Observation and Quasilinear Modeling of Rotation Reversal Hysteresis in Alcator C-Mod Plasmas

Analysis and modeling of a new set of rotation reversal hysteresis experiments unambiguously show that changes in turbulence are responsible for the intrinsic rotation reversal and the Linear to Saturated Ohmic Confinement (LOC/SOC) transition on Alcator C-Mod. Plasmas on each side of the transition exhibit different toroidal rotation profiles and different turbulent fluctuations, despite having profiles of density and temperature that are indistinguishable within measurement uncertainty – suggesting a bifurcation process. This process is modelled using a linear mode quasilinear transport approximation, with quasilinear weights calculated through linear gyrokinetic simulation. The deactivation of subdominant (in linear growth rate) ion-scale ITG and intermediate-scale TEM-like instabilities is identified as the only possible change in turbulence across the reversal which is consistent with the experimentally measured profiles and the inferred heat and particle fluxes. Thus, this work suggests a path for understanding the LOC/SOC transition and rotation reversal hysteresis through the dynamics of linearly subdominant, rather than dominant, modes and by changes in relative mode saturation levels.