Comparison of energy confinement saturation in L, I, and H-mode plasmas with strong electron heating at ASDEX Upgrade

This work presents a comparative analysis of three confinement regimes - ohmically-heated L-mode, auxiliary-heated (ECRH and NBI) I-mode and ELMy H-mode - and demonstrates that each exhibits a saturation in energy confinement as the core plasma effective collisionality increases in conditions with strong electron heating. One possible explanation is that similar changes in core turbulence drive are occurring in each confinement regime. Measurements with the Correlation Electron Cyclotron Emission (CECE) diagnostic across the Linear Ohmic Confinement to Saturated Ohmic Confinement (LOC-SOC) transition [1] indicate changes in turbulence behavior from low to high collisionality. Two additional effects - the strength of the NBI particle source relative to transport effects and fast ion activity - are also correlated with the observed trends. Future work will perform Correlation Reflectometer Radiometer (CRR) diagnostic measurements in these plasmas to investigate changes in turbulence behavior more deeply. The capabilities of the CRR diagnostic are demonstrated across a transition from small-ELM to ELM-free operation [2].

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

[2] C. Yoo et al., Rev. Sci. Instrum. 96, 033504 (2025)