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Avalanches and the edge of chaos in subcritical gyrokinetic systems

par Caroline CHAMPENOIS - publié le

jeudi 19 juillet à 10h30 - service 361.

Ben McMillan

University of Warwick, Royaume-Uni

abstract :
In tokamak plasmas, plasmas with sheared flows are often linearly stable in regimes where 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 manifold separating trajectories that decay to laminar states and those that lead to turbulence is known as the ‘edge of chaos’ : characterising this boundary gives insight into the nonlinear processes allowing subcritical turbulence. The edge of chaos, like the turbulence, displays avalanche-like bursts, but in a simpler form. Interestingly, for sufficiently large values of shear flow, excitations propagate only in one direction, and the turbulence is transient when viewed at a fixed spatial location. The system is thus only convectively unstable, and in a bounded physical tokamak, the plasma will eventually return to a quiescent state. This nonlocality may explain the triggering of the plasma edge by core turbulence, even if the edge region is locally quiescent.

Contact : Yann Camenen


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