Experiences with Carbon Divertor Operation in LHD

Abstract

The Large Helical Device (LHD) has employed a graphite helical divertor in a stainless steel vacuum vessel since the beginning of its third experimental campaign in 1999. Initially, installation of the carbon divertor resulted in a dramatic decrease in iron impurities as evidenced by the hollowing out of the radiation profiles and by spectroscopic measurements compared to the previous campaign using a stainless steel divertor. However core radiation and radiation from iron spectral lines gradually increased during the third campaign, which was attributed to sputtering of the stainless steel walls during glow discharge cleaning and redeposition on the carbon divertor tiles.Studies have shown that the graphite tiles were eroded at the divertor leg strike points to a depth of 5 µm during one experimental campaign (~10,000 shots), which is in agreement with numerical calculations using the EDDY code. Carbon is redeposited at and near the strike points along with metallic impurities forming a mixed layer, which may be contributing to iron impurities in the plasma.In LHD radiative collapse resulting from an edge thermal instability caused by radiation of gasses desorbed from the divertor during long pulse discharges terminates the discharge at high density. This radiative collapse is one of the limitations for long pulse discharges up to 150 s.