Nonlinear Saturation of Kinetic Ballooning Modes by Zonal fields in Toroidal Plasmas

TL;DR

This paper presents global gyrokinetic simulations showing that kinetic ballooning modes in tokamaks saturate nonlinearly through zonal fields, avoiding singularities predicted by ideal MHD, and involve rapid localized current growth.

Contribution

It demonstrates the nonlinear saturation mechanism of KBMs via zonal fields using gyrokinetic simulations, highlighting kinetic effects in plasma turbulence.

Findings

Kinetic ballooning modes develop into an exponential growth phase.

Saturation is regulated by spontaneously generated zonal fields.

Rapid growth of localized current sheets is observed during nonlinear evolution.

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.