The Levitated Dipole Experiment (LDX) was an experiment investigating the generation of
fusion power using the concept of a
levitated dipole
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 Eart ...
. The device was the first of its kind to test the levitated dipole concept and was funded by the
US Department of Energy
The United States Department of Energy (DOE) is an executive department of the U.S. federal government that oversees U.S. national energy policy and manages the research and development of nuclear power and nuclear weapons in the United State ...
. The machine was also part of a collaboration between the
MIT Plasma Science and Fusion Center
The Plasma Science and Fusion Center (PSFC) at the Massachusetts Institute of Technology (MIT) is a university research center for the study of Plasma (physics), plasmas, Nuclear fusion, fusion science and technology.
It was originally founded i ...
and
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 ...
, where another dipole experiment, the Collisionless Terrella Experiment (CTX), was located.
LDX ceased operations in November 2011 when its funding from the Department of Energy ended as resources were being diverted to
tokamak research.
Concept and development
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 des ...
was first theorized by
Akira Hasegawa
is a theoretical physicist and engineer who has worked in the US 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 tha ...
in 1987. The concept was later proposed as an experiment by Jay Kesner of
MIT 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 ...
in 1997. 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
''Nature Physics'' is a monthly peer-reviewed scientific journal published by Nature Portfolio. It was first published in October 2005 (volume 1, issue 1). The chief editor is Andrea Taroni, who is a full-time professional editor employed by this ...
.
The Machine
The Dipole
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 Rutherford may refer to:
Places Australia
* Rutherford, New South Wales, a suburb of Maitland
* Rutherford (Parish), New South Wales, a civil parish of Yungnulgra County
Canada
* Mount Rutherford, Jasper National Park
* Rutherford, Edmonton ...
(common in superconducting magnets). These looped around inside an
Inconel magnet; a magnet that looked like an oversized donut. The donut was charged using
induction
Induction, Inducible or Inductive may refer to:
Biology and medicine
* Labor induction (birth/pregnancy)
* Induction chemotherapy, in medicine
* Induced stem cells, stem cells derived from somatic, reproductive, pluripotent or other cell t ...
. 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.
["Design and Fabrication of the Cryostat for the Floating Coil of the Levitated Dipole Experiment (LDX)" A. Zhukovsky, M. Morgan, D. Garnier, A. Radovinsky, B. Smith, J. Schultz, L. Myatt, S. Pourrahimi, J. Minervini.] This superconductor was encased inside a liquid helium, which kept the electromagnet below 10
kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phys ...
s.
This design is similar to the
D20 dipole
D, or d, is the fourth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''dee'' (pronounced ), plural ''dees''.
History
The ...
experiment at
Berkeley
Berkeley most often refers to:
*Berkeley, California, a city in the United States
**University of California, Berkeley, a public university in Berkeley, California
* George Berkeley (1685–1753), Anglo-Irish philosopher
Berkeley may also refer ...
and the RT-1 experiment at the University of Tokyo.
Chamber
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
Induction, Inducible or Inductive may refer to:
Biology and medicine
* Labor induction (birth/pregnancy)
* Induction chemotherapy, in medicine
* Induced stem cells, stem cells derived from somatic, reproductive, pluripotent or other cell t ...
. 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
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
, essentially microwaving the plasma in a 17-kilowatt field.
The Diagnostics
![The Mechanics of a Magnetic flux loop](https://upload.wikimedia.org/wikipedia/commons/7/79/The_Mechanics_of_a_Magnetic_flux_loop.png)
The machine was monitored using diagnostics fairly standard to all of fusion. These included:
# A
Flux loop A flux loop is a loop of wire placed inside a plasma at a right angle. Changes in the field create a current in the loop, which may be interpreted to measure the properties of the plasma. Flux loops are key diagnostics in fusion power research.
...
. 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.
["X-Ray Diagnostics for the Levitated Dipole Experiment" Jennifer L. Ellsworth, Master's Thesis, MIT 2004] 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
Synchrotron radiation (also known as magnetobremsstrahlung radiation) is the electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity (). It is produced artificially in ...
at high speeds. If the reason is deflection by another particle, plasma radiates X-rays, known as
Bremsstrahlung
''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
radiation.
# An X-ray Camera.
["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] This can read lower energy X-rays.
# A Conventional Video Camera
# An emissive
Langmuir probe
A Langmuir probe is a device used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between ...
. 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 Langmuir probe is a device used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between ...
# A dozen
Langmuir probe
A Langmuir probe is a device used to determine the electron temperature, electron density, and electric potential of a plasma. It works by inserting one or more electrodes into a plasma, with a constant or time-varying electric potential between ...
s grouped together
Behavior
![Plasma in the Levitating Dipole Experiment](https://upload.wikimedia.org/wikipedia/commons/7/72/Plasma_in_the_Levitating_Dipole_Experiment.png)
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 much 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
These had been proposed by
Nicholas Krall in the 1960s.
Tritium Suppression
In the case of
deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
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.
["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] 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.
References
External links
MIT's LDX website
{{Fusion power
Magnetic confinement fusion devices
Columbia University
Massachusetts Institute of Technology