Studies in JET diverters of varied geometry. II: Impurity seeded plasmas

In current large tokamaks, non-intrinsic seeded impurities have been used t o produce divertor power loads which would be considered acceptable when ex trapolated to ITER. Many devices have achieved the goals of high fractional radiated powers, small frequent ELMs and detachment which are characterist ic of radiative H mode regimes. The influence of divertor geometry on these characteristics is described. It has been a matter of concern that the Z(e ff) associated with the seeded impurities may exceed that allowable in ITER and also that the degradation in energy confinement may be unacceptable. C onfidence can only be built in the prediction of these parameters in ITER i f reliable scalings are available for impurity content and energy confineme nt which have a sound physics basis. Work is described at JET in this area whilst using multimachine data to characterize the size scaling and provide a context for the JET data. Predicted levels for the impurity content of s eeded ITER plasmas appear to be of marginal acceptability. Discharges run i n the JET Mark I, Mark IIA and Mark IIAP diverters are compared and indicat e that increased divertor closure has brought relatively minor benefits in highly radiative discharges. The acceptability of the energy confinement of radiation for ITER remains unclear. Dimensionless parameter scaling experi ments have been conducted in which beta, q(25), fractional radiated power a nd Z(eff) are held constant for a range of rho*. The price paid for high ed ge radiation and small ELMs appears to be a 25% loss in total stored energy as a result of edge pedestal degradation. However, the underlying energy c onfinement scaling may still be consistent with gyro-Bohm scaling, which wo uld give an adequate margin for ITER. This conclusion is, however, sensitiv e to the scaling of confinement with collisionality, which is difficult to determine due to the coupling between rho* and nu* which is a consequence o f radiation dominated regimes.