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 deuterium fuelled plasmas exhibits a strong explicit dependence on the averaged outer divertor target electron temperature, ne,sep ∼ Te,ot−1/2. This dependence is reproduced by analytic reversed two point model (rev-2PM), and arises from parallel pressure balance, as well as the ratio of the power and momentum volumetric loss factors, (1 − fcooling)/(1 − fmom-loss). 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 (fueling) ramps in the outer horizontal, VH(C), and vertical target, VV, divertor configurations. Rev-2PM ne,sep estimates from the extended H-mode and more limited L-mode datasets are recovered to within ±25% of the measurements, with a scaling factor applied to account for use of , 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 JET-ILW ne,sep trends can be attributed to a significant influence of main chamber recycling, 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 trends can, however, be broken if large modifications to the divertor geometry (e.g. complete removal of the outer divertor baffle structure) are introduced in the model. The more pronounced high-field side high density region formation in the VH(C) configuration with reduced clearance to the separatrix does not appear to have a significant influence on the outer midplane separatrix and pedestal parameters when mapped to , although conditions at the inner midplane could not be assessed.