Electron density evolution after L-H transitions and the L-H/H-L cycle in ASDEX Upgrade

The development of the electron density profile and its dependences after the L–H transition have been investigated. Only electron cyclotron resonance heated H-modes have been analysed to exclude core particle fuelling. While the density gradient in the edge transport barrier increases significantly after the L–H transition, the pedestal top temperature rises continuously with the applied heating power and shows no pronounced change at the transition. The H-mode density saturates at a level which correlates with the neutral gas density in the divertor prior to the L–H transition. Although the density build-up varies with the available deuterium inventory, the initial increase in the edge density gradient is similar. This has been observed independent of the L-mode plasma collisionality (ν * ≈ 2.8–5.5). The analysis of electron density and temperature profiles reveals that L–H and H–L transitions occur at similar pedestal top pressures, but the pedestal top densities are always higher at the time of the H–L back transition.