Fuelling efficiency of hydrocarbons in TJ-II plasmas

Carbon fuelling experiments have been performed in ECRH TJ-II plasmas by injecting C2H4 (ethylene) at several radial positions through gas injection ports located on poloidal diagnosted limiters. Simultaneous plasma edge characterization has been performed with atomic beams (He, Li) and by emission spectroscopy. From comparisons of plasmas having He and H as the main species it has been determined that physical sputtering in the interaction region between the plasma and the vacuum vessel is the source of intrinsic carbon contamination. Target plasmas with a range of magnetic configurations and having different edge topologies have been also used. In particular, configurations with rational numbers that result in island chains at the edge have been explored for possible application as 'divertor-type' plasmas. Fuelling efficiencies of a few percent for injected carbon have been deduced and clear enhanced screening for chemically bounded carbon is observed to occur in the presence of peripheral magnetic islands.