Turbulence and sheared flow structures behind the isotopic dependence of the L-H power threshold on DIII-D. (18th September 2017)

• Record Type: Journal Article
• Authors: Yan, Z.; Gohil, P.; McKee, G.R.; Eldon, D.; Grierson, B.; Rhodes, T.; Petty, C.C.
• Is Part Of: Nuclear fusion. Volume 57:Number 12(2017:Dec.)
• Publication Date: 2017-09-18
• Subjects: L-H transition -- transition power threshold -- turbulence dynamics -- Reynolds stress; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/aa82c9
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: Measurements of long wavelength ( k ⊥ ρ 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 × 10 19 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 × 10 19 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 × 10 19 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.