Boundary condition for toroidal plasma flow imposed at the separatrix in high confinement JT-60U plasmas with edge localized modes and the physics process in pedestal structure formation

Abstract

Modulation charge eXchange recombination spectroscopy measurements with high spatial and temporal resolution have made the evaluation of the toroidal plasma flow of fully stripped carbon impurity ions (VϕC6+) in the JT-60U tokamak peripheral region (including, in particular, the separatrix) possible with a better signal-to-noise ratio. By comparing co- and counter-neutral beam injection discharges experimentally, we have identified the boundary condition of VϕC6+ and radial electric field shear (∇Er) imposed at the separatrix in high confinement (H-mode) plasmas with edge localized modes (ELMs). The VϕC6+ value at the separatrix is not fixed at zero but varies with the momentum input direction. On the other hand, the ∇Er value is nearly zero (or very weakly positive) at the separatrix. Furthermore, the edge localized mode perturbation does not appear to affect both VϕC6+ and ∇Er values at the separatrix as strongly as that in the pedestal region. The above experimental findings based on the precise edge measurements have been used to validate a theoretical model and develop a new empirical model. A better understanding of the physical process in the edge transport barrier (ETB) formation due to the sheared Er formation is also discussed.