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On deterministic nature of the intermittent geodesic acoustic mode observed in L- mode discharge near tokamak edge, Part I Zonal flow driven by ion temperature gradient turbulence | | Z. Y. Liu
; Y. Z. Zhang
; S. M. Mahajan
; A. D. Liu
; T. Xie
; C. Zhou
; T. Lan
; J. L. Xie
; H. Li
; G. Zhuang
; W. D. Liu
; | | Date: |
1 Jul 2020 | | Abstract: | Through a carefully designed numerical experiment, we demonstrate that a
transition between two distinct phases of energy concentration in a zonal
flow-drift wave system (caviton and instanton) may play a key role in the
intermittent excitation of geodesic acoustic mode (GAM) that are observed in
tokamaks. The two energy structures - the caviton, a slowly breathing spatial
local structure of negative energy, and the instanton, a fine radial structure
of short lifetime in rapid propagation, were recently identified in [Y. Z.
Zhang, Z. Y. Liu, T. Xie, S. M. Mahajan, and J. Liu, Physics of Plasmas 24,
122304 (2017)]; the preceding work is based on the micro-turbulence associated
with ion temperature gradient (ITG) mode, and slab-based phenomenological model
of zonal flow. When toroidal effect are introduced into the system, two
branches of zonal flow emerge: the torus-modified low frequency zonal flow
(TLFZF), and GAM, necessitating a unified exploration of GAM and TLFZF. Indeed,
we observe that the transition (decay) from the caviton to instanton is
triggered by a rapid zero-crossing of radial group velocity of drift wave and
is found to be strongly correlated with the GAM onset; it happens if and only
if the intensity of TLFZF is above a certain level. Many features peculiar to
intermittent GAMs, observed in real machines, are thus identified in the
numerical experiment; the results will be displayed in figures and in a movie.
Since one cannot make a foolproof case that the experimentally observed zonal
flows must originate in ITG driven micro-turbulence (Appendix A), we have also
explored the possibility that this phenomenon could be caused by electron drift
waves, instead; the latter will make part II of this work. | | Source: | arXiv, 2007.0250 | | Services: | Forum | Review | PDF | Favorites | |
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