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Title:
Simple advecting structures and the edge of chaos in subcritical tokamak plasmas
Authors:
McMillan, Ben F.; Pringle, Chris C. T.; Teaca, Bogdan
Affiliation:
AA(Centre for Fusion, Space and Astrophysics, Department of Physics, Warwick University, Coventry, UK), AB(Applied Mathematics Research Centre, Coventry University, Coventry CV1 5FB, UK), AC(Applied Mathematics Research Centre, Coventry University, Coventry CV1 5FB, UK)
Publication:
Journal of Plasma Physics, Volume 84, Issue 6, article id. 905840611, 18 pp.
Publication Date:
12/2018
Origin:
CUP
Keywords:
fusion plasma, plasma nonlinear phenomena
Abstract Copyright:
(c) 2018: © Cambridge University Press 2018
DOI:
10.1017/S0022377818001216
Bibliographic Code:
2018JPlPh..84f9011M

Abstract

In tokamak plasmas, sheared flows perpendicular to the driving temperature gradients can strongly stabilise linear modes. While the system is linearly stable, regimes with persistent nonlinear turbulence may develop, i.e. the system is subcritical. A perturbation with small but finite amplitude may be sufficient to push the plasma into a regime where nonlinear effects are dominant and thus allow sustained turbulence. The minimum threshold for nonlinear instability to be triggered provides a criterion for assessing whether a tokamak is likely to stay in the quiescent (laminar) regime. At the critical amplitude, instead of transitioning to the turbulent regime or decaying to a laminar state, the trajectory will map out the edge of chaos. Surprisingly, a quasi-travelling-wave solution is found as an attractor on this edge manifold. This simple advecting solution is qualitatively similar to, but simpler than, the avalanche-like bursts seen in earlier turbulent simulations and provides an insight into how turbulence is sustained in subcritical plasma systems. For large flow shearing rate, the system is only convectively unstable, and given a localised initial perturbation, will eventually return to a laminar state at a fixed spatial location.
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