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

Abstract

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.

Impact of 3D magnetic field structure on boundary and divertor plasmas in stellarator/heliotron devices

“…There have been many observations of impurity screening or core decontamination in tokamaks with RMP application [76,77,78], and in helical devices with a stochastic layer [79,80] or with the island divertor configurations [81,82]. It is not clear yet at all whether these phenomena are caused by the edge impurity screening or attributed to core transport or some other effects such as a change of source characteristic, etc.…”

Measured and Modeled Radiation Profiles for LHD Plasma - A Comparison during Plasma Detachment

“…This assumption is valid since physical sputtering yields are an order of magnitude less as compared to the chemical sputtering yields [7]. The reason for considering only atomic carbon is well justified since no significant radiation from the high Z impurities like Fe has been observed experimentally [8]. The fraction of radiation from the edge region computed for the amount of Fe reported in Ref.…”

Images of Impurity Line Emission in the Extreme Ultraviolet Region From the Large Helical Device With an Edge Stochastic Magnetic Field Layer

“…Recently, it has been found that the radiative divertor based on the impurity radiation from intrinsically existing impurity ions can be triggered by applying m/n = 1/1 magnetic field perturbation, even if the noble gas is not introduced [1]. On the other hand, it has also been found that the impurity screening reducing the impurity content in the plasma core becomes sufficiently effective due to the presence of the ergodic layer and becomes further effective for heavier impurity ions [2]. Therefore, the impurity transport study in the Manuscript ergodic layer is crucially important for steady-state operation of fusion devices.…”