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A magnetic island is a | A magnetic island is a closed magnetic [[Flux tube|flux tube]] (cf. [[Flux surface]]), bounded by a [[Separatrix|separatrix]], isolating it from the rest of space. | ||
Its topology is toroidal | Its topology is toroidal. | ||
In the context of magnetic confinement fusion, the basic magnetic field configuration consists of toroidally nested [[Flux surface|flux surfaces]], while each flux surface is characterised by a certain value of the [[ | In the context of magnetic confinement fusion, the basic magnetic field configuration consists of toroidally nested [[Flux surface|flux surfaces]], while each flux surface is characterised by a certain value of the [[Rotational transform|rotational transform]] or safety factor ''q''. Magnetic islands can appear at flux surfaces with a rational value of the safety factor ''q = m/n''. | ||
<ref> | <ref>J.H. Misguich, J.-D. Reuss, D. Constantinescu, G. Steinbrecher, M. Vlad, F. Spineanu, B. Weyssow, R. Balescu, ''Noble internal transport barriers and radial subdiffusion of toroidal magnetic lines'', [[doi:10.1051/anphys:2004001|Ann. Phys. Fr. '''28''' (2003) 1]]</ref> | ||
Subsidiary islands can appear within an island. | |||
== | == Island birth == | ||
The rupture of the | The rupture of the assumed initial topology of toroidally nested flux surfaces needed to produce the island requires the [[reconnection]] of magnetic field lines, which can only occur with finite resistivity. | ||
<ref>[ | <ref>F.L. Waelbroeck, ''Theory and observations of magnetic islands'', [[doi:10.1088/0029-5515/49/10/104025|Nucl. Fusion '''49''' (2009) 104025]]</ref> | ||
[[Stellarator]]s may have a vacuum magnetic field structure that already contains some islands (so-called 'natural islands'). | |||
Since these are completely determined by the external magnetic field, they are static. | |||
== Island growth and saturation == | |||
The prediction of the non-linear saturated state of islands is the goal of [[Neoclassical transport|Neoclassical]] Tearing Mode (NTM) theory. | |||
This theory has been developed to a considerable level of sophistication, although discrepancies with experimental observations remain. | |||
<ref>H. Lütjens and J.-F. Luciani, ''Saturation levels of neoclassical tearing modes in International Thermonuclear Experimental Reactor plasmas'', [[doi:10.1063/1.2001667|Phys. Plasmas '''12''' (2005) 080703]]</ref> | |||
== Island rotation == | |||
Islands can rotate within and/or with respect to the ambient plasma. | |||
The observation of such rotating 'MHD modes' is ubiquitous in fusion plasmas with typical frequencies of the order of several tens of kHz. | |||
The detection of such modes is possible by measuring perturbations of the magnetic field, or the electron density, temperature, or pressure. | |||
If the ambient magnetic field (produced by external coils) has an appropriate structure, the island can also lock onto that structure. | |||
<ref>F.L. Waelbroeck and R. Fitzpatrick, ''Rotation and Locking of Magnetic Islands'', [[doi:10.1103/PhysRevLett.78.1703|Phys. Rev. Lett. '''78''' (1997) 1703–1706]]</ref> | |||
Locked islands often lead to a [[Disruption|disruption]] (complete loss of confinement) in [[Tokamak|tokamaks]]. | |||
== Transport effects == | == Transport effects == | ||
It is generally assumed that the temperature is rapidly equilibrated along the magnetic field lines inside the island, so that radial transport is effectively short-circuited across the islands, decreasing the effective size of the main plasma. | It is generally assumed that the temperature is rapidly equilibrated along the magnetic field lines inside the island, so that radial transport is effectively short-circuited across the islands, decreasing the effective size of the main plasma. | ||
<ref> | <ref>ITER Physics Expert Group on Confinement and Transport et al, ''Chapter 2: Plasma confinement and transport'', [[doi:10.1088/0029-5515/39/12/302|Nucl. Fusion '''39''' (1999) 2175-2249]]</ref> | ||
However, | However, it is possible to qualify this statement somewhat by taking into account the ratio between parallel and perpendicular transport within an island. | ||
<ref> | <ref>B.Ph. van Milligen, A.C.A.P. van Lammeren, N.J. Lopes Cardozo, F.C. Schüller, and M. Verreck, ''Gradients of electron temperature and density across m=2 islands in RTP'', [[doi:10.1088/0029-5515/33/8/I03|Nucl. Fusion '''33''' (1993) 1119]]</ref> | ||
The interaction of neighbouring island chains causes the magnetic field to become stochastic (according to the Chirikov criterion <ref>B.V. Chirikov, ''A universal instability of many-dimensional oscillator systems'', [[doi:10.1016/0370-1573(79)90023-1|Phys. Rep. '''52''', Issue 5 (1979) 263]]</ref>), resulting in enhanced (anomalous) radial transport. | |||
<ref>C.W. Horton, Y.H. Ichikawa, ''Chaos and structures in nonlinear plasmas'', World Scientific, 1996 {{ISBN|9789810226367}}</ref> | |||
== Island control == | |||
Island control is possible by tailoring the ''q''-profile, external magnetic fields, | |||
<ref>S.R. Hudson et al, ''Free-boundary full-pressure island healing in stellarator equilibria: coil-healing'', [[doi:10.1088/0741-3335/44/7/323|Plasma Phys. Control. Fusion '''44''' (2002) 1377]]</ref> | |||
and the pressure profile, or by spinning up the plasma. | |||
<ref>H. Zohm et al,''MHD limits to tokamak operation and their control'', [[doi:10.1088/0741-3335/45/12A/012|Plasma Phys. Control. Fusion '''45''' (2003) A163]]</ref> | |||
Pressure effects can lead to 'island healing'. | |||
<ref>R. Kanno et al, ''Formation and healing of n = 1 magnetic islands in LHD equilibrium'', [[doi:10.1088/0029-5515/45/7/006|Nucl. Fusion '''45''' (2005) 588]]</ref> | |||
Active control of islands by external means - in particular, Electron Cyclotron Heating and Current Drive - is also possible. | |||
<ref>[http://www.rijnhuizen.nl/en/node/195 Seek and Destroy System for magnetic island control]</ref> | |||
<ref>A. Isayama et al, ''Neoclassical tearing mode control using electron cyclotron current drive and magnetic island evolution in JT-60U'', [[doi:10.1088/0029-5515/49/5/055006|Nucl. Fusion '''49''' (2009) 055006]]</ref> | |||
<ref>B. Ayten et al., ''Modelling of tearing mode suppression experiments in TEXTOR based on the generalized Rutherford equation'', [[doi:10.1088/0029-5515/51/4/043007|Nucl. Fusion '''51''' (2011) 043007]]</ref> | |||
== See also == | == See also == | ||
* [[MHD equilibrium]] | * [[MHD equilibrium]] | ||
* [[Island Divertor]] | |||
== References == | == References == | ||
<references /> | <references /> |
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