Confinement and collisionality in I-mode and L-mode decoupled from edge Weakly Coherent Mode at ASDEX Upgrade

The I-mode confinement regime is a promising operational scenario for future fusion reactors because it features high energy confinement without high particle confinement [1].  The edge-localized Weakly Coherent Mode (WCM) in I-mode is often used to contrast I-mode and L-modes. However there has been observation of WCM precursors in L-modes preceding I-modes [2], which call into question the role of the WCM in determining the transport properties of I-mode.  Novel I-mode experiments with higher than typical collisionality (ν_ei) at ASDEX Upgrade have been performed that unambiguously show that the WCM is present in both L-mode and I-mode. Furthermore, the quality of the confinement of the discharge phases is found to be independent of the presence and  amplitude of the WCM or WCM-like feature, but inversely dependent on pedestal density and collisionality.  Electron temperature fluctuation (δT_e/T_e) measurements in the plasma outer core and pedestal (ρ_pol=0.85-1.0) are made with a new 24-channel high radial resolution Correlation Electron Cyclotron Emission radiometer [3].  The WCM in I-mode is located in the pedestal region (ρ_pol=0.97-1.0) with δT_e/T_e=2-4%.  A lower frequency and more coherent WCM-like feature also appears in the L-mode.  Linear gyrokinetic analysis performed with the CGYRO code suggests that dominant turbulent modes in the outer core are electrostatic and ion directed while the edge and pedestal modes nearer the WCM location are electron directed and electromagnetic in nature.

[1] D.G. Whyte et al, Nucl. Fusion 50 (2011) 105005

[2] T. Happel et al, Nucl. Mater. Energy 18 (2019) 159-165

[3] A.J. Creely et al, Rev. Sci. Instrum. 89 (2018) 053503