Temperature fluctuation measurements in negative triangularity plasmas in ASDEX Upgrade*

Negative triangularity (NT) experiments in TCV [1] and DIII-D [2] have shown H-mode level confinement without Edge Localised Modes (ELMs), setting a path for NT as an operating scenario for burning plasmas. At ASDEX Upgrade, NT plasmas with |δ_av| < 0.2 can be achieved, and these plasmas can undergo the L to H-mode transition depending on ∇B drift configuration and heating power [3]. In this work, we present Correlation ECE measurements in NT plasmas of T_e fluctuation spectra, fluctuation amplitude, and correlation lengths in the outer core and the edge.  We compare power-matched discharges with low (|δ_av| ≈ 0) and moderate (|δ_av| ≈ 0.2) average triangularity.  At moderate |δ_av|, the plasma enters H-mode with ELMs. The low |δ_av| discharge stays in L-mode for the same heating power, contrary to the current understanding that higher negative |δ_av| means H-mode avoidance.  H-mode phases of ECRH-heated and NBI-heated NT plasmas are also compared.  An increase in T_e fluctuation amplitude with radius is observed in both plasmas, but the details of spectral features differ. Initial gyrokinetic simulations are performed for the ECRH and NBI discharges in the outer core and pedestal top.

[1] Camenen et al 2007 Nucl. Fusion 47 510

[2] Austin et al, 2019 Phys. Rev. Lett. 122, 115001 

[3] Happel et al 2023 Nucl. Fusion 63 016002