TRANSPORT EFFECTS OF LOW (M,N) MHD MODES ON TFTR SUPERSHOTS

Supershots in TFTR often suffer a performance deterioration characteri zed by a gradual decrease of the DD fusion neutron yield and plasma st ored energy after several hundred milliseconds of auxiliary heating. T he correlation between this performance deterioration and the developm ent of low m (the poloidal mode number), n (the toroidal mode number) MHD modes is studied through shot-to-shot comparisons and statistical data analyses. A good correlation is observed between performance dete rioration and the appearance of strong 3/2 and 4/3 macroscopic modes. The magnetic island structures are observed using Mirnov and ECE diagn ostics. The measured T-e, T-i and n(e) profiles show that development of the islands corresponds to a nearly constant decrement of these qua ntities over the core region r < r(s), where r(s) is the mode rational surface, on a transport time-scale (t > tau(E)). The observed energy deterioration scaling, delta W/W proportional to w/a, where w is the m agnetic island width and a is the plasma minor radius, agrees with a l ocal transport model. Numerical simulations based on the local transpo rt model reveal many features consistent with the experiments. Besides the MHD effect, it is found that a continuous increase of edge recycl ing rate during the neutral beam injection phase also has a large effe ct on the performance deterioration.