Density limit in ASDEX under clean plasma conditions

The understanding of the density limit and its parametric dependence is an important prerequisite to determine the operational range of the next step tokamaks presently being designed. The maximum attainable densities on ASDEX and many other tokamaks were found to be strongly dependent on the plasma impurity level. Boronization of the ASDEX vessel resulted in a significant improvement of the plasma purity, especially the oxygen content during a discharge was reduced by about a factor of 5 with Zeff reaching values of 1 - 1.5 at higher densities. Under these well defined conditions the density limit of ASDEX divertor discharges has been investigated over an extended range of qa for ohmic heating and with additional heating by neutral injection (NI). Special emphasis was put on the documentation of the plasma radiation and the development of the divertor plasma parameters when approaching the density limit. (orig.)