Pellet fuelling and ELMy H mode physics in JET

Abstract

As the reference operating regime for ITER, ELMy H mode investigations have received high priority in the JET experimental programme. Recent experiments have concentrated in particular on operation simultaneously at high density and high confinement using high field side (HFS) pellet launch. The enhanced fuelling efficiency of HFS pellet fuelling is found to scale favourably to a large machine such as JET. The achievable density of ELMy H mode plasmas in JET has been significantly increased using HFS fuelling, although at the expense of confinement degradation down to L mode levels. Initial experiments using control of the pellet injection frequency have shown that density and confinement can simultaneously be increased close to the values necessary for ITER. The boundaries of the available ELMy H mode operational space have also been extensively explored. The power necessary to maintain the high confinement normally associated with ELMy H mode operation is found to be substantially higher than the H mode threshold power. The compatibility of ELMy H modes with divertor operation acceptable for a fusion device has been studied. Narrow energy scrape-off widths are measured which place stringent limits on divertor power handling. Deuterium and tritium co-deposition profiles are measured to be strongly in-out asymmetric. Successful modelling of these profiles requires inclusion of the (measured) scrape-off layer flows and of the production in the divertor of hydrocarbon molecules with sticking coefficients below unity. Helium exhaust and compression are found to be within the limits sufficient for a reactor.