The effect of divertor tile material on radiation profiles in LHD

In the large helical device (LHD) radiation profiles measured using arrays of resistive metal, foil bolometers are used to investigate the change in radiation resulting from a replacement of stainless steel divertor plates with graphite tiles between the second and third experimental campaigns. In particular, for the magnetic configuration of Raxis=3.6 m and at line averaged densities below 3×1019 m−3, a reduction of the radiated power fraction from 35% in the second campaign to down to 12% in third campaign was observed. Comparing similar shots (in terms of discharge parameters) from the second and third cycles the core (0<r/a<0.79) radiation fraction was reduced from 44% to 30% of the total radiation. Spectroscopic measurements show a corresponding decrease in radiation from iron. Comparing long pulse discharges the radiated power fraction reduced from greater than 67% during the density limiting oscillation observed in the second campaign to 20% during the third campaign with a reduction in the respective core radiation fractions from 63% to 37%.