WALL CONDITIONING IN LHD

Wall conditioning in the Large Helical Device (LHD) has been conducted successively since the first experimental campaign in 1998. The effects of wall conditioning on the vacuum condition, the plasma performance, and the surface modification of the plasma-facing components have been analyzed by both macroscopic and microscopic observations such as residual gas analysis and transmission electron microscope observation, respectively. The main tools for the conditioning are mild baking (95 degrees C); glow discharges with hydrogen, helium, and neon; and wall coating with titanium and boron. Though the baking temperature is lower than in other fusion devices, it reduces impurity gases well just after the start of vacuum pumping, and it reduces retained hydrogen in plasma-facing components during the experimental campaign. Helium glow discharge was revealed to cause heavy damage on the surfaces of metallic components and the contamination of the hydrogen discharges with helium released from wall. Neon glow discharge has been conducted since it causes much less damage and hastens the conditioning of the wall. Boronization is very effective to reduce oxygen impurity in plasma, and the effects last for the whole experimental campaign in LHD.