Interaction of geodesic acoustic modes and turbulence in JFT-2M tokamak / Trapping of turbulence clumps by geodesic acoustic modes

T. Kobayashi:Shear suppression of turbulence by geodesic acoustic modes (GAMs) has been intensely studied because of its impact on plasma confinement [1,2]. GAMs can also affect turbulence profile by phase space trapping mechanism, in which turbulence can be carried radially [3]. In order to unveil self-organization mechanism of turbulence profile, investigation of the mutual interaction between GAMs and ambient turbulence is highly desirable. In JFT-2M tokamak, density and potential fluctuations of GAMs and turbulence were measured by a heavy ion beam probe system [4]. In this study, we analyze spatiotemporal feature of mutual interaction between GAMs and turbulence. Experiments were conducted in JFT-2M tokamak, which has a major radius (R) of 1.3 m and an averaged minor radius (a) of 0.3 m. The neutral beam injection (NBI) power was set to be the threshold power for the L-H transition at the line averaged electron density of n = 1.1x10 19 m -3 . Figure 1 shows the spatiotemporal propagation of the potential fluctuation of the GAM obtained by the conditional averaging analysis. At r-a ~ 3 cm in which the GAM has a maximum amplitude, the GAM propagates radially outward. At r-a ~ 0 cm, i.e., the last closed flux surface (LCFS), the amplitude of the potential fluctuation and the spatial phase delay becomes almost zero. This is interpreted as the reflection of the GAM oscillation at the LCFS. The turbulence amplitude modulation at the GAM frequency is observed as well. The turbulence modulation propagates inward.Possibility of the turbulence trapping by the reflected component of the GAM is discussed according to a theoretical model [3].M. Sasaki:Geodesic acoustic modes (GAMs), which are oscillatory zonal flows, have attracted much attention [1]. Recently, large amplitude energetic particle driven GAMs (EGAMs) have been observed in experiments, and the impact on the background plasma and turbulence could be significant [2]. The suppression of turbulence transport by GAMs has been reported [3]. Experimental study has shown that the formation of a transport barrier can be accompanied by GAMs [4]. On the other hand, the enhancement of turbulence by EGAMs has been observed in turbulence simulations [5], where EGAM destroys a transport barrier, and the turbulence increases in the turbulence stable region. In this way, GAMs can either mitigate or enhance the turbulence. This dual effect of the GAMs on turbulence requires theoretical investigation. In this study, we investigate the phase-space dynamics of spatially inhomogeneous turbulence with a transport barrier in the presence of GAMs. The phase-space dynamics results in trapping of turbulence wave-packets by GAMs. We found that the trapped turbulence wave-packets leak across the transport barrier. As a result, turbulence is enhanced by GAMs in the stable region, while turbulence suppression is obtained in the unstable region. The propagation of the turbulence is ballistic, with the phase velocity of the GAM [6]. Hence, there appears a new global characteristic velocity for turbulence dynamics, in addition to the local group velocity and that of the turbulence spreading [7]. The propagation of trapped turbulence is different from processes such as turbulence spreading and avalanches. The comparison with experiments is also discussed.