Observation of a Critical Gradient Threshold for Electron Temperature Fluctuations in the DIII-D Tokamak

Abstract

A critical gradient threshold has been observed for the first time in a systematic, controlled experiment for a locally measured turbulent quantity in the core of a confined high-temperature plasma. In an experiment in the DIII-D tokamak where L[subscript T[subscript e]][superscript -1]=|∇T[subscript e]|/T[subscript e] and toroidal rotation were varied, long wavelength (k[subscript θρs]≲0.4) electron temperature fluctuations exhibit a threshold in L[subscript T[subscript e]][superscript -1]: below, they change little; above, they steadily increase. The increase in δT[subscript e]/T[subscript e] is concurrent with increased electron heat flux and transport stiffness. Observations were insensitive to rotation. Accumulated evidence strongly enforces the identification of the experimentally observed threshold with ∇T[subscript e]-driven trapped electron mode turbulence.