View source for Physics: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                 = [[Physics:Levitated dipole|Levitated dipole]]
|city                 = Cambridge
|state                = {{wipe|Massachusetts}}
|country              = {{wipe|United States}}
|affiliation          = [[Physics:MIT Plasma Science and Fusion Center|MIT Plasma Science and Fusion Center]]
|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 [[Physics:Fusion power|fusion power]] using the concept of a [[Physics:Levitated dipole|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.<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 [[Physics:MIT Plasma Science and Fusion Center|MIT Plasma Science and Fusion Center]] and [[Organization:Columbia University|Columbia University]], where another 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 [[Physics:Tokamak|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|accessdate=June 27, 2012}}</ref>

== Concept and development ==
The concept of the levitated dipole as a [[Physics:Fusion reactor|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 [[Organization:Massachusetts Institute of Technology|MIT]] and Michael Mauel of [[Organization:Columbia University|Columbia Univesity]] in 1997.<ref>{{Cite journal|last=Kesner|first=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|pages=|via=}}</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) [[Engineering: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|accessdate=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 [[Physics: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 [[Physics:Nature Physics|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}}</ref>

== 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 (common in superconducting magnets). These looped around inside an [[Chemistry:Inconel|Inconel]] magnet; a magnet that looked like an oversized donut. The donut was charged using [[Physics:Electromagnetic induction|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.<ref>{{cite web|title=The Levitated Dipole Experiment|url=https://www-internal.psfc.mit.edu/ldx/ldx.html#1.1%20Levitated%20Ring|publisher=MIT|accessdate=7 August 2016}}</ref> The ring produced roughly a 5-tesla 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, which kept the electromagnet below 10 [[Kelvin|kelvin]]s.<ref name = "Construction"/> This design is similar to the D20 dipole experiment at [[Organization: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 mushroom-shaped vacuum chamber, which was about 5 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 [[Physics:Electromagnetic induction|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 [[Physics:Helmholtz coil|Helmholtz coil]]s, 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 [[Engineering:Radio frequency|radio frequency]], essentially microwaving the plasma in a 17-kilowatt field.<ref>"Optimization of Hot Electron Diagnostics on LDX" Nogami, Woskov, Kesner, Garnier, Mauel, 2009</ref>

=== The 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 [[Physics:Flux loop|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 [[Physics: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 [[Physics:Cyclotron radiation|Cyclotron radiation]] at low speeds and [[Physics:Synchrotron radiation|Synchrotron radiation]] at high speeds. If the reason is deflection by another particle, plasma radiates X-rays, known as [[Physics:Bremsstrahlung|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 [[Physics:Langmuir probe|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 [[Physics:Current–voltage characteristic|IV]] curve. This can be read and used to measure the density and temperature of the nearby plasma. 
# A triple [[Physics:Langmuir probe|Langmuir probe]]<ref name ="2003Diag"/>
# A dozen [[Physics:Langmuir probe|Langmuir probe]]s grouped together<ref name ="2003Diag"/>

== Behavior ==
[[File:Single Ion motion inside the LDX.png|thumbnail|right|350px|Single Ion Motion inside the LDX]]

[[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>]]

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.<ref name = "overview"/> 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.<ref name = "overview"/> This is shown here. The plasma was trapped fairly well. It gave a maximum [[Physics:Beta (plasma physics)|beta number]] of 0.26.<ref>"Improved Confinement During Magnetic Levitation in LDX", 50th Annual Meeting of the APS DDP, November 18, 2008 M Manuel</ref> A value of 1 is ideal.

=== 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.
# A more conventional Magnetohydrodynamic mode
These had been proposed by [[Biography:Nicholas Krall|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 [[Physics:Fusion power#Deuterium.2C 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, 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 ==
{{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:Fusion reactors]]

{{Sourceattribution|Levitated Dipole Experiment|1}}