Description
A validated, physics-based model of the L–H transition threshold is essential for estimating heating requirements in future burning-plasma experiments. However, no consensus has yet been reached on the mechanisms that trigger the transition, particularly regarding the generation of the initial flow shear believed to be required for turbulence suppression.
The aim of this work is to improve understanding of turbulence reduction at the L–H transition. The JET correlation reflectometer allows determination of the perpendicular velocity, $v_⊥$, and density fluctuation level, $A_n$, slightly above the outer midplane, from the SOL to inside the pedestal top. L–H transitions were induced at JET using NBI or ICRH in plasmas with different main ion masses and divertor configurations. Earlier studies found no universal edge flow shear across the JET dataset, suggesting that H-mode access is not solely set by the $v_⊥$ profile [C. Silva NF 2021]. No significant variation in $v_⊥$ was detected during the heating ramp, though fast dynamics (<10 ms) near the transition were not explored at that time.
New high-temporal-resolution results, obtained in both single-step and long-dithering L–H transitions, indicate that $v_⊥$ and $A_n$ are strongly intermittent, yet show no clear trend in the milliseconds preceding the transition. A robust feature of the L–H transition is the sharp reduction of turbulence on a timescale shorter than 50 μs, localized from the separatrix to the pedestal top. Simultaneously, the density profile steepens, though in a narrower radial region. No clear changes in the edge flow shear are observed before the L–H transition, nor immediately afterward, when turbulence suppression begins. These results highlight current limitations while also defining a path forward, namely the need to characterize turbulence in order to enable validation of physics-based models of the L–H transition.