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Transition to subcritical turbulence in a tokamak plasma

Published online by Cambridge University Press:  19 December 2016

F. van Wyk*
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, UK CCFE, Culham Science Centre, Abingdon OX14 3DB, UK STFC Daresbury Laboratory, Daresbury WA4 4AD, UK
E. G. Highcock
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, UK Chalmers University of Technology, Department of Physics, Göteborg SE-412 96, Sweden
A. A. Schekochihin
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, UK Merton College, Oxford OX1 4JD, UK
C. M. Roach
Affiliation:
CCFE, Culham Science Centre, Abingdon OX14 3DB, UK
A. R. Field
Affiliation:
CCFE, Culham Science Centre, Abingdon OX14 3DB, UK
W. Dorland
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford OX1 3NP, UK Department of Physics, University of Maryland, College Park, MD 20742-4111, USA
*
Email address for correspondence: ferdinand.vanwyk@physics.ox.ac.uk

Abstract

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.

Type
Research Article
Copyright
© Cambridge University Press 2016 

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