Nonlinear toroidal mode coupling: a new paradigm for drift wave turbulence in toroidal plasmas

Abstract

Global gyrokinetic particle simulations and nonlinear gyrokinetic theory indicate that electron temperature gradient (ETG) instability saturates via nonlinear toroidal coupling. In such nonlinear interactions, the wave energy at the unstable high toroidal-mode number domain cascades towards the more stable lower toroidal-mode number domain via scatterings off the driven lowmode number quasi-modes. During the saturation process, there is little zonal flow generation and the radial fluctuation envelopes maintain extended structures. The nonlinear coupling process depends critically on the toroidal geometry and, as such, represents a new paradigm for the spectral cascade of drift wave turbulence in toroidal systems.

Effects of parallel dynamics on vortex structures in electron temperature gradient driven turbulence

“…Particularly, in the toroidal plasmas, it has been pointed out that the compression due to the geodesic curvature and the toroidal mode coupling become more important for the saturation of the toroidal ETG instability with strong magnetic shear, where the toroidal modes overlap each other significantly. 14,15 However, in the case with weak magnetic shear, the unstable-mode rational surfaces become more distant from each other so that the toroidal mode coupling weakens. The present study on the slab ETG turbulence may contribute to fundamental understandings of the effects of the parallel compression on the long-timescale evolution of zonal flows and the related transport reduction on the neighborhood of the minimum-q surface ͑q denotes the safety factor͒ in the reversed-shear tokamaks.…”

“…10 Electron temperature gradient ͑ETG͒ modes and/or trapped electron modes are more recently investigated theoretically and numerically as a main cause of the anomalous electron heat transport. [11][12][13][14][15][16][17][18][19][20] Since the perpendicular gyromotion of ambient ions with large gyroradii shields the zonal component of potential fluctuations, the zonal flow generation and the resultant turbulence suppression are weaker than those in the ITG case. 21,22 Thus, the ETG turbulence inherently involves various vortex structures, such as turbulent vortices, zonal flows, and radially elongated streamers, which strongly depend on geometrical and plasma parameters.…”

“…Especially, the nonlinear dynamics of streamers, which may lead to substantial enhancement of the heat transport in toroidal systems, has been actively pursued. 11,[13][14][15]18 From the aspect of turbulence control with regulating the heat transport in the future fusion plasmas, it is worthwhile to understand fundamental physics behind the formation of vortex structures including zonal flows and its stability, as well as the related transport properties.…”

Overview of experimental results on HL-2A

“…Recently, simulation and theory indicate that intermediate scale quasi-modes (QM) also play an important role in determining saturation level of the turbulence and corresponding transport, especially, in cases of the absence of zonal flows such as the turbulence driven by electron temperature gradient (ETG) [26][27][28]. Fig.…”

Nonlinear Dynamics and Complex Behaviors in Magnetized Plasmas of Fusion Interest

“…(5), can be described by three degrees of freedom: the toroidal mode number n, the paraUel mode structure O {nq{r) -m,t) reflecting the radial width of a single poloidal harmonic m, and the radial mode envelope ^"(r,/). Correspondingly, nonlinear interactions can take the following three forms: mode coupling between two «s, distortion of the parallel mode structure and modulation of the radial envelope [59,60]. It has been recognized that drift wave turbulence is the channel through which turbulent transport occurs; at the same time, however, in the description of turbulent transport processes, it is crucially important to account for the radial structures that are spontaneously generated by turbulence itself and regulate turbulence intensity and turbulent transport [61].…”