Transition to subcritical turbulence in a tokamak plasma
Artikel i vetenskaplig tidskrift, 2016

Tokamak turbulence, driven by the ion-temperature gradient and occurring in the presence of flow shear, is investigated by means of local, ion-scale, electrostatic gyrokinetic simulations (with both kinetic ions and electrons) of the conditions in the outer core of the Mega-Ampere Spherical Tokamak (MAST). A parameter scan in the local values of the ion-temperature gradient and flow shear is performed. It is demonstrated that the experimentally observed state is near the stability threshold and that this stability threshold is nonlinear: sheared turbulence is subcritical, i.e. the system is formally stable to small perturbations, but, given a large enough initial perturbation, it transitions to a turbulent state. A scenario for such a transition is proposed and supported by numerical results: close to threshold, the nonlinear saturated state and the associated anomalous heat transport are dominated by long-lived coherent structures, which drift across the domain, have finite amplitudes, but are not volume filling; as the system is taken away from the threshold into the more unstable regime, the number of these structures increases until they overlap and a more conventional chaotic state emerges. Whereas this appears to represent a new scenario for transition to turbulence in tokamak plasmas, it is reminiscent of the behaviour of other subcritically turbulent systems, e.g. pipe flows and Keplerian magnetorotational accretion flows.

plasma instabilities

plasma simulation

fusion plasma


F van Wyk

Culham Lab

University of Oxford

Daresbury Laboratory

Edmund Hood Highcock

Chalmers, Fysik, Subatomär fysik och plasmafysik

AA Schekochihin

Merton College

University of Oxford

C. M. Roach

Culham Lab

AR Field

Culham Lab

William D. Dorland

University of Maryland

University of Oxford

Journal of Plasma Physics

0022-3778 (ISSN) 1469-7807 (eISSN)

Vol. 82 905820609-


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Grundläggande vetenskaper


Fusion, plasma och rymdfysik