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Transition to subcritical turbulence in a tokamak plasma

F van Wyk ; Edmund Highcock (Institutionen för fysik, Subatomär fysik och plasmafysik (Chalmers)) ; AA Schekochihin ; CM Roach ; AR Field ; W Dorland
Journal of Plasma Physics (0022-3778). Vol. 82 (2016), 6, p. 905820609.
[Artikel, refereegranskad vetenskaplig]

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.

Nyckelord: fusion plasma, plasma instabilities, plasma simulation



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Denna post skapades 2017-01-08. Senast ändrad 2017-05-05.
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