Observation of W IV–W VII line emissions in wavelength range of 495–1475 Å in the large helical device

Tetsutarou Oishi,Motoshi Goto,Hongming Zhang,Xianli Huang,Shigeru Morita

doi:10.1088/0031-8949/91/2/025602

Tetsutarou Oishi, Motoshi Goto + Show 3 more

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https://doi.org/10.1088/0031-8949/91/2/025602

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Abstract

Vacuum ultraviolet spectra of line emissions from tungsten ions at lower ionization stages have been measured in the large helical device (LHD) using a high-resolution 3 m normal incidence spectrometer in the wavelength range of 495–1475 Å. Tungsten was introduced in the LHD plasma by injecting a coaxial tungsten impurity pellet. Many tungsten lines of W IV–W VII were successfully observed in low-temperature plasmas just after the tungsten pellet injection. It is found that some W VI lines are emitted with extremely high intensity and entirely isolated from other intrinsic impurity lines, in particular, W VI at 605.926 Å (5d–6p), 639.683 Å (5d–6p), 677.722 Å (5d–6p), 1168.151 Å (6s–6p) and 1467.959 Å (6s–6p). The result strongly suggests that those lines may be useful for the spectroscopic study in ITER and other magnetic fusion devices with tungsten materials as the plasma facing component. The ion temperature was also measured from Doppler broadening of W V and W VI lines. The result indicates that the measured ion temperature is clearly higher than the ionization energy of such ions. The reason is discussed with regarding to the pellet injection.

Highlights

The reason is Tungsten is regarded as a possible candidate material for the plasma facing component (PFC) in divertor region of ITER and future fusion reactors because of its high melting point, low sputtering yield, and low tritium retention [1,2,3]
One of the major concerns regarding the tungsten PFC is that the tungsten ion causes a large radiation loss due to its large atomic number of Z = 74 when the plasma is contaminated by the tungsten impurity
At the initial stage of the tungsten transport process in fusion devices with tungsten PFCs, firstly, neutral tungsten atoms sputtered and released from the divertor plates are ionized in the divertor region and start to move upstream toward X-points along magnetic field lines

Summary

1.Introduction

Tungsten is regarded as a possible candidate material for the plasma facing component (PFC) in divertor region of ITER and future fusion reactors because of its high melting point, low sputtering yield, and low tritium retention [1,2,3]. Tungsten ions at lower ionization stages existing in edge and divertor plasmas including the scrape-off layer have rarely been observed until now in the present fusion devices, while the line emission from low-ionized tungsten is necessary for accurate evaluation of tungsten influx and understanding of the tungsten transport in the. Application of the VUV spectroscopy to high temperature plasma experiments is very important for identification of line emissions from tungsten ions in lower charge states which leads to the tungsten transport study in edge plasmas and the quantitative evaluation of tungsten influx in fusion plasmas in addition to the enhancement of spectroscopic database on tungsten line emissions. Based on the background mentioned above, VUV spectra from low-ionized tungsten are measured in the Large Helical Device (LHD) for the contribution to the tungsten transport study in edge plasmas of ITER and other tungsten-divertor fusion devices and for the expansion of experimental database of tungsten line emissions

Tungsten pellet injection experiment in LHD

VUV spectra of W IV-W VII in wavelength range of 495-1475Å

W V and W VI lines useful for tungsten diagnostics

Summary