Effects of divertor geometry and chemical sputtering on impurity behaviour and plasma performance in JET

The effects of increased geometrical closure on the behaviour of the recycling and intrinsic impurities are investigated in JET Mark I, Mark IIA and Mark IIGB pumped diverters. Increasing the divertor closure leads to a significant improvement in exhaust for both deuterium and recycling impurities. However, the impurity enrichment in the exhaust gases remains unchanged due to a simultaneous increase in deuterium and impurity compression in the divertor. A comparison is made for helium, neon and argon under different plasma conditions. In addition, the operation of the Mark II and Mark IIGB diverters has shown that Z(eff) is reduced with the improved divertor closure in the L mode discharges, although no obvious changes in the Z(eff) values have been observed in the ELMy H modes. The divertor target surface temperature has a strong influence on intrinsic carbon production. The carbon source in the Mark II and Mark IIGB diverters is significantly higher than that in the Mark I divertor, which is attributed to enhanced chemical sputtering at the increased divertor tile temperature of the Mark II and Mark IIGB diverters (related to the divertor cooling system), as opposed to the increased closure. The consequences of this elevated yield for plasmas under different operation conditions are discussed, and further evidence, obtained from a specific wall/divertor temperature reduction experiment, is presented. The effect of the divertor screening on the chemically produced impurities is investigated using the EDGE2D/NIMBUS/DIVIMP codes for the different recycling regimes and comparisons are made with experimental observations from the Mark I, Mark IIA and Mark IIAP diverters taking into account the change in chemical sputtering yield due to the different tile temperatures of these diverters.