Confinement properties of high density impurity seeded ELMy H-mode discharges at low and high triangularity on JET

The design value for ITER is based on operation at n/n(GW) = 0.85, beta(n) = 1.8 and H98(y, 2) = 1. These values have been routinely achieved in JET in argon seeded ELMy H-mode discharges in different divertor configurations and with different triangularities. Two main scenarios are emerging from the experiments. First, low triangularity (delta(u) = 0.19) in septum configuration. In this case large D-2 fuelling rates lead to confinement degradation towards L-mode. The seeding of Ar during the D-2 fuelling phase gives rise to a density close to the Greenwald value. After the switch-off of the D-2 gas fuelling ('afterpuff ' phase), the confinement recovers to H-mode quality whereas the density stays near the value reached at the end of the main fuelling phase and Z(eff) stays close to or below 2. Acting on the refuelling of Ar and D-2 in the 'afterpuff 'phase allows us to improve the stationarity of the high performance phase while maintaining up to the end of the heating phase the good confinement, density and radiation level. Second, high triangularity (delta(u) = 0.45) in vertical target configuration. In this case large fuelling rates do not lead to strong confinement degradation and the D-2 fuelling is applied continuously throughout the discharge. A radiated power fraction of up to 70%, H98(y, 2) = 0.9 at beta(n) = 2.1 and n = 1.15n(GW)-together with the formation of a radiating mantle and moderate Z(eff)-are achieved in this scenario. Furthermore, there are indications of significantly reduced heat load on the divertor target plates.