Connections between confinement saturation, profile peaking, and collisionality in L, I, and H-mode plasmas at ASDEX Upgrade

Higher plasma densities are correlated with saturated energy confinement in ohmically-heated L-mode plasmas [1] and I-mode plasmas [2] and degraded confinement in H-mode plasmas [3]. In this work, we show evidence that the quality of confinement of tokamak plasmas saturates at high density due to a universal collisionality-driven mechanism regardless of edge characteristics. We first examine connections between global changes in plasma confinement and local changes in collisionality and turbulence in the core of auxiliary (NBI and ECRH) heated ELMy H-mode plasmas at ASDEX Upgrade (AUG) using measurements from the Correlation Electron Cyclotron Emission (CECE) diagnostic. We then compare correlations between density peaking and global changes in confinement in the H-mode dataset to correlations between these same parameters across the LOC-SOC transition in ohmically-heated L-mode plasmas as well as auxiliary-heated I-mode plasmas. We find that the onset of confinement saturation occurs when the peaking of the density profiles is maximized in all three operating regimes.

[1] J.E. Rice et al., Nucl. Fusion 60, 105001 (2020)

[2] D. Silvagni et al., Nucl. Fusion 63, 084001 (2023)

[3] M. Greenwald, Plasma Phys. Control. Fusion 44, R27 (2002)