Improved confinement in high-density Ohmic discharges due to impurity puffing

Impurity puffing experiments have been performed in ultra-clean ASDEX discharges (via wall boronization) in order to explore the role of impurities for the confinement in high-density Ohmic discharges. Successful changes in confinement are achieved by neon puffing where edge radiation is increased such that the power load onto the divertor plates takes its minimum value. This low value correlates with a reduced separatrix pressure and hence may trigger a peaking of the density profile leading to favourable transport behaviour in the bulk plasma. Therefore, the access to improved Ohmic confinement can now be understood in terms of impurity radiation and recycling fluxes.