Correlation of the scrape-off layer (SOL) power width with edge electron pressure gradients at ASDEX Upgrade

Understanding how the scrape-off layer (SOL) power decay length λq scales with plasma parameters is essential for designing fusion reactors and developing a power exhaust solution. Previously, several scaling laws have been proposed to predict λq values in ITER [1, 2]. They are based on engineering or global plasma parameters, which on the one hand make the λq prediction easier, but on the other hand could hide the underlying physics mechanism setting λq. To shed additional light on this, a cross-regime (L-mode, I-mode and H-mode) database combining λq measurements from infrared cameras and upstream SOL and pedestal gradient lengths has been assembled at ASDEX Upgrade. It is found that λq exhibits a clear correlation with both SOL and pedestal electron pressure gradient lengths, which in turn are remarkably well correlated to each other. Consequently, as the pedestal width is mainly constant in the analyzed discharges, a cross-regime λq scaling is well described by the edge electron pressure evaluated at ρpol = 0.95.

The main implication of these findings is that, across the confinement regimes analyzed, it is not possible to combine a large λq with a steep pedestal electron pressure gradient. Since the achievement of high pedestal top pressures - and hence high core pressures - is tightly bound to the appearance of steep pedestal gradients, this result may be an obstacle to finding a stable scenario that couples high core performances and a power exhaust solution in ITER.