H-MODE AND VH-MODE CONFINEMENT IMPROVEMENT IN DIII-D: INVESTIGATIONS OF TURBULENCE, LOCAL TRANSPORT AND ACTIVE CONTROL OF THE SHEAR IN THE E × B FLOW

摘要

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 to H transition. This same hypothesis is the best explanation to date for the further confinement improvement seen in the plasma core when the plasma goes from H-mode to VH-mode. Consequently, the most fundamental question for H-mode studies now is: How is the electric field E_r formed? The radial force balance equation relates E_r to the main ion pressure gradient ▽P_i, poloidal rotation v_(θi), and toroidal rotation v_(θi). In the plasma edge, direct measurements show that ▽P_i, and v_(θi) are the important terms at the L to H transition, with ▽P_i, being the dominant, negative term throughout most of the H-mode. Since E_r is observed to change before the change in ▽P_i, it is inferred that main ion rotation, probably v_(θi), changes first, triggering the transition. E_r is seen to change before the change in fluctuations, consistent with E×B shear causing the change in fluctuations and transport. In the plasma core, E_r is primarily related to v_(Φi). 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 H-mode to VH-mode. Direct manipulation of v_(Φi) and E × B shear using the drag produced by a non-axisymmetric 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 to H and H to VH transitions are discussed.