Theory-based modeling of LOC–SOC transitions in ASDEX Upgrade

Modeling of the linear-to-saturated ohmic confinement transitions is performed on the ohmic discharges database collected at ASDEX Upgrade. Ion and electron temperatures and densities are predicted using the TGLF turbulence transport model. Self-consistent classical energy equipartition is retained, as well as the impact of low-Z impurities, the core concentration of which is largest at the lowest plasma densities and decreases with increasing electron density. The simulation results are then compared to the experimentally estimated confinement time behavior, and locally to the energy diffusivities behavior across the density ramps. Both density and plasma current are scanned. The key result, which somewhat confirms previous studies, is that the ion energy transport channel is dominated by the ITG instability, which becomes stronger at higher densities due to the reduction in impurity content (as such, reduction in dilution), as well as a moderate increase in ion temperature normalized gradient. A direct impact of the transition between electron modes (TEM) and ITG on the LOC–SOC is not observed. However, this is necessary to explain the behavior of the density profile. The linear phase of the confinement is simply attributed to the decrease of electron heat transport as the electron and ion temperatures become more coupled.