Observations of main-ion toroidal rotation and turbulence fluctuations across the ITG/TEM transition in DIII-D and comparison to gyrokinetic simulations*

New full-profile main-ion rotation measurements and gyrokinetic simulations across the transition from ion temperature gradient (ITG) to trapped electron mode (TEM) transport show that main-ion toroidal rotation smoothly progresses from deeply hollow to flat. Furthermore, the impurity toroidal rotation is clearly peaked in the TEM regime in contrast to the very flat main-ion rotation. Main-ion rotation, rather than impurity rotation, is representative of the bulk angular momentum transport and this critical distinction has significant implications on intrinsic rotation studies when only impurity measurements are available. These main-ion measurements were obtained during ohmic deuterium plasma discharges by controlled ramp-down of the plasma density, with electron cyclotron heating in the low-density conditions to access the TEM regime. DBS measurements show long wavelength turbulence in the high density ITG regime and absence of long wavelength fluctuations in the low density TEM regime. In the spatial region where the main-ion rotation gradient changes, GYRO simulations show the linear stability spectrum transitions from ion to electron direction, indicative of an ITG to TEM transition. In the ITG regime, uncertainty analysis indicates close proximity to the ITG/TEM boundary, showing this may be a mixed-mode regime where the most unstable mode depends sensitively on the plasma radius and temperature scale length. However, turbulence measurements indicate the presence of ion-scale fluctuations at this time in the discharge, bolstering the simulation results.  Beyond local linear stability, radially global nonlinear simulations have been performed with GTS, and predict hollow and flat profiles in the ITG and TEM regimes, respectively. These measurements and simulations show the need for direct main-ion rotation measurements for validating predictive models of intrinsic plasma rotation.