Experimental Application and Numerical Study of Reflectometry in the Alcator C-Mod Tokamak PDF Download
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Author: Yijun Lin Publisher: ISBN: Category : Languages : en Pages : 182
Book Description
The amplitude modulated ordinary-mode reflectometer in the Alcator C-Mod tokamak is used to study the quasi-coherent (QC) continuous edge fluctuations in enhanced Da (EDA) H-modes. Reflectometer data show that the QC fluctuations are localized near the center of the density pedestal. The radial width (FWHM) is usually in the range of 0.1 - 0.3 cm. The width increases with the increase of resistivity rl. The line-integrated fluctuation level approximately scales with (v*q95).56. This result indicates that higher q95, higher density, and lower temperature are favored for the QC fluctuations. Neither the location nor the width changes significantly in an EDA H-mode period, while the frequency and level vary. A 2-D full-wave code has been developed to simulate and quantitatively interpret reflectometry signals. The code uses the finite-difference time-domain method to solve Maxwell's equations in two dimensions. Perfectly-matched layers are used as the boundary. The Huygens source technique is used to generate Gaussian beams and separate the reflected waves from the total field. Simulations based on realistic 2-D geometry of the Alcator C-Mod reflectometer provide a calibration curve by which we can relate the QC fluctuations in reflectometry signals to plasma density fluctuations. Results indicate that the line-integrated fluctuation level derived from reflectometry is similar to that measured by the phase contrast imaging system. Simulations also indicate that plasma curvature extends the reflectometry response to fluctuations of high poloidal wavenumber. A preliminary study indicates that reflectometry can be used to estimate the correlation length of the turbulence in Alcator C-Mod provided that the fluctuation level is small.
Author: Yijun Lin Publisher: ISBN: Category : Languages : en Pages : 182
Book Description
The amplitude modulated ordinary-mode reflectometer in the Alcator C-Mod tokamak is used to study the quasi-coherent (QC) continuous edge fluctuations in enhanced Da (EDA) H-modes. Reflectometer data show that the QC fluctuations are localized near the center of the density pedestal. The radial width (FWHM) is usually in the range of 0.1 - 0.3 cm. The width increases with the increase of resistivity rl. The line-integrated fluctuation level approximately scales with (v*q95).56. This result indicates that higher q95, higher density, and lower temperature are favored for the QC fluctuations. Neither the location nor the width changes significantly in an EDA H-mode period, while the frequency and level vary. A 2-D full-wave code has been developed to simulate and quantitatively interpret reflectometry signals. The code uses the finite-difference time-domain method to solve Maxwell's equations in two dimensions. Perfectly-matched layers are used as the boundary. The Huygens source technique is used to generate Gaussian beams and separate the reflected waves from the total field. Simulations based on realistic 2-D geometry of the Alcator C-Mod reflectometer provide a calibration curve by which we can relate the QC fluctuations in reflectometry signals to plasma density fluctuations. Results indicate that the line-integrated fluctuation level derived from reflectometry is similar to that measured by the phase contrast imaging system. Simulations also indicate that plasma curvature extends the reflectometry response to fluctuations of high poloidal wavenumber. A preliminary study indicates that reflectometry can be used to estimate the correlation length of the turbulence in Alcator C-Mod provided that the fluctuation level is small.
Author: Aaron Craig Bader Publisher: ISBN: Category : Languages : en Pages : 220
Book Description
In this thesis we discuss measurements and modeling of minority heated fast-ion distributions in the Ion Cyclotron Range of Frequencies (ICRF) on the Alcator C-Mod tokamak. Analysis of fast-ions >100Te is important for both ITER and a future fusion reactor as both will have a significant population of 3.5 MeV alpha particles generated in fusion reactions. Fast particles in this energy range can drive unstable modes such as Toroidal Alfvén Eigenmodes (TAEs) and Reversed Shear Alfvén Eigenmodes (RSAEs). Furthermore, energetic ions may display plasma properties that differ from the bulk plasma. It is crucial to benchmark current simulation codes with measurements from highly energetic fast-ions on current devices. This thesis will focus on measurements of the fast-ion distribution made on C-Mod with an upgraded Compact Neutral Particle Analyzer (CNPA). Measurements of the fast-ion distributions will reveal strong dependences of the fast-ion effective temperature on both electron density and plasma current. For further analysis, we use the simulated distributions generated by the coupled full-wave spectral solver AORSA, with the zero orbit-width bounce-averaged Fokker-Planck code CQL3D. A new synthetic diagnostic integrated into CQL3D is used to make direct comparisons with the CNPA. We find that for plasmas that have a steady-state fast-ion distribution (df /dt = 0) the simulation and the experiment have good agreement. However, in simulations where the fast-ion distribution is evolving in time (df/dt =/ 0) we find a discrepancy between the simulation and the experimental results. The simulation is seen to evolve much slower than the experiment. Various reasons for the discrepancy are explored, including the possibility of a violation of the quasi-linear theory used in CQL3D.
