Levitated Dipole Experiment | |
---|---|
Device type | Levitated dipole |
Location | Cambridge, Massachusetts, United States |
Affiliation | MIT Plasma Science and Fusion Center, Columbia University |
Technical specifications | |
Major radius | 0.34 m (1 ft 1 in) |
History | |
Year(s) of operation | 2004–2011 |
Related devices | Collisionless Terrella Experiment (CTX) |
Links | |
Website | The Levitated Dipole eXperiment website |
The Levitated Dipole Experiment (LDX) was an experiment investigating the generation of fusion power using the concept of a levitated dipole. The device was the first of its kind to test the levitated dipole concept and was funded by the US Department of Energy. [1] The machine was also part of a collaboration between the MIT Plasma Science and Fusion Center and Columbia University, where another (non-levitated) dipole experiment, the Collisionless Terrella Experiment (CTX), was located. [2]
LDX ceased operations in November 2011 when its funding from the Department of Energy ended as resources were being diverted to tokamak research. [3] [4]
The concept of the levitated dipole as a fusion reactor was first theorized by Akira Hasegawa in 1987. [5] The concept was later proposed as an experiment by Jay Kesner of MIT and Michael Mauel of Columbia University in 1997. [6] 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. [7] The LDX team has since successfully conducted several levitation tests, including a 40-minute suspension of the superconducting coil on February 9, 2007. [8] Shortly after, the coil was damaged in a control test in February 2007 and replaced in May 2007. [9] 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. [10]
This experiment needed a special free-floating electromagnet, which created the unique "toilet-bowl" magnetic field. The magnetic field was originally made of three coils. Each coil contained a 19-strand niobium-tin Rutherford cable (common in low-temperature superconducting magnets). These looped around inside an inconel structure; creating 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 560 kilograms [11] and levitated 1.6 meters above a superconducting ring. [12] The ring produced a 5.7 T peak field. [13] This superconductor was encased inside a liquid helium cryostat, which kept the electromagnet below 10 kelvins. [13] This design is similar to the D20 dipole experiment at Berkeley and the RT-1 experiment at the University of Tokyo. [14]
The dipole was suspended inside a "squashed-pumpkin"-shaped vacuum chamber, which was about 5.2 meters in diameter and ~3 meters high. [15] At the base of the chamber was a charging coil. This coil is used to charge the dipole, using induction. Next, the dipole is raised into the center of the chamber using a launcher-rather system running through the bore of the dipole magnet. A copper magnet fixed on top of the chamber produced a magnetic field which attracted the floating dipole magnet. This external field would interact with the dipole field, suspending the dipole. The magnetic field produce by the floating dipole magnet is used to confine the plasma. The plasma forms around the dipole and inside the chamber. The plasma is formed by heating a low pressure gas using a radio frequency, essentially microwaving the plasma in a ~15-kilowatt field. [16]
The machine was monitored using diagnostics fairly standard to all of fusion. These included:
The plasma is confined by the dipole magnetic field. Single particles corkscrew along the field lines of the dipole magnet at the cyclotron resonance frequency while completing poloidal orbits. The electron population was shown to have a peaked pressure and density profile as a result of the turbulent pinch phenomenon. [10]
There were two modes of operation observed: [21]
These had been proposed by Nicholas Krall in the 1960s. [22]
In the case of deuterium [ broken anchor ] 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:
In this machine, the secondary tritium could be partially removed, a unique property of the dipole. [23] 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, increasing difficulties with storage and handling. Finally, tritium and deuterium produces fast neutrons which means any reactor burning it would require heavy radiation shielding for its magnets. As the floating dipole magnet cannot have services (such as cooling) connected from the outside world, this makes thermal management of the floating magnet much harder in a D-T machine.
A stellarator is a device that confines plasma using external magnets. Scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions. The name refers to stars as fusion also occurs in stars such as the Sun. It is one of the earliest fusion power devices, along with the z-pinch and magnetic mirror.
A tokamak is a device which uses a powerful magnetic field generated by external magnets to confine plasma in the shape of an axially-symmetrical torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. The tokamak concept is currently one of the leading candidates for a practical fusion reactor.
A magnetic mirror, also known as a magnetic trap or sometimes as a pyrotron, is a type of magnetic confinement fusion device used in fusion power to trap high temperature plasma using magnetic fields. The mirror was one of the earliest major approaches to fusion power, along with the stellarator and z-pinch machines.
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 designed to harness this energy are known as fusion reactors. Research into fusion reactors began in the 1940s, but as of 2024, no device has reached net power, although net positive reactions have been achieved.
