Nonlinear Excitation of Subcritical Instabilities in a Toroidal Plasma.

M Lesur,M Osakabe,A Shimizu,M Sasaki,The Lhd Experiment Group The Lhd Experiment Group,S.-I Itoh,T Ido,K Ida,K Itoh,S Inagaki,K Ogawa

doi:10.1103/physrevlett.116.015003

Abstract

In a collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase space. However, nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave-wave coupling can provide a seed, which is significantly below the threshold, but can still grow by (and only by) the collaboration of fluid and kinetic nonlinearities. By modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave-wave coupling equations, it is shown that this new kind of subcritical instability can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. The model is applied to the bursty onset of geodesic acoustic modes in a LHD experiment. The model recovers several key features such as relative amplitude, time scales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, driven by this mechanism of combined fluid and kinetic nonlinearities.

Highlights

[14] Since the specific mechanism of mode-mode coupling is unknown, we use the vague term of ”fluid coupling”
In this Letter, we report the first theoretical analysis that interprets an experimental observation as a subcritical instability with an essential role of kinetic nonlinearity
We develop a new reduced model, which combines the kinetic description of the daughter mode with the nonlinear fluid coupling [14] between mother and daughter

Summary

Introduction

[14] Since the specific mechanism of mode-mode coupling is unknown, we use the vague term of ”fluid coupling”. Theory predicts that these structures can lead to subcritical instabilities, where the kinetic nonlinearity enable the growth of a mode that is linearly damped We develop a new reduced model, which combines the kinetic description of the daughter mode with the nonlinear fluid coupling [14] between mother and daughter.

Results

Conclusion