Exploring the role of sheared flows in the L-H transition

Transport in toroidally confined plasmas is primarily by turbulent transport, which leads to substantial losses. This is partially mitigated when operating in the H-mode regime, which is characterized by steep edge gradients and a marked reduction in turbulent transport. Due to its superior confinement properties, H-mode is regarded as the preferred operational regime for fusion reactors. Consequently, extensive efforts have been devoted to understanding the mechanisms behind the L-H transition, with a particular focus on the role of sheared flows in suppressing turbulence.

The first goal of this work was to evaluate whether a threshold value for perpendicular velocity shear is required to access H-mode. Doppler backscattering measurements were analyzed and the perpendicular velocity profiles fitted using least squares splines, with average shear flow calculated across different regions at the plasma edge. Results indicate that no consistent threshold value exists across the dataset studied.

The second goal of this study was to leverage the dithered L-H transition to achieve enhanced radial and temporal resolution of the transition, allowing detailed tracking of the evolution of the radial electric field, turbulence, and density profiles throughout the L-H-L cycle. Dithers were synchronized and conditionally averaged, providing robust statistical significance to the analysis.

Our findings reveal that the reduction in fluctuation levels is accompanied first by a steepening of the density profile, followed only later by a deepening of the perpendicular velocity well. These results challenge the traditional view that turbulence suppression is driven primarily by an increase in sheared flows.