Effects of edge conditions on plasma confinement in ASDEX

Edge conditions have been varied on ASDEX by altering the divertor configuration and wall conditioning. Introduction of bypasses which allow neutral gas to enter the main chamber from the divertor by paths other than through the divertor neck greatly increased the power threshold for the H-mode and led to poorer H-mode quality. At the same time a regime of improved ohmic confinement (IOC) made its appearance. Closing the bypasses and boronization of the walls restored good H-mode behavior in a wide parameter range. The degree of improved ohmic confinement was found to be a function of recycling and plasma purity. The IOC effect with all its facets could be provoked in clean discharges by neon puffing. An explanation is provided in a simple theory which links impurity radition and power flow Psol into the scrape-off layer (SOL) to the edge density. In like manner, observed isotopic effects of density at the separatrix can be related to an isotopic dependence of Psol. A subset of ASDEX density-limit discharges may also be understood in terms of edge parameters and Psol, thereby sheading light on the weak power scaling observed for the density limit with auxiliary heating. (orig.)