Turbulence and sheared flow structures behind the isotopic dependence of the L-H power threshold on DIII-D

Abstract

Measurements of long wavelength ( < 1) density fluctuation characteristics in the edge of both Deuterium (D) and Hydrogen (H) plasmas across the L-H transition on DIII-D demonstrate the existence of single or double bands of low-wavenumber turbulence observed near the edge of H and D plasmas. These are strongly correlated with the L to H-mode transition power threshold (PLH) and can help explain the isotopic and density dependence of PLH, and how the PLH difference is reduced at higher density. Understanding and accurately predicting the L-H power threshold is critical to accessing to H-mode, and operating and achieving high confinement in burning plasmas such as ITER. Above about ne ~ 4 × 1019 m−3, PLH is seen to converge for H and D, and increases for both with higher density. Surprisingly, the PLH increases significantly at low density in H but not in D plasmas. Two distinct frequency bands of density fluctuations are observed in the D plasmas at low density, ne ~ 1.2–1.5 × 1019 m−3, but not in H plasmas with similar density, which appears to be correlated to the much lower power threshold in D at low density. Consistently, E × B shear in the region of r/a ~ 0.95–1.0 is larger in D plasmas than in H plasmas at low density; as the PLH increases with increasing density, the dual mode structure disappears while E × B shear becomes similar and small for both D and H plasmas at higher density, ne ~ 5 × 1019 m−3, where PLH is similar for both D and H plasmas. The increased edge fluctuations, increased flow shear, and the dual-band nature of edge turbulence correlating with lower PLH may account for the strong isotope and density dependencies of PLH and support current L-H transition theories but suggest a complex behavior that can inform a more complete model of the L-H transition threshold.