On the role of the edge density profile for the L–H transition power threshold in ASDEX Upgrade.

The L–H transition power threshold () in full tungsten (W) wall discharges is lower by 25% compared to those with graphite (C) mix tungsten walls in ASDEX Upgrade (Ryter et al 2013 Nucl. Fusion 53 113003). The lower power threshold in the full tungsten wall discharges has been found to correlate with higher edge density as well as steeper edge density gradient. An estimate of the minimum in the neoclassical radial electric field well inside the separatrix yields a constant value for all analyzed L–H transitions at fixed toroidal magnetic field (). The decrease of the threshold power is explained by the steeper edge density gradient in the discharges with full tungsten wall. [ABSTRACT FROM AUTHOR]