Geodesic–acoustic-mode in JFT-2M tokamak plasmas

Abstract

The characteristics of geodesic-acoustic-mode (GAM) are investigated through direct and simultaneous measurement of electrostatic and density fluctuations with a heavy ion beam probe.The amplitude of the GAM changes in relation to the radial position; it is small near the separatrix, reaches a local maximum at 3 cm inside the separatrix and then decreases again to 5 cm inside the separatrix. The frequency is constant in the range, though the predicted GAM frequency varies according to the temperature gradient. The correlation length is about 6 cm and comparable to the structure of the amplitude of the GAM. The results indicate the GAM has a radial structure which reflects the local condition at about 3 m inside the separatrix.The phase relation between the GAM oscillation indicates that the GAM is a radial propagating wave.The interaction between the GAM and the ambient density fluctuation is shown by the high coherence between the GAM oscillation and the temporal behaviour of the ambient density fluctuation. Moreover, the phase relation between the electric field fluctuation of the GAM ( Ẽr,GAM ) and the amplitude of the density fluctuation indicates that the modulation of the ambient density fluctuation delays the Ẽr,GAM . The causality between the GAM and the modulation of the density fluctuation is revealed.

Geodesic Acoustic Eigenmodes

“…This is consistent with the observation that GAM oscillations are observed as radial eigenmodes [11]. The radial wavelength has a dependence of ρ 2/3 i L 1/3 T , showing that GAMs are mesoscale fluctuations.…”

“…In tokamaks and other toroidal plasmas, the plasma temperature changes in the radial direction, so that the dispersion relation ω = ω G , which is provided by the local theory, predicts different frequencies at different radii. In contrast, fluctuations with a common frequency are observed within a region which has a substantial width in radial direction [10,11]. This indicates that the GAM oscillation appears as an eigenmode.…”

“…The geodesic acoustic mode (GAM) is a kind of zonal flow, which has finite real frequency owing to the geodesic curvature of a toroidal magnetic field [2], and is driven by microscopic turbulence [3,4]. Measurements of GAMs have been recently reported [5][6][7][8][9][10][11]. It has been known that the GAMs have real frequency ω G = √ 2c s /R in tokamaks (c s : ion sound velocity, R: majour radius).…”

Experimental Study of Nonlinear Processes in Edge Turbulence of Toroidal Plasmas

“…The phase difference between the potential fluctuation and the density perturbation is of interest for identifying the fluctuation mode. The phase and amplitude difference betweenñ andφ have been measured by the use of the HIBP [28,31]. The phase difference betweenñ andφ was found to be con-041-4 sistent with the prediction by the theory for GAMs, and the phase difference in the range of 50 -100 kHz was around (0.1 -0.2)π, confirming the predictions for drift waves.…”

The role of plasma elongation on the linear damping of zonal flows

“…GAMs have been observed on DIII-D ͑Ref. 1͒ by Beam Emission Spectroscopy, in JIPT-IIU, 2 JFT-2M, 3 and T-10, 4 by using heavy ion beam probes, in the ASDEX-Upgrade by Doppler reflectometry 5 and in the Heliac H1 with an array of Langmuir probes. 6 The interest in GAMs is due to the role they play in the turbulence saturation mechanism.…”