Spectral features of the geodesic acoustic mode and its interaction with turbulence in a tokamak plasma

Abstract

The three-dimensional wavenumber and frequency spectrum for the geodesic acoustic mode (GAM) has been measured in the HuanLiuqi-2A tokamak for the first time. The spectrum provides definite evidence for the GAM, which is characterized by kθ=kϕ=0 and krρi≈0.04−0.09 with the full width at half-maximum Δkrρi≈0.03−0.07. The localized GAM packet is observed to propagate outward in the radial direction with nearly the same phase and group velocity. The envelopes of the radial electric field and density fluctuations are observed to be modulated by the GAM. By comparing the experimental result with that of the envelope analysis using model signals, the mechanism of the envelope modulation has been identified. The results strongly suggest that the envelope modulation of the Ẽr fluctuations is dominantly caused by the direct regulation of the GAM during the GAM generation in the energy-conserving triad interaction, and the envelope modulation of the density fluctuations is induced by the GAM shearing effect, which transfers the fluctuation energy from low to high frequencies. In addition, the cross- and auto-bicoherences for interactions between the GAM and turbulent fluctuations show a similar peaked feature that may reflect the resonant property in the nonlinear coupling between the GAM and turbulent fluctuations.

Overview of experimental results on HL-2A

“…6(a) and 6(b), showing that over a large frequency range from 0.5 kHz to ~15 kHz, the correlation coefficients are high and the mode numbers are round zero. The coherent mode at 8.5 kHz has been identified as the GAM [22]. The low frequency (LF) feature with the potential structure of 0 m n = = is consistent with the theoretical prediction and simulation result for the LFZF [23].…”

Zonal flow energy ratio evolution during L-H and H-L transitions in EAST plasmas

“…The characteristic of H mode is that the turbulence level is reduced compared to L mode, a strong radial electric field  E r ( ) shear exists and the ion pressure gradient develops in the plasma edge region [5,6]. Zonal flows [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] must also be considered in regulating turbulence and triggering the L-H transition. Zonal flows include the low frequency zonal flow (LFZF) and the geodesic acoustic mode (GAM).…”

Landau damping of geodesic acoustic mode in toroidally rotating tokamaks

“…It was experimentally observed in many tokamak devices (see, for examples, Refs. [3][4][5] and intensively investigated in terms of theoretical analyses [6][7][8][9][10][11][12][13]and numerical simulations [14][15][16][17]. GAM is basically a sort of electostatic perturbation with toroidally symmetrical and poloidally nearly symmetrical structure.…”