Electromagnetic gyrokinetic turbulence in finite-beta helical plasmas

Abstract

A saturation mechanism for microturbulence in a regime of weak zonal flow generation is investigated by means of electromagnetic gyrokinetic simulations. The study identifies a new saturation process of the kinetic ballooning mode (KBM) turbulence originating from the spatial structure of the KBM instabilities in a finite-beta Large Helical Device (LHD) plasma. Specifically, the most unstable KBM in LHD has an inclined mode structure with respect to the mid-plane of a torus, i.e., it has a finite radial wave-number in flux tube coordinates, in contrast to KBMs in tokamaks as well as ion-temperature gradient modes in tokamaks and helical systems. The simulations reveal that the growth of KBMs in LHD is saturated by nonlinear interactions of oppositely inclined convection cells through mutual shearing as well as by the zonal flow. The saturation mechanism is quantitatively investigated by analysis of the nonlinear entropy transfer that shows not only the mutual shearing but also a self-interaction with an elongated mode structure along the magnetic field line.

Microturbulence studies of pulsed poloidal current drive discharges in the reversed field pinch

“…All RFP modeling work has thus far shown no evidence of KBMs, a mode known to arise in finite-b simulations in the tokamak at a typical b of 0.6%-2%. 23,27,28 The absence of KBMs in the RFP may be explained as a consequence of two geometric properties: high magnetic shear and low safety factor. In the tokamak, KBMs are seen to emerge at some considerable fraction (generally about 70%-100%) of the ideal ballooning mode critical b.…”

Magnetic compressibility and ion-temperature-gradient-driven microinstabilities in magnetically confined plasmas

“…With some notable exceptions [4][5][6][7][8][9][10], electromagnetic perturbations have been considered mainly from a numerical standpoint [11][12][13][14][15], and the attempt to understand their role in ITG stability has resulted in a patchy collection of numerical findings rather than in a coherent physical picture. Moreover, most studies have neglected magnetic compressibility and have thus neglected the magnetic perturbations parallel to the equilibrium magnetic field, δB , that are generated by the instability to maintain perpendicular pressure balance.…”

Strongly driven surface-global kinetic ballooning modes in general toroidal geometry

“…The effect of k 1 must be carefully considered for each non-axisymmetric machine under consideration, depending on the global shear of its configurations. For instance, a finite k 1 seems to be crucial to capture the most unstable KBM in LHD [see Fig. (1) of Ishizawa et al (2014)]. In the case of W7-X, k 1 could have a less prominent role (Aleynikova et al 2018).…”