Studies in JET divertors of varied geometry. III: Intrinsic impurity behaviour

Abstract

Impurity production and plasma impurity concentrations are surveyed over a wide range of operating conditions but with emphasis on ELMy H mode operation using NB heating. The Mark I divertor campaigns with carbon and beryllium targets are compared with the Mark II campaign, which had carbon targets and a more closed divertor geometry. The beryllium target campaign has a significantly lower central impurity concentration than that with carbon targets, although the Zeff value is still dominated by carbon. The divertor geometry appears to have little effect on the central impurity level, but it is found that Zeff and the carbon and nickel concentrations are correlated with the ELM frequency, with maxima at a frequency of around 5-15 Hz. This occurs whether the ELM frequency is varied by changing the plasma triangularity or the input power, or by increasing the fuelling rate. The absolute impurity influxes from the inner and outer targets and from the inner wall have been estimated from the C II and C III line intensities. These fluxes are compared with the central carbon concentration, and an attempt is made to correlate the influxes and the concentrations in the confined plasma, both in their time evolution and in their absolute values.