Investigation of collisionality as a mechanism for confinement saturation in L-mode, I-mode, and H-mode plasmas with strong electron heating at ASDEX Upgrade

This work presents evidence of similarities in confinement saturation that occurs as the local effective collisionality in the plasma core is increased in an ohmically heated low-confinement (L-) mode plasma and auxiliary-heated improved-confinement (I-) mode and high-confinement (H-) mode plasmas featuring strong electron heating at ASDEX Upgrade (AUG). The relationship between changes in energy confinement, effective collisionality, density peaking, and turbulence is first examined across the Linear Ohmic Confinement to Saturated Ohmic Confinement (LOC–SOC) transition using an ohmically heated L-mode discharge. Comparisons are then made to I-mode and H-mode plasmas featuring mixed electron cyclotron resonance heating and neutral beam injection (NBI) heating using two large-scale databases. A viable mechanism for the observed global trends in all three confinement regimes featuring plasmas with strong electron heating is found to be a collisionality-driven change in core physics, including changes in the core normalized logarithmic gradients, electron–ion temperature coupling, and impurity content. Linear stability analysis shows some similarities in changes to the balance of electron and ion turbulent instabilities from low to high collisionality in the three confinement regimes. Comparisons are made to turbulence measurements in the ohmically heated discharge. The results collectively indicate the existence of a universal Linear Confinement to Saturated Confinement transition occurring across all confinement regimes with dominant electron heating at AUG regardless of plasma boundary conditions and encompassing the LOC–SOC transition in ohmically heated plasmas. Additional mechanisms, including changes in the strength of the NBI particle source relative to transport and fast ion activity, are found to contribute to the observed global trends.