Magnetic island

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A magnetic island is a closed magnetic flux tube (cf. Flux surface), bounded by a separatrix, isolating it from the rest of space. Its topology is toroidal.

In the context of magnetic confinement fusion, the basic magnetic field configuration consists of toroidally nested flux surfaces, while each flux surface is characterised by a certain value of the rotational transform or safety factor q. Magnetic islands can appear at flux surfaces with a rational value of the safety factor q = m/n. [1] Subsidiary islands can appear within an island.

Island birth

The rupture of the original 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. [2]

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. [3] Locked islands often lead to a disruption (complete loss of confinement) in tokamaks.

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. [4] However, is is possible to qualify this statement somewhat by taking into account the ratio between parallel and perpendicular transport within an island. [5]

The interaction of neighbouring island chains causes the magnetic field to become stochastic (according to the Chirikov criterion [6]), resulting in enhanced (anomalous) radial transport. [7]

Island control

Island control is possible by tailoring the q-profile, external magnetic fields, [8] and the pressure profile, or by spinning up the plasma. [9] Pressure effects can lead to 'island healing'. [10] Active control of islands by external means is also possible (cf. the 'Seek and destroy' technique, under development [11]).

See also

References