Effective screening of iron impurities in the ergodic layer of the Large Helical Device with a metallic first wall.

In the Large Helical Device (LHD) operated with a metallic (stainless steel) first wall, it is found that the iron density, nFe, at the plasma core is fairly low (nFe ≤ 108 cm-3) in general neutral beam (NB)-heated discharges, while the iron quickly increases with the appearance of impurity accumulation when a multi-hydrogen ice pellet is injected or the NB input power is largely reduced. Although the highest iron density (nFe ≤ 1010 cm-3) at the plasma centre in the LHD is observed from such discharges, it suggests a still low iron concentration (nFe/ne < 10-3). Therefore, the edge iron transport in the ergodic layer, which determines the iron influx to the core plasma, is studied to clarify why the iron density in the core plasma is low. A line ratio of Fe XV located in the vicinity of the last closed flux surface to Fe VIII (or Fe IX) located in the ergodic layer decreases with density. The two-dimensional (2D) edge iron emission of Fe XVI and Fe IX is enhanced in the vicinity of the X-point with a larger number of magnetic field lines directly connected to divertor plates, which suggests that iron ions from the first wall move downstream. The density of edge Fe15+ ions giving the iron influx to the core plasma is analysed with the 2D distribution. The analysis also shows that the iron influx to the core plasma decreases with density. These results clearly indicate that the screening effect developed in the ergodic layer works well for iron ions coming from the first wall. A three-dimensional edge transport simulation with EMC3-EIRENE can also predict an effective impurity screening for heavy impurities compared to light impurities. [ABSTRACT FROM AUTHOR]