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Author: Oleg Krutkin Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Plasma turbulence is nowadays believed to be responsible for the anomalous transport and consequently the degradation of discharge conditions in magnetic confined fusion devices, such as tokamaks. Since a good energy confinement time is crucial for achieving a positive energy yield, understanding and control of turbulent processes is currently one of the major goals of the Magnetic Confinement Fusion research. To study the plasma turbulence, experimental tools that are able to provide information about its characteristics are necessary. Such tools include microwave diagnostics and, in particular, Doppler reflectometry and radial correlation Doppler reflectometry. While these non-invasive diagnostics benefit from the simplicity of the setup, there are a number of unresolved issues when it comes to the interpretation of the experimental data. Issues such are small-angle scattering and plasma curvature effects limit the range of applicability of the simple interpretation of the measurements, while nonlinear scattering effects make it inapplicable altogether. These problems make it necessary to validate the interpretation of experimental data. Thus, the primary goal of this thesis was to create a synthetic Doppler reflectometry and radial correlation Doppler reflectometry diagnostic for the interpretation of the FT-2 tokamak experimental results. This goal is achieved by applying full-wave IPF-FD3D code to the results of gyrokinetic plasma modelling with ELMFIRE code to obtain the synthetic signals, which are then benchmarked with experimental measurements. The synthetic diagnostic is also used for a more general study of the possibility of nonlinear effects influencing the experimental measurements. Finally, the secondary goal of this thesis was to perform an analytical research of plasma curvature effects, nonlinear scattering and a novel technique for turbulence structures' characterization. The first principles analytical study was performed by considering the Helmholtz equation and obtaining an analytical expression for the experimental signals. The results for the latter two topics were numerically validated with the partial use of specially developed linear numerical model and the full-wave IPF-FD3D code.
Author: Oleg Krutkin Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Plasma turbulence is nowadays believed to be responsible for the anomalous transport and consequently the degradation of discharge conditions in magnetic confined fusion devices, such as tokamaks. Since a good energy confinement time is crucial for achieving a positive energy yield, understanding and control of turbulent processes is currently one of the major goals of the Magnetic Confinement Fusion research. To study the plasma turbulence, experimental tools that are able to provide information about its characteristics are necessary. Such tools include microwave diagnostics and, in particular, Doppler reflectometry and radial correlation Doppler reflectometry. While these non-invasive diagnostics benefit from the simplicity of the setup, there are a number of unresolved issues when it comes to the interpretation of the experimental data. Issues such are small-angle scattering and plasma curvature effects limit the range of applicability of the simple interpretation of the measurements, while nonlinear scattering effects make it inapplicable altogether. These problems make it necessary to validate the interpretation of experimental data. Thus, the primary goal of this thesis was to create a synthetic Doppler reflectometry and radial correlation Doppler reflectometry diagnostic for the interpretation of the FT-2 tokamak experimental results. This goal is achieved by applying full-wave IPF-FD3D code to the results of gyrokinetic plasma modelling with ELMFIRE code to obtain the synthetic signals, which are then benchmarked with experimental measurements. The synthetic diagnostic is also used for a more general study of the possibility of nonlinear effects influencing the experimental measurements. Finally, the secondary goal of this thesis was to perform an analytical research of plasma curvature effects, nonlinear scattering and a novel technique for turbulence structures' characterization. The first principles analytical study was performed by considering the Helmholtz equation and obtaining an analytical expression for the experimental signals. The results for the latter two topics were numerically validated with the partial use of specially developed linear numerical model and the full-wave IPF-FD3D code.
