Investigation of Dynamical Constraints of Transport Barriers in DIII-D (SVR)

In simplified theoretical geometries, it has been shown that the flux constraint—where the charge-weighted particle flux due to fluctuations must vanish—plays a critical role in the formation and sustainment of transport barriers (TBs) in magnetized plasmas [1]. This constraint, together with free energy dynamics, governs which plasma instabilities can be excited. Even in the presence of abundant free energy, certain instabilities may be suppressed if their associated fluctuations violate the flux constraint. In some cases, this constraint alone can significantly reduce or even eliminate ITG/TEM-driven transport, independent of velocity shear.

In this work, we explore how experimental DIII-D pedestal profiles may be better interpreted via the broad conclusions established in [1]. We analyze data from earlier experimental campaigns and the recent Shape Volume Rise (SVR) campaign to understand how changes in magnetic geometry influence pedestal structure and overall confinement. A key part of this study is to determine how the flux constraint affects the instability threshold under experimental conditions. By identifying the specific parameters that most strongly impact the flux constraint and associated thresholds, we aim to clarify the mechanisms responsible for transport barrier formation and improved edge confinement.



This work is supported by US DOE under DE-FC02-04ER54698.



1) M. Kotschenreuther, X. Liu, S.M. Mahajan, D.R. Hatch, G. Merlo. “Transport barriers in magnetized plasmas- general theory with dynamical constraints.” Nuclear Fusion, vol. 64, no. 7, 4 June 2024, p. 076033, https://doi.org/10.1088/1741-4326/ad4c75.