Study of impurity retention in the scrape-off layer of the FTU tokamak

A new approach to obtaining information about the impurity dynamics along the open field lines in the scrape-off layer of a tokamak plasma is described. This technique exploits a movable metallic target that allows the study of the impurity influx from the target and of the impurity content in the main plasma as a function of the plasma parameters at the radial location of the target. A simple 1-D model is presented, which is able to reproduce the observed relationship between those two quantities in these experiments in FTU. The results obtained indicate that the friction and electrostatic forces associated with the plasma flow to the target dominate the impurity dynamics along open field lines. The influence of the connection length on the screening of the impurities, predicted by the model, has been verified experimentally