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Development of high-speed vacuum ultraviolet imaging camera system for high-temperature plasma diagnostics
https://ir.soken.ac.jp/records/3574
https://ir.soken.ac.jp/records/3574c50a70f7-42ae-40e1-a6b6-f1ccae48e621
| 名前 / ファイル | ライセンス | アクション |
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要旨・審査要旨 (410.5 kB)
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| Item type | 学位論文 / Thesis or Dissertation(1) | |||||
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| 公開日 | 2013-05-24 | |||||
| タイトル | ||||||
| タイトル | Development of high-speed vacuum ultraviolet imaging camera system for high-temperature plasma diagnostics | |||||
| タイトル | ||||||
| タイトル | Development of high-speed vacuum ultraviolet imaging camera system for high-temperature plasma diagnostics | |||||
| 言語 | en | |||||
| 言語 | ||||||
| 言語 | eng | |||||
| 資源タイプ | ||||||
| 資源タイプ識別子 | http://purl.org/coar/resource_type/c_46ec | |||||
| 資源タイプ | thesis | |||||
| 著者名 |
明, 廷〓
× 明, 廷〓 |
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| フリガナ |
ミン, ティンフン
× ミン, ティンフン |
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| 著者 |
MING, Tingfeng
× MING, Tingfeng |
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| 学位授与機関 | ||||||
| 学位授与機関名 | 総合研究大学院大学 | |||||
| 学位名 | ||||||
| 学位名 | 博士（学術） | |||||
| 学位記番号 | ||||||
| 内容記述タイプ | Other | |||||
| 内容記述 | 総研大甲第1544号 | |||||
| 研究科 | ||||||
| 値 | 物理科学研究科 | |||||
| 専攻 | ||||||
| 値 | 10 核融合科学専攻 | |||||
| 学位授与年月日 | ||||||
| 学位授与年月日 | 2012-09-28 | |||||
| 学位授与年度 | ||||||
| 値 | 2012 | |||||
| 要旨 | ||||||
| 内容記述タイプ | Other | |||||
| 内容記述 | In order to achieve economical fusion reactors, a high beta operation of the plasma is required. When the plasma beta, i.e. the plasma pressure divided by the magnetic pressure, increases, the induced pressure gradient drives MHD instabilities. Therefore, the understanding of the MHD instabilities is one of the key issues to realize high beta plasmas. The spatial structure of the instabilities is essential in understanding of their characteristics. Computed tomographic reconstruction (CTR) using the soft X-ray emission is one of the standard techniques for that purpose. In the Large Helical Device (LHD), the arrangement of the detectors suitable for the CTR is difficult; due to the superconducting coil systems, it is not possible to arrange the detectors in a way that the plasma is observed from different directions. The tangentially viewing imaging system has been thus used for the observation of the core MHD activities. And such tangentially viewing imaging systems have been also developed on many devices around the world. However, those imaging systems are using soft X-ray emission, which are good for observations of the core plasma activities but not good for studies of the edge plasma activities. A newly developed high-speed imaging system using vacuum ultraviolet (VUV) emission from the edge LHD plasma is presented in this study. The VUV imaging system is composed of a telescope made of Mo/Si multilayer mirrors, micro-channel plate (MCP) and a high-speed visible CMOS camera. With Mo/Si mirrors, VUV photons with a wavelength of 13.5 nm can be selectively measured. A telescope optics rather than pin-hole optical system is selected; the solid angle of the mirror viewed from the plasma is much larger than the solid angle of a pinhole system. Therefore, faster optics, suitable for fluctuation study can be constructed. A Zr filter is installed to cut-off the low energy VUV photons which can also be reflected by the Mo/Si mirrors. The image is then detected by MCP and recorded by the high-speed camera. By measuring the CVI line emission around 13:5nm, edge MHD activities are directly visualized by this device. Additionally, since the time evolution of line emission of C VI is selectively measured, it is possible to carry out carbon impurity transport study with this imaging system. Data analysis methods for the imaging data have been developed in this thesis. Since the imaging data are line-integrated ones, tomographic reconstruction of local emission profile from the line-integrated data are required. Construction of the so-called geometry matrix by which the local emission is related with lineintegrated image is discussed. If an arbitrary three-dimensional emission profile is assumed, reconstruction from a two-dimensional image is not possible. An assumption that the emission along the magnetic field line is constant is made. Thereby a new method is developed to construct the geometry matrix. In this method, sight lines are projected to curved sight lines on a cross-section (horizontally elongated section in this thesis). The line elements in a sight line are connected to the line elements in the curved sight line along the magnetic field lines. Then the CTR problem for tangentially viewing case is reduced to a standard 2D tomography problem with peculiar sight lines. HINT2 equilibrium code is used to estimate the magnetic field lines. With HINT2 code, magnetic field line traces can be made with finite beta condition. Magnetic field lines outside the last closed magnetic surface (LCFS) can be also estimated. This is good for the VUV imaging diagnostics since it is possible that C VI emission may come from the stochastic region which is outside the LCFS. After the construction of geometry matrix, synthetic images can be calculated according to the relation of line-integrated image and local emission profile. The performance of the tomographic reconstruction method is