Particle and energy transport during the first tritium experiments on JET

Abstract

The particle and energy transport properties of the high fusion performance JET pulses that were obtained before and during the first tritium experiments are discussed. The particle diffusion coefficient of tritium is determined by monitoring the decay of a small quantity of injected tritium in a deuterium background plasma. 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 in the discharge. As the tritium concentration of these D-T discharges is small (varying from <1% to 10%), no isotopic dependence was expected and indeed 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 try and establish the reason for the improved confinement and its transient nature. Similarities between these pulses and DIII-D VH mode discharges have been noticed, and common characteristics are discussed. In particular, the expansion of the region with access to the second stability regime certainly appears to be a possibility for the enhanced confinement. The stabilization of the ηi mode by the peaked density profile seems unlikely to be the cause of the improved confinement. Finally, for the discharge with a high concentration of tritium, it has been suggested that alpha particle driven instabilities could affect the energy confinement. A comparison is made with tile stability threshold of toroidicity induced Alfven eigenmodes (TAE), which appear to have been stable. The alpha particle statistics are also presented