Particle and energy transport during the first tritium experiments on JET

Particle and energy transport of the high fusion performance JET pulses obtained before and during the first tritium experiments are discussed. The particle diffusion of tritium is determined by monitoring the decay of a small quantity of injected tritium in a deuterium background. A good simulation of the measured 14 MeV neutron emissivity profile is obtained throughout the decay phase if the mixing of the two species is described by a model in which the tritium diffusion coefficient is similar to that of deuterium. The energy confinement of these low density, hot ion, H mode discharges is found to have both improved central and edge confinement over the conventional medium to high density H mode discharges, regardless of the presence or absence of tritium. As the tritium concentration of these D-T discharges is small, no isotopic dependence was expected and none is observed. Enhancement factors of at least twice the value predicted by H mode scaling expressions are observed but only transiently. A local transport analysis is completed to find the reason for the improved confinement and its transient nature. Similarities with DIII-D VH mode discharges have been noticed and are discussed. The expansion of the region with access to the second stability regime certainly appears to be a possibility for the enhanced confinement. Stabilization of the ηi mode by the peaked density profile seems unlikely to be the cause. For the discharge with a high concentration of tritium, it has been suggested that α particle driven instabilities could affect energy confinement. A comparison is made with the stability threshold of toroidicity induced Alfven eigenmodes (TAE), which appear to have been stable. The α particle statistics are also presented. 39 refs, 20 figs, 2 tabs