Laser ablation/ionization mass spectrometry on tokamak deposition probes

Laser ablation-ionization mass spectrometric surface analysis was applied in plasma diagnostic experiments on the MT-1M tokamak. Non-intrinsic impurities were injected into the discharges using laser blow-off in order to study the impurity transport processes. High purity silicon samples were used as deposition probes in the edge plasma of the tokamak. The distributions of the impurities deposited on the probe surfaces during the discharges were measured after the exposures by laser ionization mass spectrometry. In the first experiments with sodium impurities the surface distributions showed a considerable poloidal dependence besides the well known decrease with increasing minor radius. The poloidal dependence showed mirror-like symmetry on the probes facing the ion and electron sides. In the second experimental series carried out with tungsten impurities even more localized deposit distributions were obtained. The results are explained assuming that the impurities propagate in confined packets along the magnetic field lines even for several toroidal turns.