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Study of edge localized modes using mm-wave interferometer array and electron cyclotron emission imaging diagnostics

AYUB, MUHAMMAD KHAWAR (POSTECH)

원문보기

  • 발행기관 POSTECH
  • 지도교수Prof. Gunsu Yun
  • 발행년도2017
  • 학위수여년월2017. 2
  • 학위명박사
  • 학과 및 전공일반대학원 물리학과
  • 세부전공Fusion Plasmas
  • 원문페이지121
  • 실제URIhttp://www.dcollection.net/handler/postech/000002330122
  • 본문언어영어
  • 저작권포항공과대학교 논문은 저작권에 의해 보호받습니다.
초록 moremore
The study of edge localized modes (ELMs) is one of the most important and interesting subject in the field of fusion plasma research. As the ELM bursts involve rapid loss of considerable particles and heat flux from the edge of plasma, control and physical understanding of ELM dynamics are essential...
The study of edge localized modes (ELMs) is one of the most important and interesting subject in the field of fusion plasma research. As the ELM bursts involve rapid loss of considerable particles and heat flux from the edge of plasma, control and physical understanding of ELM dynamics are essential for the steady-state and safe operation of future magnetic fusion devices such as international thermonuclear experimental reactor (ITER). The recently visualized ELM dynamics are found to be very complicated due to its non-linear and non-axis symmetric nature. It is speculated that the linear theory based on peeling-ballooning (PB) model may not be applicable and non-ideal effects such as damping, diffusion and nonlinear saturation come into play during the evolution of an ELM. Therefore, detailed study of the non-ideal effects and accurate measurement of ELM properties such as radial size and perturbation amplitude are essential for the complete understanding of underlying physics of ELM dynamics. The electron cyclotron emission imaging (ECEI) system on the KSTAR, an advanced 2D imaging diagnostics based on the principle of conventional ECE radiometry can visualize the ELM dynamics, using large aperture optics and microwave diagnostic technique, but it has certain limitations. In particular, perturbations with very small radial size (< 2 cm) are difficult to resolve using the ECEI system due to the fundamental limitation in the radial resolution (~ 2 cm at the plasma edge of a typical plasma discharge with toroidal magnetic field ~ 2 T) imposed by the relativistic broadening of ECE. However, at the edge of tokamak plasmas the exact interpretation of ECE signals has intrinsic complexity due to rapid change in the optical thickness from thick to thin. For the optically marginal plasma region (which separates optically thick and optically thin plasma while the optical thickness ~ < 1), e.g., the steep gradient region where the ELMs usually appear during a typical H-mode plasma, ECE signal not only involves the electron temperature fluctuations but also has a significant contribution from electron density fluctuations making difficult the exact estimation of the amplitude. Therefore, the accurate measurement of the radial size and density fluctuation amplitude of the ELMs as well as the interpretation of ECE signals from the optically marginal region of plasma at the edge is very important and need especial attention. A novel technique to estimate the range of radial size and density fluctuation amplitude of ELMs in the KSTAR tokamak plasma is presented. A microwave imaging reflectometry (MIR) system is used as a multi-channel mm-wave interferometer array (MIA) to measure the density fluctuations associated with ELMs, while ECEI system is used as a reference diagnostics to confirm the MIA observation. Two dimensional full-wave (FWR2D) simulations integrated with optics simulation are performed to investigate the Gaussian beam propagation and reflection through the plasma as well as the MIA optical components and obtain the interferometric phase undulations of individual channels at the detector plane due to ELM perturbation. The simulation results show that the amplitude of the phase undulation depends linearly on both radial size and density perturbation amplitude of ELM. For a typical discharge with ELMs, it is estimated that the ELM structure observed by the MIR system has density perturbation amplitude in the range ~ 7 % to 14 % while radial size in the range ~ 1 to 3 cm. The interpretation of ECE signal from the optically marginal region of plasma is also presented. For this purpose, the density and temperature fluctuations associated with the ELMs observed in the KSTAR tokamak are estimated by assuming the ELM filamentary structure as a flux tube bulged out like ballooning mode instability. Synthetic ECE signals from the rotating ELM are calculated based on the measured electron temperature profile and an assumed electron density profile constrained by the measured line averaged density, yielding ~ 0.02 relative fluctuation level in agreement with the experimental observations. The measured ECE signal is nearly in phase with the density modulation associated with the rotating ELM. This implies that the ECE signal corresponding to the ELM filaments has a significant contribution from the density fluctuations. This study can be helpful for the complete understanding of underlying ELMs physics.
목차 moremore
I.Introduction 1
1.1 Magnetic confinement and tokamak concept 4
1.2 High confinement mode and edge localized modes (ELMs) 8
...
I.Introduction 1
1.1 Magnetic confinement and tokamak concept 4
1.2 High confinement mode and edge localized modes (ELMs) 8
1.3 Purpose of my Thesis 9

