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

Abstract

Abstract. 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 behavior across the transition from linear to saturated Ohmic confinement (LOC/SOC). At low collisionailty 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 toward the co-current direction occurs. Linear gyrokinetic calculations indicate that these reversals can not 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 normalised rotation gradient, u', depends strongly on local plasma parameters, in particular on R/L ne . Taken together with turbulent particle transport theory, these results suggest that the coto 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 ne 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 .

Impurity transport, turbulence transitions and intrinsic rotation in Alcator C-Mod plasmas

“…In these works, no clear ITG to TEM transition appears responsible, but rather, more subtle changes in the turbulence correlate with the rotation and particle transport changes. The current work presented here is consistent with these findings [48,18,47,20].…”

“…The nonlinear gyrokinetic analysis of C-Mod plasmas presented here suggests that a clear transition from ITG to TEM does not accompany changes in particle transport and rotation profiles as previously reported on ASDEX-Upgrade [22]. We note that the relationship between changes in the the toroidal rotation and ITG to TEM transition has also been studied recently in Ohmic plasmas and auxiliary heated plasmas at Alcator C-Mod [47] and ASDEX-Upgrade [48,18,20]. In these works, no clear ITG to TEM transition appears responsible, but rather, more subtle changes in the turbulence correlate with the rotation and particle transport changes.…”

Quantitative comparison of electron temperature fluctuations to nonlinear gyrokinetic simulations in C-Mod Ohmic L-mode discharges

“…11,12 It has been suggested that a unifying hypothesis for these changes is a change in the underlying turbulence from dominant trapped electron mode (TEM) to ion temperature gradient (ITG) across the LOC/SOC transition, 11,12 which will be a continuous transition in terms of their relative amplitudes, not discrete, in reality. There is counter evidence from C-Mod 13 and from ASDEX Upgrade, 14 suggesting that the LOC/SOC transition occurs without a change in turbulence from TEM to ITG. It is also noteworthy that we do not understand clearly all these changes are coupled together.…”

“…Although rotation reversal occurs robustly with the LOC/SOC transition in C-Mod, 15 it has been also observed that these two phenomena are not correlated in Tore-Supra 16 and ASDEX-Upgrade. 14,17 These conflicting observations motivated the current work. In addition to studying the physics of the LOC/SOC transition, validation of gyrokinetic codes in a variety of plasmas is of interest.…”

Main-Ion Intrinsic Toroidal Rotation Profile Driven by Residual Stress Torque from Ion Temperature Gradient Turbulence in the DIII-D Tokamak

“…In helium main-ion plasmas on DIII-D the radial location of the helium velocity hollowing was seen to be dependent on the deposition location of the electron heating [10,11], which ties the reversal phenomenon to the energy flux. Variations of the rotation gradient with changes of the local density gradient have been also been observed [12] indicating the need for local profile investigation to organize the observations. Investigations into the type of linearly unstable turbulence between ion and electron modes that correlate with the observations have been explored [12][13][14], indicating that other processes beyond the dominant turbulence mode, such as the collisionality scaling of non-Maxwellian equilibria [15], are required to explain the wide range of observed phenomenology.…”