Behavior of metallic impurity in divertor configuration of Large Helical Device

摘要

Numerical transport study predicts that the edge surface layer in ergodic layer of Large Helical Device (LHD) has a favorable capability of impurity screening for materials of not only divertor plates but also vacuum vessel. In order to demonstrate the theoretical prediction, the density of iron originating in the LHD vacuum vessel made of stainless steel, which is not covered by carbon plates like tokamaks, is accurately determined with its radial profile using a space-resolved extreme ultraviolet (EUV) spectrometer, of which absolute intensity calibration is done with bremsstrahlung continuum. For the purpose effective intensity coefficients are precisely calculated for iron ions based on a collisional-radiative model. The iron ion density profiles of Fe¹⁴⁺, Fe¹⁵⁺, Fe²²⁺ and Fe²³⁺ are then evaluated with the radial emissivity profile reconstructed from chord-integrated profile and the effective intensity coefficient. The ratio of iron density to electron density integrated over the whole plasma volume can be finally calculated by fitting the iron density profile using one-dimensional impurity transport code. Thus, the analysis on the ratio gives a typical value of 8×10⁻⁷ in experimental campaign at last year. The entirely small value of the iron density demonstrates the theoretical prediction. The radial structure of transport coefficients are also obtained from the impurity transport code, showing a large inward convection velocity.