Wall conditioning strategies in the stellarator TJ-II

This paper summarizes the different wall conditioning strategies applied in the TJ-II stellarator since the operational starting in 1997. In a first stage (1997–2001) the all-metal machine (stainless steel) was conditioned by He glow discharge. This procedure allowed an acceptable density control in low power ECRH plasmas. The boronization of TJ-II (starting in 2001) led to obtain low Z plasmas with high ECRH injected power, but the density control under NBI injection was not possible. Finally, the coating of the boronized walls with a lithium thin film has allowed to obtain high reproducible NBI plasmas with good density control.