Edge turbulence velocity preceding the L-H transition in NSTX

The low-to-high mode or L-H transition in tokamaks involves a sudden reduction in the edge turbulence level and a decrease in the edge plasma transport. The mechanism for the L-H transition is widely believed to be associated with changes in the poloidally averaged poloidal turbulence velocity. Using a gas puff imaging (GPI) diagnostic, the poloidally averaged poloidal turbulence velocity ⟨Vpol⟩pol was measured for a set of 16 shots with L-H transitions in NSTX. These measurements were focused –2 cm inside the separatrix where the relative fluctuation level as seen in GPI decreases significantly and consistently at the transition. The magnitude of this poloidal velocity preceding the transition varied widely, ranging from |Vpol| ∼ 1–4 km/s during the last 1 ms before the transition, and the magnitude of the radial gradient also had a wide shot-to-shot range of |grad Vpol| ∼ 0.34 to 1.9 km/s cm during this same time. The frequency spectrum of ⟨Vpol⟩pol during the 30 ms before the transition had a near-coherent peak at ∼2–5 kHz, but only in about half of these shots. In general, there were no clear and consistent changes in the poloidal velocity of the turbulence preceding the L-H transition in this database.