Quantification of Turbulent Driving Forces for the Geodesic Acoustic Mode in the JFT-2M Tokamak

We investigate spatial structures of turbulence and turbulent transport modulated by the geodesic acoustic mode (GAM), from which the excitation mechanism of the GAM is discussed. The GAM is found to be predominantly excited through a localized Reynolds stress force, rather than the dynamic shearing force. The evaluated growth rate is larger than the linear damping coefficients and is on the same order of magnitude as the effective growth rate evaluated from time evolution in the GAM kinetic energy.

Figure 1

(a) Schematic of the heavy ion beam probe and (b) time evolution of the potential fluctuation spectrum at $r-a\sim -3\mathrm{cm}$.

Figure 2

Radial profiles of (a) the frequency dependent potential power spectrum density, (b) the GAM amplitude and phase, (c) the skewness of GAM amplitude, and (d) the time evolution of the GAM potential fluctuation and its envelope at $r-a\sim -1\mathrm{cm}$.

Figure 3

Amplitude and phase profiles of the turbulence and turbulent transport modulation at the GAM frequency: (a) turbulence amplitude, (b) radial wave number, (c) Reynolds stress, and (d) particle flux. The left and right vertical axes scale amplitude and phase, respectively.

Figure 4

Radial profiles of (a),(b) the modulation amplitude, (c),(d) the modulation phase, and (e),(f) the expected growth rate for the Reynolds stress force and dynamic shearing force, respectively. An effective growth rate evaluated from the time evolution of the GAM kinetic energy is overplotted in (e).