Simple advecting structures and the edge of chaos in subcritical tokamak plasmas

Ben McMillan, Chris Pringle, Bogdan Teaca

    Research output: Contribution to journalArticlepeer-review

    11 Citations (Scopus)
    29 Downloads (Pure)

    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.
    Original languageEnglish
    Article number905840611
    Number of pages18
    JournalJournal of Plasma Physics
    Volume84
    Issue number6
    Early online date6 Dec 2018
    DOIs
    Publication statusPublished - Dec 2018

    Keywords

    • nonlinear phenomena
    • fusion plasma
    • plasma

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