A levitated dipole is a type of nuclear fusion reactor design using a superconducting torus which is magnetically levitated inside the reactor chamber. The name refers to the magnetic dipole that forms within the reaction chamber, similar to Earth's or Jupiter's magnetospheres. It is believed that such an apparatus could contain plasma more efficiently than other fusion reactor designs.
The Levitated Dipole Experiment (LDX) was funded by the US Department of Energy's Office of Fusion Energy. The machine was run in a collaboration between MIT and Columbia University. Funding for the LDX was ended in November 2011 to concentrate resources on tokamak designs.
The Earth's magnetic field is generated by the circulation of charges in the Earth's molten core. The resulting magnetic dipole field forms a shape with magnetic field lines passing through the Earth's center, reaching the surface near the poles and extending far into space above the equator. Charged particles entering the field will tend to follow the lines of force, moving north or south. As they reach the polar regions, the magnetic lines begin to cluster together, and this increasing field can cause particles below a certain energy threshold to reflect, and begin travelling in the opposite direction. Such particles bounce back and forth between the poles until the collide with other particles. Particles with greater energy continue towards the Earth, impacting the atmosphere and causing the aurora.
This basic concept is used in the magnetic mirror approach to fusion energy. The mirror uses a solenoid to confine the plasma in the center of a cylinder, and then two magnets at either end to force the magnetic lines closer together to create reflecting areas. One of the most promising of the early approaches to fusion, the mirror ultimately proved to be very "leaky", with the fuel refusing to properly reflect from the ends as the density and energy were increased. Annoyingly, it was the particles with the most energy, those most likely to undergo fusion, that preferentially escaped. Research into large mirror machines ended in the 1980s as it became clear they would not reach fusion breakeven in a practically sized device.
The levitated dipole can be thought of, in some ways, as a toroidal mirror, much more similar to the Earth's field than the linear system in a traditional mirror. In this case, the confinement area is not the linear area between the mirrors, but the toroidal area around the outside of the central magnet, similar to the area around the Earth's equator. Particles in this area that move up or down see increasing magnetic density and tend to move back towards the equator area again. This gives the system some level of natural stability. Particles with higher energy, the ones that would escape a traditional mirror, instead follow the field lines through the hollow center of the magnet, recirculating back into the equatorial area again.
This makes the Levitated Dipole unique when compared with other magnetic confinement machines. In those experiments, small fluctuations can cause significant energy loss. By contrast, in a dipolar magnetic field, fluctuations tend to 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.
Adapting this concept to a fusion experiment was first proposed by Dr. Jay Kesner (MIT) and Dr. Michael Mauel (Columbia) in the mid to late nineties. The pair assembled a team and raised money to build the machine. They achieved first plasma on Friday, August 13, 2004 at 12:53 PM. First plasma was done by (1) successfully levitating the dipole magnet and (2) RF heating the plasma. 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 in May 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 very special free-floating electromagnet, which created the unique "toilet-bowl" magnetic field. The magnetic field was originally made of two counter-wound rings of currents. Each ring contained a 19-strand niobium-tin Rutherford cable (common in superconducting magnets). These looped around inside a Inconel magnet; a magnet that looked like an oversized donut. The donut was charged using induction. 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 kelvins. 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 the base of the chamber was a charging coil. This coil is used to charge the dipole, using induction. The coil exposing the dipole to a varying magnetic field. Next, the dipole is raised into the center of the chamber. This could be done with supports or using the field itself. 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 forms around the dipole and inside the chamber. The plasma is formed by heating a low pressure gas. The gas is heated using a radio frequency, 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. This is a loop of wire. The magnetic field passes through the wire loop. 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 cord (or line out) inside the machine. 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.
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