View source for Levitated Dipole Experiment

{{Infobox fusion devices
|name                 = LDX
|fullname             = Levitated Dipole Experiment
|image                = Outside the LDX chamber.png
|imagetitle           = A picture of the LDX chamber on 25 Jan 2010
|type                 = [[Levitated dipole]]
|city                 = [[Cambridge, Massachusetts|Cambridge]]
|state                = [[Massachusetts]]
|country              = [[United States]]
|affiliation          = [[MIT Plasma Science and Fusion Center]], [[Columbia University]]
|major_radius         = {{cvt|0.34|m}}
|minor_radius         = <!-- {{cvt|00|m}} -->
|volume               = <!-- {{val|00|ul=m3}} -->
|field                = <!-- {{cvt|00|T}} -->
|heating              = <!-- {{val|00|ul=MW}} -->
|power                = <!-- {{val|00|ul=MW}} -->
|time                 = <!-- {{val|00|ul=s}} -->
|current              = <!-- {{val|00|ul=MA}} -->
|temperature          = <!-- {{val|00|e=6|ul=K}} -->
|construction_date    = 
|operation_start_year = 2004
|operation_end_year   = 2011
|prev                 = 
|next                 = 
|related              = Collisionless Terrella Experiment (CTX)
|website              = {{URL|https://www-internal.psfc.mit.edu/ldx/| The Levitated Dipole eXperiment website}}
|other_links          = 
}}

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 [[United States Department of Energy|US Department of Energy]].<ref>{{Cite web|title=Levitated Dipole Experiment|url=https://www-internal.psfc.mit.edu/ldx/|access-date=2020-06-22|website=www-internal.psfc.mit.edu}}</ref>  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.<ref>{{Cite web|title=CTX|url=http://sites.apam.columbia.edu/CTX/index.html|access-date=2020-06-22|website=sites.apam.columbia.edu}}</ref>

LDX ceased operations in November 2011 when its funding from the Department of Energy ended as resources were being diverted to [[tokamak]] research.<ref name="fundingcut">{{cite web|title=LDX funding canceled|url=http://www.fusenet.eu/node/216|url-status=dead|archive-url=https://web.archive.org/web/20130117084532/http://fusenet.eu/node/216|archive-date=2013-01-17|access-date=June 27, 2012}}</ref><ref name="fundingcut2">{{cite journal|last=Feder|first=Toni|year=2011|title=US narrows fusion research, joins German stellarator|journal=Physics Today|volume=64|issue=9|pages=30|doi=10.1063/PT.3.1252}}</ref>

== Concept and development ==
{{See also|Levitated dipole}}
The concept of the levitated dipole as a [[fusion reactor]] was first theorized by [[Akira Hasegawa]] in 1987.<ref>{{Cite journal|last=Hasegawa|first=Akira|date=1987|title=A dipole field fusion reactor|url=http://inis.iaea.org/Search/search.aspx?orig_q=RN:21025528|journal=Comments on Plasma Physics and Controlled Fusion|language=en|volume=11|issue=3|pages=147–151|issn=0374-2806}}</ref> The concept was later proposed as an experiment by Jay Kesner of [[Massachusetts Institute of Technology|MIT]] and Michael Mauel of [[Columbia University]] in 1997.<ref>{{Cite journal|last1=Kesner|first1=J|last2=Mauel|first2=M|date=1997|title=Plasma Confinement in a Levitated Magnetic Dipole|url=https://dspace.mit.edu/bitstream/handle/1721.1/95330/97ja013_full.pdf?sequence=1|journal=Plasma Physics Reports|volume=23|issue=9|page=742|bibcode=1997PlPhR..23..742K}}</ref> 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) [[Radio frequency|RF]] heating the plasma.<ref name="20040813plasma">{{cite web|date=13 August 2004|title=LDX begins first plasma experiments|url=https://www-internal.psfc.mit.edu/ldx/reports/FirstPlasma.html|access-date=7 August 2016|website=Levitated Dipole Experiment}}</ref> The LDX team has since successfully conducted several levitation tests, including a 40-minute suspension of the [[Superconductor|superconducting]] coil on February 9, 2007.<ref name="20070209levitation">{{cite web|date=9 March 2009|title=First flight and damaged L-coil|url=https://www-internal.psfc.mit.edu/ldx/reports/status_0702.html|website=Levitated Dipole Experiment}}</ref> Shortly after, the coil was damaged in a control test in February 2007 and replaced in May 2007.<ref>{{cite web|date=21 May 2007|title=Levitation coil replacement|url=https://www-internal.psfc.mit.edu/ldx/reports/status_0705.html|website=Levitated Dipole Experiment}}</ref> 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]].<ref name="NaturePhysResults">{{cite journal|last1=Boxer|first1=A. C|last2=Bergmann|first2=R|last3=Ellsworth|first3=J. L|last4=Garnier|first4=D. T|last5=Kesner|first5=J|last6=Mauel|first6=M. E|last7=Woskov|first7=P|year=2010|title=Turbulent inward pinch of plasma confined by a levitated dipole magnet|journal=Nature Physics|volume=6|issue=3|pages=207|bibcode=2010NatPh...6..207B|doi=10.1038/nphys1510|doi-access=free}}</ref>