ITER is an international nuclear fusion research and engineering megaproject aimed at creating energy through a fusion process similar to that of the Sun. It is being built next to the Cadarache facility in southern France. Upon completion of construction of the main reactor and first plasma, planned for 2033–2034, ITER will be the largest of more than 100 fusion reactors built since the 1950s, with six times the plasma volume of JT-60SA in Japan, the largest tokamak operating today.
Inertial electrostatic confinement, or IEC, is a class of fusion power devices that use electric fields to confine the plasma rather than the more common approach using magnetic fields found in magnetic confinement fusion (MCF) designs. Most IEC devices directly accelerate their fuel to fusion conditions, thereby avoiding energy losses seen during the longer heating stages of MCF devices. In theory, this makes them more suitable for using alternative aneutronic fusion fuels, which offer a number of major practical benefits and makes IEC devices one of the more widely studied approaches to fusion.
Plasma diagnostics are a pool of methods, instruments, and experimental techniques used to measure properties of a plasma, such as plasma components' density, distribution function over energy (temperature), their spatial profiles and dynamics, which enable to derive plasma parameters.
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 concept of the levitated dipole as a fusion reactor was first theorized by Akira Hasegawa in 1987.
The tesla is the unit of magnetic flux density in the International System of Units (SI).
Magnetic confinement fusion (MCF) is an approach to generate thermonuclear fusion power that uses magnetic fields to confine fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of controlled fusion research, along with inertial confinement fusion.
Alcator C-Mod was a tokamak that operated between 1991 and 2016 at the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center (PSFC). Notable for its high toroidal magnetic field, Alcator C-Mod holds the world record for volume averaged plasma pressure in a magnetically confined fusion device. Until its shutdown in 2016, it was one of the major fusion research facilities in the United States.
The National Spherical Torus Experiment (NSTX) is a magnetic fusion device based on the spherical tokamak concept. It was constructed by the Princeton Plasma Physics Laboratory (PPPL) in collaboration with the Oak Ridge National Laboratory, Columbia University, and the University of Washington at Seattle. It entered service in 1999. In 2012 it was shut down as part of an upgrade program and became NSTX-U, for Upgrade.
The polywell is a proposed design for a fusion reactor using an electric and magnetic field to heat ions to fusion conditions.
The Helically Symmetric Experiment, is an experimental plasma confinement device at the University of Wisconsin–Madison, with design principles that are intended to be incorporated into a fusion reactor. The HSX is a modular coil stellarator which is a toroid-shaped pressure vessel with external electromagnets which generate a magnetic field for the purpose of containing a plasma. It began operation in 1999.
Magnetized Target Fusion (MTF) is a fusion power concept that combines features of magnetic confinement fusion (MCF) and inertial confinement fusion (ICF). Like the magnetic approach, the fusion fuel is confined at lower density by magnetic fields while it is heated into a plasma. As with the inertial approach, fusion is initiated by rapidly squeezing the target to greatly increase fuel density and temperature. Although the resulting density is far lower than in ICF, it is thought that the combination of longer confinement times and better heat retention will let MTF operate, yet be easier to build. The term magneto-inertial fusion (MIF) is similar, but encompasses a wider variety of arrangements. The two terms are often applied interchangeably to experiments.
The Lockheed Martin Compact Fusion Reactor (CFR) was a fusion power project at Lockheed Martin’s Skunk Works. Its high-beta configuration, which implies that the ratio of plasma pressure to magnetic pressure is greater than or equal to 1, allows a compact design and expedited development. The project was active between 2010 and 2019, after that date there have been no updates and it appears the division has shut down.
The ARC fusion reactor is a design for a compact fusion reactor developed by the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center (PSFC). ARC aims to achieve an engineering breakeven of three. The key technical innovation is to use high-temperature superconducting magnets in place of ITER's low-temperature superconducting magnets. The proposed device would be about half the diameter of the ITER reactor and cheaper to build.
Akira Hasegawa is a Japanese theoretical physicist and engineer who has worked in the U.S. and Japan. He is known for his work in the derivation of the Hasegawa–Mima equation, which describes fundamental plasma turbulence and the consequent generation of zonal flow that controls plasma diffusion. Hasegawa also made the discovery of optical solitons in glass fibers, a concept that is essential for high speed optical communications.
The history of nuclear fusion began early in the 20th century as an inquiry into how stars powered themselves and expanded to incorporate a broad inquiry into the nature of matter and energy, as potential applications expanded to include warfare, energy production and rocket propulsion.