Investigation of the Influence of Divertor Recycling on Global Plasma Confinement in JET

The impact of the divertor geometry on the global plasma confinement in type I ELMy H-mode has been investigated in the JET tokamak. Discharges have been performed in which the position of the strike-points was changed while keeping the bulk plasma's equilibrium essentially unchanged. Large variations of the global plasma confinement have been observed, the H98 factor changing from typically 0.7 when the outer strike-point is on the vertical target to 0.9 when it is located in the pump duct entrance. Profiles are mainly impacted in the pedestal but core profiles' peaking is also modified. Although substantial differences are observed in the divertor conditions, none seem to correlate directly with the confinement. Modelling with the EDGE2D-EIRENE and SOLEDGE2D-EIRENE transport codes exhibits differences in the energy losses due to neutrals inside the separatrix, but orders of magnitude are too low to explain simply the impact on the confinement.
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