Nonlinear Excitation of Subcritical Instabilities in a Toroidal Plasma

M. Lesur, K. Itoh, T. Ido, M. Osakabe, K. Ogawa, A. Shimizu, M. Sasaki, K. Ida, S. Inagaki, S.-I. Itoh, and the LHD Experiment Group
Phys. Rev. Lett. 116, 015003 – Published 8 January 2016

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.

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  • Received 11 May 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.015003

© 2016 American Physical Society

Physics Subject Headings (PhySH)

Plasma Physics

Authors & Affiliations

M. Lesur1,*, K. Itoh2,3, T. Ido2, M. Osakabe2,4, K. Ogawa2,4, A. Shimizu2, M. Sasaki1,3, K. Ida2,4, S. Inagaki1,3, S.-I. Itoh1,3, and the LHD Experiment Group2

  • 1Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan
  • 2National Institute for Fusion Science, Toki 509-5292, Japan
  • 3Research Center for Plasma Turbulence, Kyushu University, Kasuga 816-8580, Japan
  • 4SOKENDAI (The Graduate University for Advanced Studies), Toki 509-5292, Japan

  • *Present address: Institut Jean Lamour, UMR 7198, Lorraine University, 54506 Vandoeuvre-les-Nancy, France.

See Also

Strong Destabilization of Stable Modes with a Half-Frequency Associated with Chirping Geodesic Acoustic Modes in the Large Helical Device

T. Ido, K. Itoh, M. Osakabe, M. Lesur, A. Shimizu, K. Ogawa, K. Toi, M. Nishiura, S. Kato, M. Sasaki, K. Ida, S. Inagaki, S.-I. Itoh, and the LHD Experiment Group
Phys. Rev. Lett. 116, 015002 (2016)

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Vol. 116, Iss. 1 — 8 January 2016

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