Effect of changes in separatrix magnetic geometry on divertor behaviour in DIII-D

Results and interpretation of recent experiments on DIII-D designed to evaluate divertor geometries favourable for radiative heat dispersal are presented. Two approaches examined here involved lengthening the parallel connection in the scrape-off layer, L_∥, and increasing the radius of the outer divertor separatrix strike point, R_OSP, with the goal of reducing target temperature, T_TAR, and increasing target density, n_TAR. From one-dimensional (1D) two-point modelling based on conducted parallel heat flux, it is expected that: n_TAR ∝ R _OSP² L_∥^6/7n_SEP ³ and T_TAR ∝ R_OSP² L _∥^-4/7n_SEP^-2, where n _SEP is the midplane separatrix density. These scalings suggest that conditions conducive to a radiative divertor solution can be achieved at low n_SEP by increasing either R_OSP or L_∥. Our data are consistent with the above L_∥ scalings. On the other hand, the observed dependence of n_TAR and T_TAR on R _OSP displayed a more complex behaviour, under certain conditions deviating from the above scalings. Our analysis indicates that deviations from the R_OSP scaling were due to the presence of convected heat flux, driven by escaping neutrals, in the more open configurations of the larger R_OSP cases. A comparison of 'open' versus 'closed' divertor configurations for the H-mode plasmas in this study show that the 'closed' case provides at least 30% reduction in the peaked heat flux at common density with the 'open' case and partial divertor detachment at lower plasma density.