Modifications in turbulence and edge electric fields at the L–H transition in the DIII-D tokamak

TLDR

The L–H transition in the DIII‑D tokamak is a key confinement transition in fusion plasmas. The study investigates precursors to various edge localized mode (ELM) types. The transition is marked by an abrupt suppression of edge density fluctuations within ~100 µs and a simultaneous increase in edge poloidal rotation velocity, indicating a more negative radial electric field. Spectroscopic measurements reveal that the L–H transition creates a 3–5 cm wide sheared Er and poloidal flow region coincident with the fluctuation‑suppression zone, extending to the H‑mode edge transport barrier, and that this shear layer persists throughout quiescent H‑mode while reducing density fluctuations and improving core confinement, in agreement with recent theory.

Abstract

The L to H transition in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. I, p. 159] is associated with two clear signatures: edge density fluctuations are abruptly suppressed (in ≊100 μsec), while the edge poloidal rotation velocity vθ increases, implying that the radial electric field Er becomes more negative. Detailed new spectroscopic profile measurements show that the changes in vθ and Er generate a region of sheared electric field and poloidal flow of width ≊3–5 cm. This region develops simultaneously with, and has the same spatial extent as, the edge fluctuation suppression zone as measured using a reflectometer system. Furthermore, the radial extent of the shear and fluctuation suppression zones encompass the location of the H-mode edge transport barrier. These observations are consistent with recent theoretical models of the L–H transition, and a comparison with these theories is presented. Data are also presented on the evolution of edge parameters and density fluctuations after the transition: the shear and fluctuation suppression layers are maintained for the duration of the quiescent H-mode phase, while relative density fluctuation levels decrease and interior plasma confinement gradually improves. Precursors to several different types of edge localized mode (ELMs) are also discussed.