formed two shells around the dipole: a low density shell, occupying a large volume and a high density shell, closer to the dipole. This is shown here. The plasma was trapped fairly well. It gave a maximum beta number of 0.26. A value of 1 is ideal.
Modes of Operation
There were two modes of operation observed:
- Hot electron interchange: a lower density, mostly electron plasma.
- A more conventional Magnetohydrodynamic mode
In the case of deuterium fusion (the cheapest and most straightforward fusion fuel) the geometry of the LDX has the unique advantage over other concepts. Deuterium fusion makes two products, that occur with near equal probability:
D + D → T + 1H D + D → 3He + n
In this machine, the secondary tritium could be partially removed, a unique property of the dipole. Another fuel choice is tritium and deuterium. This reaction can be done at lower heats and pressures. But it has several drawbacks. First, tritium is far more expensive than deuterium. This is because tritium is rare. It has a short half-life making it hard to produce and store. It is also considered a hazardous material, so using it is a hassle from a health, safety and environmental perspective. Finally, tritium and deuterium produces fast neutrons which means any reactor burning it would require heavy shielding.
- "MIT tests unique approach to fusion power". MIT News, David Chandler, MIT News Office, March 19, 2008. Accessed March 2008
- "LDX funding canceled". Retrieved June 27, 2012.
- Hasegawa, A., Comments on Plasma Physics and Controlled Fusion, 1987, vol. 1, p. 147.
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- "LDX begins first plasma experiments". Levitated Dipole Experiment. Retrieved April 3, 2007.
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- "Turbulent Transport in a Laboratory Magnetospheric Dipole" European Physical Society 38th Conference on Plasma Physics, Strasbourg, France June 28, 2011
- presentation"LDX Machine Design and Diagnostics" APS DPP meeting 1998, Garnier and Mauel
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- "X-Ray Diagnostics for the Levitated Dipole Experiment" Jennifer L. Ellsworth, Master's Thesis, MIT 2004
- J. Larmor, "On a dynamical theory of the electric and luminiferous medium", Philosophical Transactions of the Royal Society 190, (1897) pp. 205–300 (Third and last in a series of papers with the same name).
- "Diagnostic setup for spatial and temporal measurements of plasma fluctuations using electric probes in the LDX" E Ortiz, M Mauel, D Garnier, 45th DPP meeting, October 2003
- "Overview of LDX Results" Jay Kesner, A. Boxer, J. Ellsworth, I. Karim, Presented at the APS Meeting, Philadelphia, November 2, 2006, Paper VP1.00020
- "Improved Confinement During Magnetic Levitation in LDX", 50th Annual Meeting of the APS DDP, November 18, 2008 M Manuel
- "Helium Catalyzed D-D Fusion in a Levitated Dipole" Presentation Kesner, Catto, Krasheninnikova APS 2005 DPP Meeting, Denver
- "Stabilization of Hot Electron Plasma by a Cold Background" N Krall, Phys. Fluids 9, 820 (1966)
- "Fusion Technologies for Tritium-Suppressed D-D Fusion" White Paper prepared for FESAC Materials Science Subcommittee, M. E. Mauel and J. Kesner, December 19, 2011