Divertor modelling of septum assessment experiments in JET MkIIGB

We present results obtained from modeling the septum assessment experiments of the JET MkIIGB divertor with the B2-Eirene code package. Similar L-mode discharges have been performed with and without septum, varying density and gas puffing location. High and low inner wall clearance simulations are also compared, as this parameter has been shown experimentally to play an important role in the final density limit. According to the simulations, the neutrals ionization pattern is not affected by the septum, consistent with experiment. The septum does improve the pumped particle flux in the outer divertor, the inner divertor results depending on plasma-wall clearance, but this only concerns a small fraction of the recycling flux at the targets (less than 2%) and does not influence the overall divertor plasma very much. Comparison between inner and outer gas fuelling with a moderate gas puff rate shows no difference in the simulations without septum, while a slight asymmetry is observed in the simulations with septum, showing that the septum allows acting separately on the inner and outer divertor. We conclude that a neutral flow exists linking outer and inner diveriors with particles travelling around the plasma in the main chamber. (C) 2004 WILEY-VCH Verlag, GmbH & Co. KGaA. Weinheim.