Author: Yunxing Ma Publisher: ISBN: Category : Languages : en Pages : 210
Book Description
Usually when sufficient heating power is injected, tokamak plasma will make an abrupt transition into a state with improved confinement, known as the high-confinement mode, or H-mode. Given the greatly enhanced fusion yield, H-mode is foreseen as the baseline scenario for the future plasma operation of the International Thermonuclear Experimental Reactor (ITER). Many research efforts have been given to understand the criteria for H-mode access. To further contribute to this research, a primary focus of this thesis is characterizing the H-mode access conditions in the Alcator C-Mod tokamak, across a broad range of plasma density, magnetic field, and plasma current. In addition, dedicated experiments were designed and executed on C-Mod, to explore the effects of divertor geometry, ICRF resonance location, and main ion species on H-mode access conditions. Results from these experiments will be included in this thesis. The underlying physics of H-mode access is very complex, and the critical mechanisms remain largely unresolved. To promote our understanding, some models proposed for the H-mode transition are tested, using well documented local plasma conditions, obtained in C-Mod experiments. In particular, this thesis pioneers the test of a recently developed model for H-mode threshold power predictions.
Author: William Davis Lee Publisher: ISBN: Category : Languages : en Pages : 205
Book Description
(Cont.) Data taken during ITB discharges have shown rotation profiles that evolve from flat co-current to hollow, with rotation slightly counter-current inside the barrier foot. Radial electric field profiles have been infered using neoclassical theory; these profiles are found to be highly sheared around the barrier foot. A lower limit to the gradient of the radial electric field ([partial derivatice]E[sub]r/[partial derivative]r) approximately] 183 kV/m2 has been set, and a lower limit on the shearing frequency has been approximated as [omega] [sub]E [approximately] krad/s.
Author: Mark Alan Chilenski Publisher: ISBN: Category : Languages : en Pages : 485
Book Description
This thesis presents two new techniques for analyzing data from impurity transport experiments in magnetically confined plasmas, with specific applications to the Alcator C-Mod tokamak. The objective in developing these new techniques is to improve the quality of the experimental results used to test simulations of turbulent transport: better characterization of the uncertainty in the experimental results will yield a better test of the simulations. Transport codes are highly sensitive to the gradients of the background temperature and density profiles, so the first half of this thesis presents a new approach to fitting tokamak profiles using nonstationary Gaussian process regression. This powerful technique overcomes many of the shortcomings of previous spline-based data smoothing techniques, and can even handle more complicated cases such as line-integrated measurements, computation of second derivatives, and 2d fitting of spatially- and temporally-resolved measurements. The second half of this thesis focuses on experimental measurements of impurity transport coefficients. It is shown that there are considerable shortcomings in existing point estimates of these quantities. Next, a linearized model of impurity transport data is constructed and used to estimate diagnostic requirements for impurity transport measurements. It is found that spatial resolution is more important than temporal resolution. Finally, a fully Bayesian approach to inferring experimental impurity transport coefficient profiles which overcomes the shortcomings of the previous approaches through use of multimodal nested sampling is developed and benchmarked using synthetic data. These tests reveal that uncertainties in the transport coefficient profiles previously attributed to uncertainties in the temperature and density profiles are in fact entirely explained by changes in the spline knot positions. Appendices are provided describing the extensive work done to determine the derivatives of stationary and nonstationary covariance kernels and the open source software developed as part of this thesis work. The techniques developed here will enable more rigorous benchmarking of turbulent transport simulations, with the ultimate goal of developing a predictive capability.
Author: Yijun Lin
Author: Yijun Lin
Author: Aaron Craig Bader
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Author: 
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Author: Yunxing Ma
Author: William Davis Lee
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Author: Mark Alan Chilenski
Author: Michael Anthony Graf