Studies of dust transport in long pulse discharges in the Large Helical Device

A stereoscopic fast framing cameras observed the three-dimensional trajectories of dusts in plasmas in the Large Helical Device (LHD). It proved that the dusts locate in the peripheral plasma and the most of the dusts move along the magnetic field lines with acceleration which direction corresponds to that of the plasma flow in the peripheral plasma. Release of a large amount of dusts was often observed just before the plasma termination in recent ICRF heated long pulse discharges, which were synchronized with sparks in a closed divertor region. After the experimental campaign, the traces of exfoliation of carbon rich mixed-material deposition layers were found in the divertor region. The transport of the dusts is investigated using a dust transport simulation code, which is modified to be applied to a fully three-dimensional geometry such as the LHD configuration. The simulation shows the shielding effect of the peripheral plasma against dust penetration into the main plasma. It can explain the observations of the most of the dust trajectories, and indicates that the control of the dust radii to less than about 1 mm is necessary to prevent the plasma termination due to the dust penetration for the long pulse discharges. (author)