Core intrinsic rotation behaviour in ASDEX Upgrade ohmic L-mode plasmas

摘要

The ASDEX Upgrade intrinsic rotation database has been expanded to include a large number of measurements from ohmic L-mode discharges covering a wide range of plasma densities, currents and magnetic fields. This database was then used to study the rotation behaviour across the transition from linear to saturated ohmic confinement (LOC/SOC). At low collisionality the plasma is in the LOC regime and the toroidal rotation profile is in the co-current direction. As the collisionality is increased and the plasma transitions from LOC to SOC, the core rotation decreases resulting in a hollow, counter-current profile. Even deeper in the SOC regime, however, a second reversal back towards the co-current direction occurs. Linear gyrokinetic calculations indicate that these reversals cannot be explained by a transition from a trapped electron mode (TEM) to an ion temperature gradient (ITG) dominated regime. Rather, the analysis indicates that the intrinsic normalized rotation gradient, u', depends strongly on local plasma parameters, in particular on R/L_(nc). Taken together with turbulent particle transport theory, these results suggest that the co- to counter-current directed rotation reversal occurs in the TEM regime due to profile changes and not due to transition from TEM to ITG. The second reversal back to the co-current direction is explained by the reduction of R/L_(nc) observed in ITG dominated plasmas. Lastly, a simple linear model for the residual stress assuming a fixed poloidal tilt angle of the turbulent eddies was applied to the database. This model is able to capture the main parameter dependencies observed in the data and demonstrates that at least two processes contributing to the poloidal tilt of the turbulent eddies are needed to reproduce the experimentally observed u' values: one proportional to the sign of the turbulence propagation and the second independent of ω_r.