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

Measurements of long wavelength ($k_{\perp }{\rho }_i$  <  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 (P _LH) and can help explain the isotopic and density dependence of P _LH, and how the P _LH 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 n _e ∼ 4  ×  10¹⁹ m⁻³, P _LH is seen to converge for H and D, and increases for both with higher density. Surprisingly, the P _LH 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, n _e ∼ 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 P _LH 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, n _e ∼ 5  ×  10¹⁹ m⁻³, where P _LH 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 P _LH may account for the strong isotope and density dependencies of P _LH 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. (paper)