Abstract
For Large Helical Device, LHD, first cycle and second cycle experiments were conducted in 1998. Before each cycle experiments, SS316L samples were installed at the inner wall along poloidal direction. After the experiments, these samples were extracted, and the changes of surface morphology, atomic composition and gas desorption were investigated.
After the first cycle experiments, dominant erosion and deposition of wall materials were observed, and the oxygen impurity level at the surface was considerably large. After the second cycle experiments, erosion and deposition almost disappeared, and the thickness of oxide layer largely decreased. In addition, the amount of desorbed impurity gas was reduced, compared with the case of first cycle experiments. In the second cycle experiments, it was seen that the plasma stored energy largely increased due to the reduction of the radiation power. The impurity level at the inner wall was largely reduced by the second cycle experiments.
