Role of the radial electric field in the transition from L (low) mode to H (high) mode to VH (very high) mode in the DIII-D tokamak*

TLDR

The study seeks to determine how the radial electric field Er is generated and to assess its implications for the L–H and H–VH transition theories. Er is calculated from the radial force balance equation, linking it to the ion pressure gradient, poloidal rotation, and toroidal rotation. Measurements show that E×B shear exceeds the threshold needed to suppress turbulence, accounting for the confinement gains at the L–H and H–VH transitions; at the edge the ion pressure gradient and poloidal rotation dominate Er, while in the core toroidal rotation is primary, and deliberate manipulation of vφi and shear via magnetic perturbations alters transport in agreement with the shear‑stabilization hypothesis.

Abstract

The hypothesis of stabilization of turbulence by shear in the E×B drift speed successfully predicts the observed turbulence reduction and confinement improvement seen at the L (low)–H (high) transition; in addition, the observed levels of E×B shear significantly exceed the value theoretically required to stabilize turbulence. Furthermore, this same hypothesis is the best explanation to date for the further confinement improvement seen in the plasma core when the plasma goes from the H mode to the VH (very high) mode. Consequently, the most fundamental question for H-mode studies now is: How is the electric field Er formed? The radial force balance equation relates Er to the main ion pressure gradient ∇Pi, poloidal rotation vθi, and toroidal rotation vφi. In the plasma edge, observations show ∇Pi and vθi are the important terms at the L–H transition, with ∇Pi being the dominant, negative term throughout most of the H mode. In the plasma core, Er is primarily related to vφi. There is a clear temporal and spatial correlation between the change in E×B shear and the region of local confinement improvement when the plasma goes from the H mode to the VH mode. Direct manipulation of the vφi and E×B shear using the drag produced by a nonaxisymmetric magnetic perturbation has produced clear changes in local transport, consistent with the E×B shear stabilization hypothesis. The implications of these results for theories of the L–H and H–VH transitions will be discussed.