九州大学学術情報リポジトリ
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Study on thickness profile of the deposited film forming on the plasma-facing wall in the QUEST tokamak
王, 正興
https://doi.org/10.15017/1931953
出版情報：九州大学, 2017, 博士（工学）, 課程博士 バージョン：
権利関係：
（様式３）Form 3
氏 名 ： 王 正興
Name WANG ZHENGXING
論 文 名 ： Study on thickness profile of the deposited film forming on the
plasma-facing wall in the QUEST tokamak
Title （QUESTにおけるプラズマ対向壁に堆積される膜の厚さ分布の研究）
区 分 ： 甲
Category
論 文 内 容 の 要 旨
Thesis Summary
The issues of plasma-surface interaction (PSI) play an important role in long duration steady-state operation of plasma in the fusion devices. The typical PSI issues involve particle recycling in the plasma-facing wall (PFW), physical sputtering, chemical sputtering and so on. There are some neutral particles and ions escaping from magnetic confinement. They have a strong interaction with the PFW in the range of several tens of nanometers from the PFW surface. These processes make the wall materials release and the released materials become impurities of plasma. The mixture of all kinds of impurities forms a deposited film on the surface of the PFW. The thickness of deposited film affects strongly the plasma performance.
Different PFW materials including carbon (C)-based materials and metal materials have been investigated to improve the performance of plasma. Nowadays, the study on PFW material of fusion devices is focused on the metal materials due to the much lower fuel inventory than the case in the C-based materials. ITER adopts a full metallic wall, beryllium as the material of the main chamber and tungsten as the material of divertor. The deposited film forming in the metal material PFW must be metal containing. The properties of this metal containing deposited film in metallic PFW fusion devices should be studied for the development of ITER project. QUEST is an all-metal experimental device. Therefore, the deposited film on the PFW of QUEST is metal containing and optically opaque. This thesis is focused on the studies of compositions, thickness measurement, and forming mechanism of the deposited film on the actual QUEST wall. The thesis is organized as following shown.
Chapter 1 is the introduction of this thesis. The background of this topic is introduced.
In chapter 2, the compositions of the deposited film forming on the specimens installed on the PFW in QUEST are measured by X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectrometry (GD-OES). We find that the compositions are C, O and some metals. This is very different with the deposited film forming in the fusion devices of carbon-based materials PFW, such as TEXTOR and DIII-D.
In chapter 3, the reflectivity profiles on the PFW of QUEST are measured with a colorimeter. The reflectivity is related to the thickness of the deposited film forming on the PFW. An innovative colorimetry-based method is developed to measure the thickness of the deposited film on the actual QUEST wall. The optical constants of the deposition film in QUEST are studied with 21 SS specimens and 6 standard Mo specimens by using the ellipsometry and transmission electron microscopy (TEM). The
solution model in ellipsometry is a uniform monolayer model. We find that the optical constants of the deposited film on the PFW are position-dependent. Except for some special regions, the value range of extinction coefficient is about 1.0-2.0 when the wavelength was 540 nm. This high extinction coefficient makes the probing light of 540 nm wavelength be easy to attenuate on the deposited film and is limited on the surface. For the thick enough deposited film, the probing light could not penetrate through it.
Therefore, the colorimetry just could give a minimum possible value of deposited film (about 40 nm) on the most positions in deposition region. This indicate the clear limitation for measuring metal-containing films forming in QUEST by colorimetry. The thickness results deriving from TEM and GD-OES could give a good cross check to that from colorimetry. Although the thickness result of colorimetry in QUEST is quite rough, it could still be benefit to some research studies of PSI in fusion devices due to quick in-situ large-area inspections.
In chapter 4, for studying the forming mechanism of the profile of deposited film along the poloidal direction of the PFW, the ion motion is investigated in QUEST. We find that the energetic fuel ions and heavy impurity ions are easy to drift and deviate from the magnetic flux line gradually. Finally, they could attack the PFW. In detail, the regular fuel ions whose energy is not so high move following the magnetic flux line strictly. They are confined by the closed magnetic surface, or attack the divertors or limiters along the open magnetic line. However, the motion orbit of energetic ions or heavy impurity ions have an apparent drift from the magnetic flux. It is easy for them to drift toward the PFW and erode the deposited film. In this chapter, we study what kind of ions could deposit on the specific position on the PFW. It finds that only the ions from the limited regions in plasma could deposit on the specific position on the PFW.
The ions’ source regions or pitch angles are different for different positions along the poloidal direction on the PFW. Naturally, the erosion effect from these ions is different on different positions on the PFW. The postmortem observations on the PFW after the campaigns could be explained well by the motion of the energetic ions or impurity ions. It finds that erosion effect of drift ions may play an important role in forming the nonuniformity of the deposited film distributing along the poloidal direction of the PFW.
In summary in Chapter 5, the innovative research works in this thesis focus on two parts. The first one is developing the colorimetry to large-area in-situ measure the thickness of the metal-containing deposited film forming on the PFW in QUEST. The second one is the study of physical mechanism of th e deposited film forming on the PFW by investigating the ion orbit in QUEST.
