Transition to subcritical turbulence in a tokamak plasma

• Published in: arXiv.org 2017-01
• Main Authors: F van Wyk, Highcock, E G, Schekochihin, A A, Roach, C M, Field, A R, Dorland, W
• Format: Article
• Language: English
• Subjects: Computational fluid dynamics; Computer simulation; Deposition; Plasmas (physics); Shear flow; Stability; Temperature gradients; Tokamak devices; Turbulence
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1607.08173

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.