Magnetic island
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
- ↑ 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, Ann. Phys. Fr. 28 (2003) 1
- ↑ F.L. Waelbroeck, Theory and observations of magnetic islands, Nucl. Fusion 49 (2009) 104025
- ↑ F.L. Waelbroeck and R. Fitzpatrick, Rotation and Locking of Magnetic Islands, Phys. Rev. Lett. 78 (1997) 1703–1706
- ↑ ITER Physics Expert Group on Confinement and Transport et al, Chapter 2: Plasma confinement and transport, Nucl. Fusion 39 (1999) 2175-2249
- ↑ 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, Nucl. Fusion 33 (1993) 1119
- ↑ B.V. Chirikov, A universal instability of many-dimensional oscillator systems, Phys. Rep. 52, Issue 5 (1979) 263
- ↑ C.W. Horton, Y.H. Ichikawa, Chaos and structures in nonlinear plasmas, World Scientific, 1996 ISBN 9789810226367
- ↑ S.R. Hudson et al, Free-boundary full-pressure island healing in stellarator equilibria: coil-healing, Plasma Phys. Control. Fusion 44 (2002) 1377
- ↑ H. Zohm et al,MHD limits to tokamak operation and their control, Plasma Phys. Control. Fusion 45 (2003) A163
- ↑ R. Kanno et al, Formation and healing of n = 1 magnetic islands in LHD equilibrium, Nucl. Fusion 45 (2005) 588
- ↑ Seek and Destroy System for magnetic island control