Tuesday, November 28, 2017
Energy confinement time in Ohmic L-mode plasmas across multiple tokamaks has been found to scale with density in two major regimes, Linear Ohmic Confinement (LOC) and Saturated Ohmic Confinement (SOC). These confinement regimes have also been found to be correlated with intrinsic rotation generated in the plasma with no external torque sources, as the core toroidal rotation direction has been found to reverse direction when the normalized collisionality ν*, evaluated at the profile minimum, passes through a critical value around 0.4. Analysis and modelling of recent rotation reversal hysteresis experiments carried out on Alcator C-Mod unambiguously show that changes in turbulence are responsible for the intrinsic rotation reversal and the Linear to Saturated Ohmic Confinement (LOC/SOC) transition. Plasmas exhibit profiles of density and temperature which are nearly identical within error bars, but with differing rotation and turbulent fluctuations, placing a very tight constraint on possible mechanisms for the LOC/SOC transition which depend on a change in mean density and temperature profiles or gradients. Through application of a quasilinear transport approximation, the deactivation of a subdominant TEM-like mode is identified to be consistent with the observed change in turbulence. This work provides strong evidence against the long-standing conjecture that the LOC/SOC transition is indicative of a change in the dominant ion-scale drift-wave instability, and suggests the importance of subdominant modes in determining the qualitative behavior of plasmas.