Insight into L-H transition physics through high resolution Er measurements in the plasma edge of ASDEX Upgrade
The formation of the edge radial electric field (Er) was investigated experimentally at ASDEX Upgrade in L-modes of favourable and unfavourable drift configuration prior to confinement transitions.
Er measurements using Doppler reflectometry (DR) and helium spectroscopy (HES) show that in L-modes with matched plasma density and heating power the Er-well in the edge of the confined plasma is shallower in unfavourable than in favourable drift configuration, whereas the main ion pressure gradient (∇rpi/eni) is similar (see Fig. 1a). Neoclassical (NC) theory can not explain this difference. The new Er measurements are also the first to show that the respective confinement transitions, L- to H-mode (LH) in favourable drift configuration and L- to I-mode (LI) and I- to H- mode (IH) in unfavourable drift configuration, occur at different Er gradients (see Fig. 1b). In the framework of a critical ExB velocity shear suppressing the edge turbulence this suggests that the strength of the underlying characteristic edge turbulence is different between the two drift configurations.
U. Plank et al., Phys. Plasmas 30, 042513 (2023); https://doi.org/10.1063/5.0102763