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

Abstract

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.