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Subcritical instabilities destabilized by kinetic nonlinearities in hot plasmas

par Caroline CHAMPENOIS - publié le


lundi 30 novembre 2015 à 11:00, service 322 du campus Saint-Jérôme

Maxime Lesur
institut Jean Lamour, Vandoeuvre-les-Nancy

Subcritical instabilities (nonlinear instabilities, which grow despite being linearly damped), or submarginal turbulence, are ubiquitous in fluids and plasmas. These include small-scale turbulence (e.g. pipe flow) and large-scale perturbations (e.g. Kelvin-Helmholtz instability). Therefore, in general, the task of predicting the stability of perturbations should involve nonlinear calculations. In collisionless plasmas in particular, it is known that in theory, linearly stable large-scale modes can be destabilized by the presence of structures in phase-space. In this presentation, we will review the theories of subcritical instabilities, with an emphasis on those driven by kinetic nonlinearities such as particle-trapping in phase-space (as opposed to those driven by fluid nonlinearities, such as parametric or modulational coupling). Going further, we will introduce our latest addition to the theory, which is applicable to large-scale electromagnetic modes driven by energetic particles. A new mechanism of subcritical instability was found, where growth requires a sustained collaboration between kinetic and fluid nonlinearities. This theory interprets a recent observation of bursts of geodesic acoustic mode (GAM) on the LHD stellerator as the first example of a subcritical instability driven by kinetic nonlinearities. We will discuss how this contributes to the understanding of the long-lasting "trigger problem", concerning the onset of abruptly growing modes.

Contact local : Yann Camenen


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