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Fusion energy has great potential to meet the ever increasing global energy demand. The most promising magnetic confinement device, the tokamak, remains the dominant candidate for fusion power generation. The generated thermal power from tokamaks is comparable to fission nuclear power, but with no possibility of catastrophic accidents (such as the melting of fuel rods in the Fukushima I reactors caused by the failure of the cooling systems) if the facilities were damaged. This safety feature arises from the fact that magnetic fusion can only be achieved under very precise circumstances, and damage to any key system will immediately result in a cessation of the fusion reaction. Our goal is to develop advanced visualization tools via imaging diagnostics to provide fundamental physics understanding and thus help determine the optimum path to achieve and sustain magnetic fusion. High resolution Terahertz (THz) imaging diagnostics are exceptionally well suited to this task due to the characteristic properties of fusion plasmas at THz frequencies. High performance electronics have been developed for the far-infrared tangential interferometer and polarimeter (FIReTIP) system on the National Spherical Tokamak Experiment (NSTX) device. This work significantly increased both the phase and temporal resolution of the FIReTIP system, thereby allowing it to monitor high frequency density and magnetic field fluctuations up to 4 MHz with unprecedented accuracy, which is of critical performance as characterizing these fluctuations are essential in understanding transport physics issues in fusion plasmas. The millimeter-wave and THz spectral region offers tremendous potential for a diverse range of applications outside of plasma diagnostics in fields such as communications, biomedical diagnostics, remote imaging and concealed weapons detection. The unique THz spectral region offers exciting and novel application opportunities arising from its inherent advantages in spatial resolution and data rate, bandwidth, and component compactness, but the lack of sufficient source power has not been adequately solved. High power radiation sources in the THz range also have considerable application in advanced fusion plasma diagnostics. Work is presented herein on the design of waveguide components and a high efficiency pulse modulator to support the development of integrated high power sources for fusion diagnostics and other applications.
The advancement of magnetic confinement nuclear fusion toward a viable source of energy on the scale of today's conventional power plants requires the development of a broad range of instruments for use in present day experimental fusion reactors. A class of plasma diagnostic systems that make use of electromagnetic emission from free electrons includes Electron Cyclotron Emission Imaging (ECEI), conceived at the University of California at Davis as an extension of ECE radiometry. A new ECEI system with unique capabilities is designed and realized for use on the Tokamak Experiment for Technology Oriented Research (TEXTOR), a toroidal plasma confinement device located at Forschungszentrum Jülich, Germany. The TEXTOR ECEI system is capable of 128 channel (16 vertical by 8 radial) 2-D imaging of electron temperature fluctuations below 1% in the poloidal plane on [mu]s time scales. Advancements in a variety of millimeter wave technologies are discussed, including the development of dual-dipole antennas and miniature elliptical substrate lenses, planar quasi-optical notch filters, dichroic plate high-pass filters, dielectric film beamsplitters, RF electronics for double down-conversion heterodyne frequency mixing and signal detection, and optical coupling of electron cyclotron emission signals and local oscillator power. Particular emphasis is given to the development of a new heuristic for the design of optical coupling systems for millimeter wave imaging arrays which has resulted in the realization of the feature of independent vertical zoom, new to ECEI, by which the vertical extent of the plasma image may be continuously varied from 20 to 35 cm. The new TEXTOR ECEI system is compared in laboratory characterization to the legacy ECEI system, which it replaced in 2008, to reveal dramatic improvements in image quality, optical performance, and system noise temperature. Finally, the installation of this diagnostic is discussed and data obtained during commissioning are presented. A look forward to continuing projects in the field of ECEI reveals an exciting future for the technology with growing international collaboration and invaluable contributions to the effort to develop energy resources that may some day eliminate mankind's dependence on fossil fuels.
Magnetic confinement thermonuclear fusion energy has long been considered a potential substitute for fossil fuels as the major long term energy source of global development. With its effective magnetic field configuration, tokamak devices have received extensive investigations with advancement in plasma diagnostic tools [1]. A comprehensive millimeter wave passive imaging diagnostic system for measurement of electron temperature fluctuations in tokamaks has been conceived and developed at the University of California at Davis utilizing the Electron Cyclotron Emission from the plasma [2,3]. HL-2A is a diverted tokamak developed and constructed by the Southwestern Institute of Physics (SWIP) in Chengdu, China based on the vacuum vessel and magnetic coil system of the former German ASDEX device. Previous millimeter wave diagnostics including reflectometry and ECE radiometry have been installed on the tokamak for electron density and temperature profile measurements [4,5]. However, there is increasing need for fluctuation measurements over the plasma volume for research into plasma confinement and instabilities. Through a collaborative effort between the Davis Millimeter Wave Research Center (DMRC) of the University of California at Davis and SWIP, a new 192 channel (24 vertical by 8 radial) Electron Cyclotron Emission Imaging system has been designed and constructed at UC Davis for a 2 dimensional coverage of the plasma temperature with high spatial resolution. A new imaging optical system with zooming capability is optimized for the available port window on HL-2A with a versatile coverage of the plasma volume ranging from a magnification ratio of 1 to 1.8. A novel local oscillator (LO) optical system is designed to maintain the optimum illumination onto the antenna array under different operating frequencies of the Backward Wave Oscillator. The RF electronics for double down-conversion heterodyne frequency mixing and signal detection is developed from the DIIID ECEI system with improved sensitivity and reduced noise. Other millimeter wave components such as the dual-dipole antenna array and dichroic plate high-pass filters are fabricated and characterized. The complete system is assembled and calibrated in the laboratory at UC Davis with extensive testing and characterization of the functionality of each subsystem.
By Thomas Sunn Pedersen.
To understand the fundamental physical phenomena of the hot confinement plasma located inside of the controlled fusion experimental devices called Tokamaks, good spatial and temporal measurements of the electron temperature and density fluctuations are needed. Two powerful plasma visualization diagnostics, Electron Cyclotron Emission Imaging (ECEI) and Microwave Imaging Reflectometry (MIR), have been developed by the plasma diagnostic group in University of California at Davis. Unlike the conventional 1-D diagnostic methods, they are able to provide 2-D high resolution image of the electron temperature and density fluctuations.This dissertation will focus on the development of the ECEI system customized for the KSTAR tokamak in Korea. The physics principle of the electron cyclotron radiation in fusion plasmas is firstly reviewed, followed by a description of the system architecture of the ECEI diagnostic. Technology advancements in a variety of the system components are discussed, including the miniature elliptical substrate lenses, dual-dipole antennas and mixers array, quasi-optical planar filters, double down-conversion heterodyne electronics, and local oscillator power coupling. Particular emphasis is given to the design methods and major innovations in the advanced optical coupling system for the KSTAR ECEI. This includes step-by-step descriptions on the imaging lens design analysis, as well as the realization of the advanced imaging features, such as independent zoom and focus control. The development of the frequency selective surface (FSS) notch filter is also discussed in detail. The FSS's basic configurations, design principles, and the numerical design tools will be discussed. The previous notch filter designs as well as the new generation multilayer configuration, which brings significant performance improvements, will be summarized. The laboratory testing platform and the fabrication considerations will also be presented. The KSTAR ECEI diagnostic, fabricated and installed in 2010, represents a new benchmark in imaging diagnostic system in terms of plasma coverage, resolution, and imaging flexibility. It also represents a major step forward in the standardization of many ECEI system components, such as dual detector array with mini lens and planar quasi-optical filter components.