Prompt core confinement improvement across the L–H transition in DIII-D: Profile stiffness, turbulence dynamics, and isotope effect

Abstract

We elaborate on the nature of the prompt core confinement improvement observed at the L–H transition in DIII-D, which is a long-standing issue unsolved for more than two decades and can impact future fusion reactor performance. Dynamic transport analysis suggests the essential role of the profile stiffness for understanding the mechanism of the prompt core confinement improvement. Beam emission spectroscopy shows that transport reduction at the core cannot be explained only by the ion scale turbulence density fluctuation suppression. Properties of nonlocal confinement improvement across the L–H transition are experimentally assessed in hydrogen (H) and deuterium (D) plasmas. Prompt core confinement improvement is found to be more rapid in the lighter hydrogen isotope.

Investigation of core transport changes in DIII-D discharges with off-axis T e profile peaks

“…Comparison with a linear transport model suggests that there is a change in core electron heat transport during L-H transition. This observation is consistent with previous studies [29,30] that observe prompt transport change and confinement improvement in the plasma core at L-H transitions. This work has implications for future tokamak devices that intend to reach the H-mode state with ECH as the dominant auxiliary heating method.…”