Transition to subcritical turbulence in a tokamak plasma
Journal article, 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.

fusion plasma

plasma simulation

plasma instabilities

Author

F van Wyk

University of Oxford

Culham Science Centre

Daresbury Laboratory

Edmund Hood Highcock

Chalmers, Physics, Subatomic and Plasma Physics

AA Schekochihin

Merton College

University of Oxford

C. M. Roach

Culham Science Centre

AR Field

Culham Science Centre

William D. Dorland

University of Maryland

University of Oxford

Journal of Plasma Physics

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

Vol. 82 6 905820609

Driving Forces

Sustainable development

Areas of Advance

Energy

Roots

Basic sciences

Subject Categories

Fusion, Plasma and Space Physics

DOI

10.1017/S0022377816001148

More information

Latest update

11/22/2021