== Machine description  ==
=== Dipole ===
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 [[Electromagnetic induction|induction]]. 
Once charged, it generated a magnetic field for roughly an 8-hour period. Overall, the ring weighed 560 kilograms<ref name="Garnier2006">{{cite journal|last1=Garnier|first1=D.T.|last2=Hansen|first2=A.K.|last3=Mauel|first3=M.E.|last4=Michael|first4=P.C.|last5=Minervini|first5=J.V.|last6=Radovinsky|first6=A.|last7=Zhukovsky|first7=A.|last8=Boxer|first8=A.|last9=Ellsworth|first9=J.L.|last10=Karim|first10=I.|last11=Ortiz|first11=E.E.|year=2006|title=Design and initial operation of the LDX facility|journal=[[Fusion Engineering and Design]]|volume=81|issue=20–22|pages=2371–2380|doi=10.1016/j.fusengdes.2006.07.002|bibcode=2006FusED..81.2371G }}</ref> and levitated 1.6 meters above a superconducting ring.<ref>{{cite web|title=The Levitated Dipole Experiment|url=https://www-internal.psfc.mit.edu/ldx/ldx.html#1.1%20Levitated%20Ring|publisher=MIT|access-date=7 August 2016}}</ref> 
The ring produced a 5.7 T peak field.<ref name = "Construction">"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.</ref> 
This superconductor was encased inside a liquid helium cryostat, which kept the electromagnet below 10 [[kelvin]]s.<ref name = "Construction"/> 
This design is similar to the [[D20 dipole]] experiment at [[Lawrence Berkeley National Laboratory|Berkeley]] and the RT-1 experiment at the University of Tokyo.<ref>"Turbulent Transport in a Laboratory Magnetospheric Dipole" European Physical Society 38th Conference on Plasma Physics, Strasbourg, France June 28, 2011.</ref>

=== Chamber ===
The dipole was suspended inside a "squashed-pumpkin"-shaped vacuum chamber, which was about 5.2 meters in diameter and ~3 meters high.<ref>presentation"LDX Machine Design and Diagnostics" APS DPP meeting 1998, Garnier and Mauel</ref> At the base of the chamber was a charging coil. This coil is used to charge the dipole, using [[Electromagnetic induction|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.<ref>"Optimization of Hot Electron Diagnostics on LDX" Nogami, Woskov, Kesner, Garnier, Mauel, 2009</ref>

=== Diagnostics ===
[[File:The Mechanics of a Magnetic flux loop.png|thumbnail|A flux loop 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.]]
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.<ref name = "X-rays">"X-Ray Diagnostics for the Levitated Dipole Experiment" Jennifer L. Ellsworth, Master's Thesis, MIT 2004</ref> 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.<ref name = "X-rays"/> 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 [[Larmor formula|changes speed]], for any reason.<ref>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).</ref> 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.<ref name ="2003Diag">"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</ref> This can read lower energy X-rays. 
# A conventional video camera <ref name ="2003Diag"/>
# 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 [[Current–voltage characteristic|IV]] curve. This can be read and used to measure the density and temperature of the nearby plasma. 
# A triple [[Langmuir probe]]<ref name ="2003Diag"/>
# A dozen [[Langmuir probe]]s grouped together<ref name ="2003Diag"/>

== Behavior ==
{{See also|Levitated dipole}}
[[File:Plasma in the Levitating Dipole Experiment.png|thumbnail|right|350px|Bulk plasma behavior inside the LDX <ref name = "overview">"Overview of LDX Results" Jay Kesner, A. Boxer, J. Ellsworth, I. Karim, Presented at the APS Meeting, Philadelphia, November 2, 2006, Paper VP1.00020</ref>]]

The plasma is confined by the dipole magnetic field. 
Single particles corkscrew along the field lines of the dipole magnet at the [[Cyclotron motion#Cyclotron resonance|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.<ref name="NaturePhysResults"/>

=== Modes of Operation ===
There were two modes of operation observed:<ref>"Helium Catalyzed D-D Fusion in a Levitated Dipole" Presentation Kesner, Catto, Krasheninnikova APS 2005 DPP Meeting, Denver</ref> 
# Hot electron interchange: a lower density, mostly electron plasma, occurring when the dipole was operated in "supported" mode (not levitated).
# A more conventional [[Magnetohydrodynamic]] mode.
These had been proposed by [[Nicholas Krall]] in the 1960s.<ref>"Stabilization of Hot Electron Plasma by a Cold Background" N Krall, Phys. Fluids 9, 820 (1966)</ref>

=== Tritium Suppression ===
In the case of [[Fusion_power#Deuterium|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:

:<chem>D + D -> T + ^1H</chem>
:<chem>D + D -> ^3He + n</chem>

In this machine, the secondary tritium could be partially removed, a unique property of the dipole.<ref>"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</ref> 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.

== References ==
{{reflist|30em}}

== External links ==
*[https://www-internal.psfc.mit.edu/ldx/ MIT's LDX website]
*[https://web.archive.org/web/20160819133354/http://videos.howstuffworks.com/discovery/33306-discovery-tech-power-from-a-floating-metal-donut-video.htm Discovery Tech report on LDX (6/08), "Power from a floating metal donut"]

{{Fusion power}}

[[Category:Magnetic confinement fusion devices]]
[[Category:Columbia University]]
[[Category:Massachusetts Institute of Technology]]