Plasma and Fusion Research

Volume 12, 1303005 (2017)

Letters

Radially Distributed Instabilities and Nonlinear Process to Generate Pressure Variation in a Torus Plasma
Naohiro KASUYA and Masatoshi YAGI1)
Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-kouen, Kasuga, Fukuoka 816-8580, Japan
1)
National Institute for Quantum and Radiological Science and Technology, 2-166 Omotedate, Obuchi, Rokkasho, Aomori 039-3212, Japan
(Received 14 May 2016 / Accepted 17 January 2017 / Published 9 February 2017)

Abstract

Distributed instabilities can successively change one after another to give accelerated radial propagation. The response of the linearly unstable distributed instabilities is identified in a gradual evolution phase as well as in a phase just after impact of modulation. Global nonlinear simulations of drift-interchange modes in helical plasmas are carried out with source modulation using a reduced MHD model. Conditional average of long time-series data with the modulation period reveals characteristic responses of the plasma. Smaller-scale variations comparable to the micro-temporal scale in this simulation are also included in the gradual evolution phase. The correlation analysis shows that the evolution of the mean pressure is strongly correlated with the strength of the nonlinear coupling. The evaluation of the energy balance to decompose the energy transfer into contribution from each three-wave coupling clarifies that a single mode coupling at each location has the dominant contribution to the smaller-scale pressure evolution in spite of self-organized mechanism with a wide range of comparable magnitude modes. Comparison of mode amplitudes does not define the dominant one, so identification of the active mode is useful for understanding the causality. This selection suggests the mechanism that gives the spreading effective in the quasi-steady state as for the ballistic propagation in the self-organized critical state.

Keywords

turbulence spreading, self-organization, global simulation, nonlinear coupling, energy transfer, source modulation, torus plasma

DOI: 10.1585/pfr.12.1303005

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