Longterm Evolution of the Impurity Composition and Impurity Events with the ITER-like Wall at JET

This paper covers the aspects of the long term evolution of impurities in the main plasma and divertor of the JET Tokamak with respect to the newly installed ITER-Like Wall (ILW). Three main aspects considered are, firstly, the changes related to the switch over from the carbon dominated JET-C (pre 2011) to the JET-ILW with beryllium as the main wall material and an all tungsten (W) divertor, secondly the evolution of impurity fluxes in the newly installed W divertor to monitor materialmigration and establishment of a steady state divertor surface composition, and thirdly the statistical analysis of irregular impurity events causing significant plasma contamination and radiation losses. The main findings comprise of a drop in carbon emissions of a factor1 20, immediate low oxygen content comparable to levels obtained previously after campaign long conditioning effects due to the beryllium main wall. Despite the attempts of reaching steady state divertor conditions after the initial 1600 plasma seconds the divertor is still evolving due to increasing heating power applied through out the campaign. The levels of carbon released are increasing while beryllium levels in the deposition dominated inner divertor are dropping, hinting at a change in Mixed Material/beryllium layer composition. For the impurity events a steady number is observed despite the increase in machine flexibility and input power. Constant plasma operation elevates part of the problem altogether.
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