Nonlinear saturation of kinetic ballooning modes by zonal fields in toroidal plasmas

Abstract

Kinetic ballooning modes (KBM) are widely believed to play a critical role in disruptive dynamics as well as turbulent transport in tokamaks. While the nonlinear evolution of ballooning modes has been proposed as a mechanism for "detonation" in tokamak plasmas, the role of kinetic effects in such nonlinear dynamics remains largely unexplored. In this work global gyrokinetic simulation results of KBM nonlinear behavior are presented. Instead of the finite-time singularity predicted by ideal MHD theory, the kinetic instability is shown to develop into an intermediate nonlinear regime of exponential growth, followed by a nonlinear saturation regulated by spontaneously generated zonal fields. In the intermediate nonlinear regime, rapid growth of localized current sheet is observed.

Global gyrokinetic simulation of turbulence driven by kinetic ballooning mode

“…Recently, nonlinear evolution of KBM has been investigated by means of global electromagnetic gyrokinetic simulations. 18 The simulation results imply the importance of the zonal mode, i.e., zonal flows, zonal magnetic fields, and zonal pressure, in the saturation of KBMs. In order to elucidate the essential mechanism of the saturation, this simulation includes only a pair of unstable KBMs with n ¼ 610 and n ¼ 0 toroidal modes.…”

Numerics and computation in gyrokinetic simulations of electromagnetic turbulence with global particle-in-cell codes

“…The principle difference of Ref. [4] comparing to our paper is the treatment of the electrons. In Ref.…”

“…There are a number of gyrokinetic codes which develop capabilities for global simulations in electromagnetic regimes, for example GEM [2], GTC [3,4], XGC [5], GENE [6], GKNET [7], GKW [8]. In this paper, we employ the well-established gyrokinetic particle-in-cell (PIC) codes ORB5 [9] and EUTERPE [10].…”

“…[7] using the Eulerian code GKNET and Ref. [4] where the gyrokinetic PIC code GTC has been used. The principle difference of Ref.…”

“…In Ref. [4], the electrons are treated in the fluid approximation ("fluid-kinetic hybrid electron model") whereas this papers uses the fully gyrokinetic description for all species. In the fluid-electron approximation, the cancellation problem is not an issue, in contrast to our simulations where the cancellation problem must be solved.…”