Experimental studies of the link between production and penetration of impurity atoms into the plasma edge and the central impurity content in TEXTOR-94

Abstract Characteristic quantities for impurity release from limiters and the penetration of atoms into the plasma boundary have been determined by optical methods on the tokamak TEXTOR-94. The velocity distribution of neon atoms released from a tungsten limiter has been measured. A drastic increase of fast particles could be observed in comparison to the velocity distribution of neon released from a carbon limiter under comparable conditions. As a result the penetration depth of neon increased by a factor of 1.7. The influence of local electron density and temperature near the limiter has been demonstrated for various species as He, C, Ne, Si and Mo comparing different central electron densities. Both penetration depth and neutral fueling efficiency decrease with increasing density. For high Z materials such as Mo prompt redeposition of singly ionized ions is found to be of major importance. The measured radial profile of the ion source distribution of molybdenum under neutral beam heated conditions has been used to calculate the Mo-density in the plasma center for different line averaged central electron densities n e , taking into account both the ionization processes inside and outside the LCFS. The decrease of the calculated Mo-density with n e is in agreement with the experimentally observed variation of the central Mo-density.