investigated in LHD configuration. Several algorithms have been tested, such as Phillips- Tikhonov (PT) method, maximum entropy method (MEM), truncated singular value decomposition (TSVD). According to the phantom test results, PT gives the best performance, and the spatial mode structure with high mode number (m ～ 10) has been shown to be reconstructed even with a complex LHD con figuration. The noise level where the reconstruction is available is investigated. The maximum noise level is about 6%. The quality of the reconstruction results is however more sensitive to the accuracy of the equilibrium magnetic field. A simple transport model has been constructed to study the behavior of the carbon impurity. In this modeling, transport process, ionization and recombination process are included. With this transport model, time evolutions of the C VI emissivity are estimated assuming a transport coecient profile. It is confirmed that the CVI emission is peaked in the edge region of LHD plasma in normal experimental condition. Using this model, the penetration depth of injected carbon pellets is estimated from the imaging data. The ionization process is dominant just after the pellet ablation. Therefore, the image just after the deposition is used to study the initial deposition profile. The estimated penetration depth is qualitatively consistent with the estimation made by the pulse width of the Hα emission signal that is caused by the carbon pellet injection. MHD activities with low-frequency (about 0:75kHz) have been successfully measured by the VUV imaging system. The amplitude of the MHD activities is fairly large and degradation of the connement is observed with this MHD modes. The poloidal mode number (m = 1) and the location of the mode (r=a ～ 0:9) is estimated. The 2D spatial structure of the m=n = 1=1 mode has been identi fied by comparing the synthetic images assuming and images measured experimentally. The estimate of the mode number is consistent with that estimated according to the magnetic probe signal and the location is consistent with q =1 rational surface. The proof of concept, this type of imaging system can detect fuctuations localized in the edge region, is shown successfully. Although the maximum sampling rate of this device is as low as 2kHz at the present moment, it is limited only by the intensity of the plasma emission. And the data analysing technique developed in this study is shown to be quite useful in analysing the MHD activities. 7.2 Outlook The challenge is to improve the framing rate up to ～ 10kHz to realize the concept that the VUV imaging system is suitable for investigating normal edge MHD instabilities in LHD. For normal discharges, the carbon impurity concentration is almost stable and very low, upgrade of the optical system is required to improve the framing rate. Since the total number collected by the imaging system is proportional to the imaging area of the mirrors, a new mirror system, which has larger imaging area, is under plan. Additionally, the carbon pellet injection is widely applied to obtain high temperature plasma in LHD. And MHD activities have been observed during the pellet injection. Since the concentration of the carbon impurity is much higher than that in normal discharges, hence, the intensity of the C VI emission can be significantly increased in such discharges. Therefore, it is possible to study such pellet induced MHD phenomena with this imaging system. There are many applications of this technique. Verification of the MHD model related with the resonant magnetic eld penetration is an important example and will be investigated in future. And the tomographic reconstruction will be tried to obtain the 2D emission profile, which enables to investigate the phenomena where asymmetric distribution exists. And the important topics to be studied using refined VUV telescope system are as follows: (1) Locked mode. It has been observed that the edge MHD instabilities slowed down and stopped. A large magnetic island is formed after the locking; the performance of the plasma degraded significantly by the mode locking. The detailed mechanism that how the rotating mode locks and how to expand to be a static island will be studied. (2) ELM-like activities and the relation of RMP field In H-mode plasma in LHD, edge MHD instabilities drive ELM-like relaxation events. The detailed process of the ELM-like collapse will be studied. When the RMP (LID) field is applied, the scale of the ELM is reduced. The spatial structure of the ELM activities with RMP field will be estimated. (3) Core density collapse event In the core density collapse events observed in the high-density plasma experiments, the ballooning mode might play an important role. One of the advantage of the imaging diagnostics is that mode structure with high mode number can be measured directory. The pre-cursor oscillations caused by the ballooning mode will be studied. (4) Physics of Island formation When the externally applied field is shielded, there is no magnetic island inside the plasma. When the magnetic field penetrates, the magnetic island is formed. The VUV telescope can monitor the formation process of the island with high framing rate. The comparison of the model of the island can be performed by the comparison of the time scale of the expansion of the magnetic island. This kind of fast measurement can be realized two dimensionally only by VUV telescope system at present in LHD. |
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| 値 | 有 | |||||
