Impurity transport at the plasma edge

Of the three major links in the chain connecting plasma edge conditions to central impurity levels - production; edge transport; and, central transport - edge transport is the weakest link in that it is the most complex, least understood and hardest to measure directly. A large number of edge impurity measurements can be made, however, which when interpreted in conjunction with an edge impurity transport model can improve understanding and increase confidence in prediction. These quantities include 3-D spatial distributions of impurity densities and impurity temperatures in the successive stages of ionization from the neutral state to full ionization. Some examples of such experiments carried out in JET and TEXTOR and their interpretation using a Monte Carlo impurity transport code are presented. The field remains in an early state of development and ultimate quantities, such as the central impurity density, can only be explained/predicted to within uncertainty factors, typically greater than two. Examples are given of modelling the central Zeff in JET ohmic discharges employing carbon or beryllium toroidal limiters. A new definition of 'impurity screening' is presented with the objective of finding a convenient quantitative relation between edge plasma conditions and central impurity levels