![Plasma in the Levitating Dipole Experiment](https://upload.wikimedia.org/wikipedia/commons/7/72/Plasma_in_the_Levitating_Dipole_Experiment.png)
A levitated dipole is a type of
nuclear fusion
Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...
reactor design using a
superconducting
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
torus
In geometry, a torus (plural tori, colloquially donut or doughnut) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis that is coplanar with the circle.
If the axis of revolution does not tou ...
which is
magnetically
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...
levitated inside the reactor chamber. The name refers to the
magnetic dipole
In electromagnetism, a magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the size of the source is reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the electric ...
that forms within the reaction chamber, similar to
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's or
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...
's
magnetospheres. It is believed that such an apparatus could contain
plasma more efficiently than other fusion reactor designs. The concept of the levitated dipole as a
fusion reactor
Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices ...
was first theorized by
Akira Hasegawa in 1987.
Concept
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 they collide with other particles. Particles with greater energy continue towards the Earth, impacting the atmosphere and causing the
aurora
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
.
This basic concept is used in the
magnetic mirror
A magnetic mirror, known as a magnetic trap (магнитный захват) in Russia and briefly as a pyrotron in the US, is a type of magnetic confinement device used in fusion power to trap high temperature plasma using magnetic fields. T ...
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
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, '' Voyager 1'', o ...
encounter with Uranus.
Examples
The concept of the levitated dipole was first realized when Jay Kesner of
MIT
The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the m ...
and Michael Mauel of
Columbia University
Columbia University (also known as Columbia, and officially as Columbia University in the City of New York) is a private research university in New York City. Established in 1754 as King's College on the grounds of Trinity Church in Manhatt ...
made a joint proposal to test the concept in 1997. This led to the development of two experiments: the
Levitated Dipole Experiment (LDX) at MIT and the Collisionless Terrella Experiment (CTX) at Columbia University.
See also
*
List of plasma physics articles
This is a list of plasma physics topics.
A
* Ablation
* Abradable coating
* Abraham–Lorentz force
* Absorption band
* Accretion disk
* Active galactic nucleus
* Adiabatic invariant
* ADITYA (tokamak)
* Aeronomy
* Afterglow plasma
* Air ...
References
{{Fusion power
Plasma physics
Magnetic confinement fusion