II.Edge Localized Modes 12
2.1 Theory of ELMs: MHD 13
2.1.1 Ideal magnetohydrodynamics 13
2.1.2 The plasma stability and energy principle 15
2.1.3 Pressure driven instabilities 17
2.1.4 Current driven instabilities 18
2.1.5 Peeling-ballooning modes 19
2.2 Background study related to ELMs 20
2.3 Comparison of experiment and theory 25
2.4 Diagnostics for ELM study in KSTAR 26
2.4.1 Mirnov coil array 26
2.4.2 H-alpha emission monitor 27
2.4.3 Interferometry 28
2.4.4 Thomson scattering system 28
2.4.5 Charge exchange recombination spectroscopy 29
2.4.6 Electron cyclotron emission (ECE) radiometry 31
2.4.7 Electron cyclotron emission imaging (ECEI) 35

III.Microwave Imaging Reflectometry 41
3.1 Reflectometry 42
3.2 Microwave imaging reflectometry (MIR) 46
3.3 Plasma waves for reflectometry 49
3.4 Properties of Gaussian beam 50
3.5 MIR system as mm-wave interferometer array (MIA) 51
3.6 MIR System on the KSTAR 53
3.6.1 Four frequency probe beam source 54
3.6.2 Detection system55
3.6.3 Launching and receiving optics 56
3.7 Recent progress in MIR 59

IV.MHD Simulation for ELM Study 61
4.1 The plasma equilibrium and Grad-Shafranov equation 61
4.1.1 The basic considerations of plasma equilibrium61
4.1.2 An axisymmetric toroidal plasma equilibrium: the Grad-Shafranovequation 63
4.2 The cold plasma model 66
4.2.1 Plane waves in a homogeneous plasma 66
4.2.2 Cold plasma dielectric permittivity 68
4.2.3 Cold plasma dispersion relation 71
4.2.4 Cutoff and Resonance 73
4.3 Plasma equilibrium using TOQ code 75
4.4 FWR2D simulation for ELMs 76
4.5 CODE V 80

V.Estimation of radial size and density fluctuation amplitude of ELMs 81
5.1 ELM observation using MIR as MIA 81
5.2 FWR2D simulation results and its comparison with MIA measurement 86

VI. Interpretation of ECE signal from optically marginal plasmas 91
6.1 ELM observation using ECEI 93
6.2 Modelling of density and temperature fluctuations associated with rotating ELM filaments 94
6.3 Interpretation of δT_ECE/〈T_ECE 〉 in the optically marginal plasma region 97
6.4 Comparison of δT_ECE/〈T_ECE 〉 with density fluctuation 99

VII.FWR2D simulations for high pressure atmospheric plasmas 104
7.1 Plasma modelling and simulation set up 104
7.2 Simulation results 105

VIII.Summary and Conclusion 108

References 111

Acknowledgements 116

Curriculum vitae 118