Ion scale nonlinear interaction triggered by disparate scale electron temperature gradient mode

Abstract

We have observed that the disparate scale nonlinear interactions between the high-frequency ($0.4 MHz) electron temperature gradient (ETG) mode and the ion-scale low-frequency fluctuations ($kHz) were enhanced when the amplitude of the ETG mode exceeded a certain threshold. The dynamics of nonlinear coupling between the ETG mode and the drift wave (DW) mode has already been reported [C. Moon, T. Kaneko, and R. Hatakeyama, Phys. Rev. Lett. (2013)]. Here, we have newly observed that another low-frequency fluctuation with f ' 3.6 kHz, i.e., the flute mode, was enhanced, corresponding to the saturation of the DW mode growth. Specifically, the bicoherence between the flute mode and the DW mode reaches a significant level when the rT e /T e strength exceeded 0.54 cm À1 . Thus, it is shown that the ETG mode energy was transferred to the DW mode, and then the energy was ultimately transferred to the flute mode, which was triggered by the disparate scale nonlinear interactions between the ETG and ion-scale low-frequency modes. V C 2015 AIP Publishing LLC. [http://dx.There is strong experimental evidence that the anomalous heat transports are attributed to electron temperature gradient (ETG) driven fluctuations (ETG mode) in magnetically confined plasmas. 1-4 However, the physical mechanism of the ETG mode-induced anomalous transport has not been fully clarified yet because the experimental observations of the ETG mode are limited due to device configurations and are based on the conventional transport process models. 5 Recently, it has been widely recognized that the nonlinear coupling with the fluctuations plays important roles in plasma transport. 6 Therefore, it is necessary to consider the nonlinear interactions with the ETG mode and the other fluctuations in order to understand clearly the mechanism of the ETG mode-induced transport. In particular, the importance of the coupling of the ETG mode with poloidally localized large-scale fluctuations, not zonal flows, has been pointed out in toroidal plasmas. 7,8 In order to understand the evolution of the ETG mode in toroidal plasmas, such a nonlinear interaction between the ETG mode and large-scale fluctuations must be studied in detail. For this purpose, an experiment on the ETG mode in the linear device plasma may provide essential information, even though the ETG mode in the slab cylindrical plasma is not identical to the toroidal ETG mode.For decades, low-frequency fluctuations, such as the drift wave (DW) mode, the flute mode, and others, have been considered to be the main cause of plasma transport. These fluctuations have been well investigated and reported. 5,9 Furthermore, it has been reported that nonlinear couplings of the ETG mode with disparate scale fluctuations 7,8 drive the meso-scale structures such as zonal flows and streamers. 10 Thus, we investigated the nonlinear phenomena of the ETG mode by precisely controlling the ETG in linear magnetized plasmas in order to clearly understand the mechanism of disparate scale nonlinear interactions in the ETG mode...