Dynamics of poloidal flows and turbulence at the H-mode transition in W7-AS

The dynamic interplay of turbulence propagation velocity v ⊥ and turbulence amplitude at the H-mode transition is studied by means of Doppler reflectometry. The optimized microwave antenna design allows for a temporal resolution of less than 10 µs. For the selected radial layers in the first few centimetres inside the separatrix v ⊥ closely follows the E × B -velocity as it is obtained from impurity spectroscopy. Both the L–H transition as well as H–L back-transition occur at a certain threshold of the velocity shear; a typical value for a position 2 cm inside the separatrix is ∇ E r = − 100 V cm −2 . As a general trend in both L- and H-mode the turbulence amplitude increases with decreasing v ⊥ corresponding to a decreasing velocity shear ∇ v ⊥ . This relation is broken at the transition itself where a sudden change of is observed within 200 µs whereas no significant discontinuity is found for v ⊥ . On a fast timescale the probed plasma layer displays a rich dynamic behaviour such as large correlated excursions of v ⊥ and with a frequency expected for geodesic acoustic modes and nearly completed transitions prior to the final one. We propose that the observed dynamics around the H-mode transition can be considered critical fluctuations around the phase transition between high- and low-rotation plasma edge.