Author: Jon Clark Hillesheim Publisher: ISBN: Category : Languages : en Pages : 307
Book Description
Understanding the turbulent transport of particles, momentum, and heat continues to be an important goal for magnetic confinement fusion energy research. The turbulence in tokamaks and other magnetic confinement devices is widely thought to arise due to linearly unstable gyroradius-scale modes. A long predicted characteristic of these linear instabilities is a critical gradient, where the modes are stable below a critical value related to the gradient providing free energy for the instability and unstable above it. In this dissertation, a critical gradient threshold for long wavelength ($k_{\theta} \rho_s \lesssim 0.4$) electron temperature fluctuations is reported, where the temperature fluctuations do not change, within uncertainties, below a threshold value in $L_{T_e}^{-1}=\nabla T_e / T_e$ and steadily increase above it. This principal result, the direct observation of a critical gradient for electron temperature fluctuations, is also the first observation of critical gradient behavior for \textit{any} locally measured turbulent quantity in the core of a high temperature plasma in a systematic experiment. The critical gradient was found to be $L_{T_e}^{-1}_{crit}=2.8 \pm 0.4 \ \mathrm{m}^{-1}$. The experimental value for the critical gradient quantitatively disagrees with analytical predictions for its value. In the experiment, the local value of $L_{T_e}^{-1}$ was systematically varied by changing the deposition location of electron cyclotron heating gyrotrons in the DIII-D tokamak. The temperature fluctuation measurements were acquired with a correlation electron cyclotron emission radiometer. The dimensionless parameter $\eta_e=L_{n_e}/L_{T_e}$ is found to describe both the temperature fluctuation threshold and a threshold observed in linear gyrofluid growth rate calculations over the measured wave numbers, where a rapid increase at $\eta_e \approx 2$ is observed in both. Doppler backscattering (DBS) measurements of intermediate-scale density fluctuations also show a frequency-localized increase on the electron diamagnetic side of the measured spectrum that increases with $L_{T_e}^{-1}$. Measurements of the crossphase angle between long wavelength electron density and temperature fluctuations, as well as measurements of long wavelength density fluctuation levels were also acquired. Multiple aspects of the fluctuation measurements and calculations are individually consistent with the attribution of the critical gradient to the $\nabla T_e$-driven trapped electron mode. The accumulated evidence strongly enforces this conclusion. The threshold value for the temperature fluctuation measurements was also within uncertainties of a critical gradient for the electron thermal diffusivity found through heat pulse analysis, above which the electron heat flux and electron temperature profile stiffness rapidly increased. Toroidal rotation was also systematically varied with neutral beam injection, which had little effect on the temperature fluctuation measurements. The crossphase measurements indicated the presence of different instabilities below the critical gradient depending on the neutral beam configuration, which is supported by linear gyrofluid calculations. In a second set of results reported in this dissertation, the geodesic acoustic mode is investigated in detail. Geodesic acoustic modes (GAMs) and zonal flows are nonlinearly driven, axisymmetric ($m=0,\ n=0$ potential) $E \times B$ flows, which are thought to play an important role in establishing the saturated level of turbulence in tokamaks. Zonal flows are linearly stable, but are driven to finite amplitude through nonlinear interaction with the turbulence. They are then thought to either shear apart the turbulent eddies or act as a catalyst to transfer energy to damped modes. Results are presented showing the GAM's observed spatial scales, temporal scales, and nonlinear interaction characteristics, which may have implications for the assumptions underpinning turbulence models towards the tokamak edge ($r/a \gtrsim 0.75$). Measurements in the DIII-D tokamak have been made with multichannel Doppler backscattering systems at toroidal locations separated by $180^{\circ}$; analysis reveals that the GAM is highly coherent between the toroidally separated systems ($\gamma> 0.8$) and that measurements are consistent with the expected $m=0,\ n=0$ structure. Observations show that the GAM in L-mode plasmas with $\sim 2.5-4.5$ MW auxiliary heating occurs as a radially coherent eigenmode, rather than as a continuum of frequencies as occurs in lower temperature discharges; this is consistent with theoretical expectations when finite ion Larmor radius effects are included. The intermittency of the GAM has been quantified, revealing that its autocorrelation time is fairly short, ranging from about 4 to about 15 GAM periods in cases examined, a difference that is accompanied by a modification to the probability distribution function of the $E \times B$ velocity at the GAM frequency. Conditionally-averaged bispectral analysis shows the strength of the nonlinear interaction of the GAM with broadband turbulence can vary with the magnitude of the GAM. Data also indicates a wave number dependence to the GAM's interaction with turbulence. Measurements also showed the existence of additional low frequency zonal flows (LFZF) at a few kilohertz in the core of DIII-D plasmas. These LFZF also correlated toroidally. The amplitude of both the GAM and LFZF were observed to depend on toroidal rotation, with both types of flows barely detectable in counter-injected plasmas. In a third set of results the development of diagnostic hardware, techniques used to acquire the above data, and related work is described. A novel multichannel Doppler backscattering system was developed. The five channel system operates in V-band (50-75 GHz) and has an array of 5 frequencies, separated by 350 MHz, which is tunable as a group. Laboratory tests of the hardware are presented. Doppler backscattering is a diagnostic technique for the radially localized measurement of intermediate-scale ($k_{\theta} \rho_s \sim 1$) density fluctuations and the laboratory frame propagation velocity of turbulent structures. Ray tracing, with experimental profiles and equilibria for inputs, is used to determine the scattering wave number and location. Full wave modeling, also with experimental inputs, is used for a synthetic Doppler backscattering diagnostic for nonlinear turbulence simulations. A number of non-ideal processes for DBS are also investigated; their impact on measurements in DIII-D are found, for the most part, to be small.
