Semi-analytical study of the tokamak pedestal density profile in a single-null diverted plasma with puffing–recycling gas sources

Abstract

The tokamak pedestal density structure is generally studied using a diffusiondominant model. Recent investigations (Stacey and Groebner 2009 Phys. Plasmas 16 102504) from first principle based physics have shown a plausible existence of large inward convection in the pedestal region. The diffusionconvection equation with rapidly varying convection and diffusion coefficients in the near edge region and model puffing-recycling neutral particles is studied in this paper. A peculiar property of its solution for the existence of the large convection case is that the pedestal width of the density profile, qualitatively different from the diffusion-dominant case, depends mainly on the width of the inward convection and only weakly on the neutral penetration length and its injection position.

Particle transport analysis of the density build-up after the L–H transition in ASDEX Upgrade

“…The measurements exhibit a decrease of the separatrix density, which is used as a boundary condition for the modelling. Since a strong edge pinch links the separatrix density with the pedestal top density, as demonstrated in reference [30], the pedestal top density in the simulations also decreases after the initial increase. In contradiction to these simulations, we observe a continuous increase of the pedestal top density in the measurements, although the separatrix density decreases.…”

“…We could not find any combination of edge pinch, diffusion coefficients constant in ρ pol and neutral gas density, which delivered satisfying agreement with the measurements, where the entire evolution is taken into account. A strong pinch relates the density at the LCFS with the pedestal top density [30]. Since the density at the separatrix remained constant or decreased after the L-H transition, the pedestal top density in the simulation with a strong pinch decreases as well after the initial increase.…”