Parameter dependencies of the separatrix density in low triangularity L-mode and H-mode JET-ILW plasmas


 The midplane electron separatrix density, ne,sep, in JET-ILW L-mode and H-mode low triangularity unseeded plasmas exhibits a strong explicit dependence on the averaged outer divertor target electron temperature, ne,sep ∝Te,ot
 -1/2, arising from parallel pressure balance, as well as on the ratio of the power and momentum volumetric loss factors, (1-fcooling)/(1-fmom-loss), in line with expectation from the analytic reversed two point model (rev-2PM). Quantifying the influence of the (1-fcooling) and (1-fmom-loss) loss factors on ne,sep has been enabled by measurement estimates of these quantities from L-mode density ramps in the outer horizontal, VH(C), and vertical target, VV, divertor configurations. Rev-2PM ne,sep estimates from H-mode and L-mode datasets are recovered to within ±25% of the measurements, with a scaling factor applied to account for use of〈Te,ot〉, an averaged quantity, rather than flux tube resolved target values. Both the (1-fcooling) and (1-fmom-loss) trends and recovery of ne,sep using the rev-2PM formatting are reproduced in EDGE2D-EIRENE L-mode-like and H-mode-like density scan simulations. The general lack of a divertor configuration effect in the ne,sep trends can be attributed to a significant influence of main chamber recycling (MCR), which has been shown in the EDGE2D-EIRENE results to moderate ne,sep with respect to changes in divertor neutral leakage imposed by changes in the divertor configuration. The unified ne,sep vs 〈Te,ot〉trends can, however, be broken if large modifications to the divertor geometry are artificially introduced in the model. Considerably higher inner target density profiles in the VH(C) configuration with reduced clearance to the separatrix do not have a significant effect on the outer midplane ne,sep and ne,ped vs 〈Te,ot〉trends. The more pronounced high-field side high density region formation in the VH(C) configuration does not therefore appear to play a strong role in fuelling the outer midplane confined plasma, although conditions at the inner midplane could not be assessed.