Transition to subcritical turbulence in a tokamak plasma

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.

arXiv comment16 pages, 5 figures, accepted to Journal of Plasma Physics

References in corpus (13)

• scikit-image: Image processing in Python
• The rise of fully turbulent flow
• Multiscale Gyrokinetics for Rotating Tokamak Plasmas: Fluctuations, Transport and Energy Flows
• Critically balanced ion temperature gradient turbulence in fusion plasmas
• Turbulent transport in tokamak plasmas with rotational shear
• Global bifurcations to subcritical magnetorotational dynamo action in Keplerian shear flow
• Transport Bifurcation in a Rotating Tokamak Plasma
• Understanding the effect of sheared flow on microinstabilities
• Zero-Turbulence Manifold in a Toroidal Plasma
• Suppression of turbulence and subcritical fluctuations in differentially rotating gyrokinetic plasmas
• Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?
• Generalized universal instability: Transient linear amplification and subcritical turbulence
• Transport Bifurcation Induced by Sheared Toroidal Flow in Tokamak Plasmas

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