Scrape-off layer (SOL) power width scaling and correlation between SOL and pedestal gradients across L, i and H-mode plasmas at ASDEX Upgrade

Abstract

A cross-regime (L-mode, I-mode and H-mode) database combining scrape-off layer (SOL) power decay length λ q divertor measurements and upstream SOL electron pressure, temperature and density decay lengths has been assembled at ASDEX Upgrade. It is found that a cross-regime λ q scaling is best described by a local edge quantity, such as the edge electron pressure evaluated at ρ pol = 0.95. Furthermore, λ q exhibits a clear correlation with edge electron pressure gradient lengths, no matter if taken inside or outside the separatrix. In addition, the database reveals that SOL and pedestal electron pressure gradients are remarkably well correlated across all confinement regimes. The physical interpretation of this observation is discussed with regard to an edge pressure critical gradient paradigm governing the edge physics and to a turbulence spreading in the SOL. Moreover, it is shown that the Spitzer-Härm electron conduction regime is a reasonable approximation to estimate λ q across different confinement regimes. The main implication of these findings is that a widening of λ q is linked to a reduction of edge electron pressure gradients.