Zachary Williams, Department of Physics, University of Wisconsin-Madison

"On The Interactions of Magnetic Fluctuations, Zonal Flows, and Microturbulence in Fusion Plasmas"

Abstract: 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 large-scale magnetic fluctuations 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. Experimental observations demonstrate a clear dependence of microturbulence levels on RMP strength. Gyrokinetic simulations confirm this dependency, and show direct connections of this behavior to zonal flow degradation via magnetic fluctuations. To approach the underlying physics from a third angle, preliminary results from simulations which include a newly-incorporated current gradient drive that allow for the self-consistent generation of tearing modes and their interactions with microinstability are presented.