On Multi-Scale Interactions Among Microturbulence, Tearing Modes, and Zonal Flows

Analytic theory and gyrokinetic simulations show that turbulence-regulating zonal flows are weakened by radial magnetic field fluctuations as a consequence of particle streaming along radial fields and shorting out cross-flux-surface potential differences. Two prominent sources of radial magnetic field fluctuations are studied here, resonant magnetic perturbations (RMPs) in tokamaks and tearing modes in reversed-field pinches (RFPs). This work focuses on understanding the inherently multi-scale nature of the interplay of microturbulence, zonal flows, and tearing modes and its effect on transport. This interplay is studied with gyrokinetics to model DIII-D tokamak and MST RFP plasmas. An imposed magnetic perturbation that mimics a tearing mode increases the level of trapped-electron-mode turbulence to a level consistent with fluctuation and transport measurements in MST plasmas. This motivated a dedicated experiment on DIII-D to study the impact of varying RMP amplitude on turbulence in inboard-limited L-mode plasmas. Highlights of the theory-experiment comparison are presented. To study the self-consistent multi-scale interaction of the tearing mode physics, nonlinear simulations containing both tearing mode (driven from equilibrium current gradients) and microinstability scales are performed in a slab geometry. The system is characterized by distinct microinstability- and tearing-dominated regimes. Within the microturbulence-dominated phase, the slow tearing mode growth corresponds directly to a decay in zonal flow and a corresponding increase in the electrostatic turbulence amplitudes. For the tearing-dominated regime, we discuss the possibility of gradient-enhanced tearing caused by the microturbulence, as well alterations to saturated island structure.