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

The effects of increased geometrical closure on the behaviour of the recycl ing and intrinsic impurities are investigated in JET Mark I, Mark IIA and M ark IIGB pumped diverters. Increasing the divertor closure leads to a signi ficant 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 di vertor. A comparison is made for helium, neon and argon under different pla sma conditions. In addition, the operation of the Mark II and Mark IIGB div erters 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 tempera ture has a strong influence on intrinsic carbon production. The carbon sour ce in the Mark II and Mark IIGB diverters is significantly higher than that in the Mark I divertor, which is attributed to enhanced chemical sputterin g at the increased divertor tile temperature of the Mark II and Mark IIGB d iverters (related to the divertor cooling system), as opposed to the increa sed closure. The consequences of this elevated yield for plasmas under diff erent operation conditions are discussed, and further evidence, obtained fr om 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 rec ycling regimes and comparisons are made with experimental observations from the Mark I, Mark IIA and Mark IIAP diverters taking into account the chang e in chemical sputtering yield due to the different tile temperatures of th ese diverters.