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**Harvard**

van Wyk, F., Highcock, E., Schekochihin, A., Roach, C., Field, A. och Dorland, W. (2016) *Transition to subcritical turbulence in a tokamak plasma*.

** BibTeX **

@article{

van Wyk2016,

author={van Wyk, F and Highcock, Edmund and Schekochihin, AA and Roach, CM and Field, AR and Dorland, W},

title={Transition to subcritical turbulence in a tokamak plasma},

journal={Journal of Plasma Physics},

issn={0022-3778},

volume={82},

issue={6},

pages={905820609},

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.},

year={2016},

keywords={fusion plasma, plasma instabilities, plasma simulation },

}

** RefWorks **

RT Journal Article

SR Electronic

ID 246692

A1 van Wyk, F

A1 Highcock, Edmund

A1 Schekochihin, AA

A1 Roach, CM

A1 Field, AR

A1 Dorland, W

T1 Transition to subcritical turbulence in a tokamak plasma

YR 2016

JF Journal of Plasma Physics

SN 0022-3778

VO 82

IS 6

AB 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.

LA eng

DO 10.1017/S0022377816001148

LK http://dx.doi.org/10.1017/S0022377816001148

OL 30