VEST 장치에서 Bursting MHD 현상에 의한 이온 가열 및 회전 가속

학위논문(박사)--서울대학교 대학원 :공과대학 에너지시스템공학부,2019. 8. 황용석.MHD instabilities in magnetically confined plasma significantly degrade the confinement and possibly leads to disruption. Plasma rotation and its shear have stabilization effects on various MHD instabilities such as NTMs (Neoclassical tearing modes) and RWMs (Resistive wall modes). On the other hand, MHD instabilities can have a significant influence on plasma rotation. While it is an important piece toward a thorough understanding of the interaction between MHD instabilities and plasma rotation, its experimental exploration has been limited to a few phenomena and still remains to be incomplete. In this thesis, detail dynamics of plasma rotation and ion temperature are investigated in bursting MHD activity on VEST (Versatile Experiment Spherical Torus). For this purpose, a novel ion Doppler spectroscopy (IDS) is newly developed with a superb temporal resolution ranging from 0.2-1 ms. This IDS capability permits to investigate the bursting MHD activity with fast crash time (~0.3ms). Interestingly, significant toroidal rotation acceleration as well as ion heating is observed in the bursting MHD activity. In order to explain the sudden spin-up phenomenon, several candidate mechanisms are discussed and compared with the experimental result. The fast IDS system is carefully designed and installed to measure the plasma rotation and ion temperature in VEST. A high throughput spectroscopic system is successfully developed by employing the transmission grating and by matching the etendue across the whole of the optical system. Fast temporal resolution can be attained to 1 ms for 10 spatial channels and 0.2 ms for one spatial channel. The diagnostic is based on the line emission of intrinsic carbon impurity and it can cover the R>0.5 m. The measured spectra by the IDS system are averaged spectra over the line-of-sight. Doppler tomographic inversion technique is employed to extract the local plasma rotation and ion temperature from the line integrated spectra. However, the inversion problem is a highly ill-posed problem and the reconstructed results sometimes have an unphysically oscillating feature. Second-order Tikhonov regularization method is utilized to properly estimate the local plasma properties which are believed to be continuous. This method is based on the minimization of the cost function which includes not only the accuracy of measurement but also the smoothness of the solution. Also, appropriate quantification of the uncertainties of the IDS system is analyzed by considering the propagation of errors of fitted parameters of spectra based on the numerical Monte Carlo (MC) method. Bursting MHD activities are usually observed in the low q Ohmic discharge with low prefill gas in VEST. These events are characterized by a spike in the plasma current, loop voltage and a burst of magnetic fluctuations with a high frequency above 10 kHz. Magnetic fluctuation shows a very fast exponential growth with a typical time constant of tens of microsecond (~100τA). Interestingly, fast IDS diagnostic reveals that significant toroidal rotation acceleration in the counter-Ip direction as well as ion heating are observed in the bursting MHD activity. The rotation and ion temperature are changed globally in the same manner in the entire of the plasma volume, which clearly shows that some kind of rotation torque and ion heating are acting on the plasma, instead of locally enhanced plasma transport. The rotation and ion temperature increase very fast with the time scale of < 300 μs and then recover slowly with the confinement time scale. Several physical mechanisms to account for the sudden spin-up are discussed in terms of temporal behavior, the direction of torque and torque amplitude. The considered mechanisms are reconnection outflows, toroidal electric field by reconnection process, enhanced electron loss, and NTV (Neoclassical toroidal viscosity) torque with offset rotation. Among them, NTV torque by non-axisymmetric magnetic fluctuation from the MHD activity seems to a most probable mechanism. Strong plasma rotation accelerations during the MHD activities are experimentally investigated for the first time. This observation will help us understand the effect of MHD event in plasma rotation and temporal dynamics of MHD event. In particular, this result suggests the possibility of the interplay between MHD instability and induced plasma rotation such as the self-organization process.Chapter 1. Introduction 1 1.1. Tokamak 3 1.2. MHD Stability and Plasma Rotation 6 1.3. Versatile Experiment Spherical Torus (VEST) 9 1.3.1. Ohmic Discharge 11 1.3.2. Diagnostics 13 1.4. Objectives of Research 15 Chapter 2. Ion Doppler Spectroscopy on VEST 16 2.1. Principle of Ion Doppler Spectroscopy (IDS) 17 2.2. Experimental Setup of IDS System 22 2.2.1. Viewing Geometry 24 2.2.2. Collecting Optics and Fibers 26 2.2.3. Spectrometer 29 2.2.4. Optical Characteristics 33 2.3. Calibration of the System 34 2.3.1. Wavelength Calibration 34 2.3.2. Intensity Calibration 37 2.3.3. Spatial Calibration 40 2.4. Data Processing and Uncertainty 43 2.4.1. Spectrum Fitting Algorithm 43 2.4.2. Doppler Tomographic Inversion 48 2.4.3. Phantom Test for the Doppler Tomography 55 2.4.4. Quantification of Uncertainty 61 Chapter 3. Bursting MHD Events on VEST 67 3.1. Tearing modes(TMs) on VEST 67 3.1.1. Characteristics of TMs on VEST 67 3.1.2. Locking Bifurcation 72 3.2. Bursting MHD Events on VEST 76 3.2.1. Characteristics of Bursting MHD Events on VEST 76 3.2.2. Comparison to the Tearing Mode 81 Chapter 4. Ion Heating and Torque from Bursting MHD Events 84 4.1. Spatio-temporal Behavior of Plasma Rotation and Ion Temperature during Bursting MHD Events 85 4.1.1. Spatial Profile Variation of Rotation and Ion Temperature 87 4.1.2. Detail Temporal Dynamics of Ion Properties in Bursting MHD 93 4.2. Ion Heating during the MHD Events 98 4.3. Possible Candidate Mechanisms for the Sudden Spin-up 105 4.3.1. Reconnection Outflows 105 4.3.2. Toroidal Electric Field by Reconnection Process 107 4.3.3. Enhanced Electron Loss Mechanism 110 4.3.4. NTV Torque with an Offset Rotation Velocity 111 Chapter 5. Conclusions and Future Work 116 5.1. Summary and Conclusions 116 5.2. Recommendations for Future Work 119 Bibliography 121 국 문 초 록 127Docto

Wolfgang Amadeus Mozart의 성악곡에서의 Ornamentation에 관한 연구 : Opera "Cosi fan tutte"를 중심으로

학위논문(석사)--서울대학교 대학원 :음악과 성악전공,1997.Maste

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