Temporal behavior of the electron density profile in the scrape-off layer of neutral-beam-heated ASDEX plasmas

Abstract

Density measurements in the peripheral region of a NI heated discharge are presented. Most of the results are obtained via a new technique utilizing a 60 keV neutral lithium beam which yields profiles — on a continuous basis — that can be related to the density profile. The initial consequence of NI is a deterioration of the particle confinement time (L-phase) which manifests itself as an ∼ 40% increase in the density e-folding length λ n outside the separatrix; possibly, there is a slight decrease in density inside the separatrix. Attainment of the H-phase with its better confinement properties is accompanied by an abrupt decrease in λ n to a value typical of the Ohmic phase; there are indications that the density gradient near the separatrix is even steeper. The subsequent increase in edge density is then intermittently interrupted by bursts which, at the conclusion of each burst, lead to a reduction in n e of the scrape-off layer and at least the region immediately inside the separatrix.