Observation of L-mode-like core electron temperature fluctuations in H-mode plasmas at ASDEX Upgrade*

Turbulence suppression is important to achieve enhanced confinement in tokamak plasmas. The Correlation Electron Cyclotron Emission (CECE) diagnostic measures turbulent electron temperature fluctuations (dTe/Te) important for transport. Prior work at DIII-D using CECE showed a large reduction in core (r/a ≈ 0.7) dTe/Te from L-mode (dTe/Te ≈ 1.3%) to H-mode (dTe/Te ≤ 0.33%) [1]. Using a new turbulence database [2], we observe significant temperature fluctuations (dTe/Te ≥ 1%) in the core (r/a ≈ 0.4) of auxiliary-heated ELMy H-modes in AUG. These results indicate that reduced core dTe/Te are not guaranteed during H-mode operation. Further, the dTe/Te appear to undergo an abrupt change in magnitude at a collisionality similar to that of Linear to Saturated Ohmic Confinement (LOC-SOC) transitions in ohmically-heated L-mode plasmas at AUG [3]. These results suggest that changes in turbulence underlying the LOC-SOC transition are not unique to ohmically-heated L-mode plasmas.

[1] L. Schmitz et al 2009 Nucl. Fusion 49 095004

[2] C. Yoo et al EPJ Web of Conferences 277, 03001 (2023)

[3] C. Angioni et al PRL 107, 215003 (2011)