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.