Author: Simon Freethy Publisher: ISBN: Category : Languages : en Pages :
Book Description
The conversion of thermally-born electrostatic waves in tokamak plasmas has the potential to be a powerful diagnostic for tokamak edge physics. Analytic theory and full wave modelling both conclude that analysis of emission in the microwave region carries with it information on the magnetic field in spatially localised areas which depend on density and frequency. Knowing these quantities and the 3D (2D + frequency) microwave emission pattern, it is in theory possible to calculate the current density which is vital to the understanding of the plasma pedestal. Motivated by a pilot experiment carried out on the Mega Ampere Spherical Tokamak (MAST), a novel microwave imaging device has been developed to obtain the first images of mode conversion in a Tokamak and to prove the principle of the synthetic aperture imaging technique on Tokamak devices. Here the design and calibration of the Synthetic Aperture Microwave Imaging (SAMI) radiometer is described, as well as the presentation and comparison of some of the first images of mode conversion with full wave simulations.
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: Thomas J. Dolan Publisher: Springer Science & Business Media ISBN: 1447155564 Category : Technology & Engineering Languages : en Pages : 816
Book Description
Magnetic Fusion Technology describes the technologies that are required for successful development of nuclear fusion power plants using strong magnetic fields. These technologies include: • magnet systems, • plasma heating systems, • control systems, • energy conversion systems, • advanced materials development, • vacuum systems, • cryogenic systems, • plasma diagnostics, • safety systems, and • power plant design studies. Magnetic Fusion Technology will be useful to students and to specialists working in energy research.
Author: I. H. Hutchinson Publisher: Cambridge University Press ISBN: 9780521675741 Category : Science Languages : en Pages : 460
Book Description
This book provides a systematic introduction to the physics of plasma diagnostics measurements. It develops from first principles the concepts needed to plan, execute and interpret plasma measurements, making it a suitable book for graduate students and professionals with little plasma physics background. The book will also be a valuable reference for seasoned plasma physicists, both experimental and theoretical, as well as those with an interest in space and astrophysical applications. This second edition is thoroughly revised and updated, with new sections and chapters covering recent developments in the field.
Author: Dr. Ching Eng (Jason) Png Publisher: CRC Press ISBN: 1498758681 Category : Technology & Engineering Languages : en Pages : 361
Book Description
This book provides a first integrated view of nanophotonics and plasmonics, covering the use of dielectric, semiconductor, and metal nanostructures to manipulate light at the nanometer scale. The presentation highlights similarities and advantages, and shows the common underlying physics, targets, and methodologies used for different materials (optically transparent materials for nanophotonics, vs opaque materials for plasmonics). Ultimately, the goal is to provide a basis for developing a unified platform for both fields. In addition to the fundamentals and detailed theoretical background, the book showcases the main device applications. Ching Eng (Jason) Png is Director of the Electronics and Photonics Department at the Institute of High Performance Computing, Agency for Science Technology and Research, Singapore. Yuriy A. Akimov is a scientist in the Electronics and Photonics Department at the Institute of High Performance Computing, Agency for Science Technology and Research, Singapore.
Author: Oleg Krutkin
Author: Oleg Krutkin
Author: Jon Clark Hillesheim
Author: Simon Freethy
Author: Yijun Lin
Author: Thomas Martin Lehecka
Author: Carolin Helma Tröster
Author: Thomas J. Dolan
Author: I. H. Hutchinson
Author: 
Author: Dr. Ching Eng (Jason) Png