JET-P(99)39

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 MkI, MkIIA and MkIIGB pumped divertors. 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 simultaneous increase in deuterium and impurity compression in the divertor. The comparison is made for He, Ne and Ar under different plasma conditions. In addition, the operation of the Mark II and Mark IIGB divertors has shown that Zeff is reduced with the improved divertor closure in the L-mode discharges, although no obvious changes in the Zeff values have been observed in the ELMy H-modes. The divertor target surface temperature has a strong influence on the intrinsic carbon production. The carbon source in the Mark II and Mark IIGB divertors is significantly higher than that in the Mark I divertor, which is attributed to the enhanced chemical sputtering at the increased divertor tile temperature of the Mark II and Mark IIGB divertors (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 for the chemically produced impurities is investigated using the EDGE2D/NIMBUS/DIVIMP codes for the different recycling regimes and the comparisons are made with the experimental observations from Mark I and Mark II taking into account the change in the chemical sputtering yield due to the different tile temperatures of the Mark I and Mark II divertors.
Name Size  
JETP99039 454.94 Kb