Impurities in TdeV with and without conditioning by trimethylboron/helium glow discharge

Abstract

Boronisation by glow discharge with 30% B(CH 3) 3 in He was applied in the TdeV tokamak. Plasma current and density scans were performed before and after the process; the impurity influxes (visible spectroscopy and mass spectrometry), the plasma contamination (VUV spectroscopy and effective ion charge Z eff ) and the radiated power P rad were measured. The density limit was investigated. The lifetime of the conditioning effect was correlated with surface analysis of wall samples. Without boronisation, Z eff , P rad and a large fraction of the particle recycling were determined by oxygen from the residual gas. The density limit was (3.5−4.0)×10 19 m -3 and a shrinkage of the plasma radius occured at low current and high density. With boronisation, oxygen is reduced several fold, and ( Z eff−1 ) and P rad are reduced by ≈60%. Also, plasma shrinkage is eliminated and the density limit is increased to ≈5×10 19 m -3 . The lifetime of the effect seems to be mostly determined by saturation of the boron layer by oxygen.