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

Abstract

The effects of increased geometrical closure on thebehaviour of the recycling and intrinsic impurities are investigatedin JET Mark I, Mark IIA and Mark IIGB pumped divertors. Increasing the divertorclosure leads to a significant improvement in exhaust for bothdeuterium and recycling impurities. However, the impurity enrichmentin the exhaust gases remains unchanged due to a simultaneous increase indeuterium and impurity compression in the divertor. A comparison ismade for helium, neon and argon under different plasma conditions. In addition,the operation of the Mark II and Mark IIGB divertors has shown thatZeff is reduced with the improved divertor closure in the L modedischarges, although no obvious changes in the Zeff values have beenobserved in the ELMy H modes. The divertor target surface temperaturehas a strong influence on intrinsic carbon production. The carbonsource in the Mark II and Mark IIGB divertors is significantly higherthan that in the Mark I divertor, which is attributed to enhancedchemical sputtering at the increased divertor tile temperature of theMark II and Mark IIGB divertors (related to the divertor coolingsystem), as opposed to the increased closure. The consequences of thiselevated yield for plasmas under different operation conditions arediscussed, and further evidence, obtained from a specificwall/divertor temperature reduction experiment, is presented. Theeffect of the divertor screening on the chemically producedimpurities is investigated using the EDGE2D/NIMBUS/DIVIMP codes forthe different recycling regimes and comparisons are made with experimental observations from the Mark I, Mark IIA and Mark IIAP divertors taking into accountthe change in chemical sputtering yield due to the different tiletemperatures of these divertors.