Advancements in Langmuir Probe Diagnostic for Measurements in RF Sheath and in Modelling of the ICRF Slow Wave PDF Download
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Author: Mariia Usoltceva
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Coupling power to the plasma with ion cyclotron range of frequencies (ICRF) waves is a promising method for heating tokamak plasmas to fusion relevant temperatures. For high efficiency, the ICRF antenna must be placed close to the plasma, but they enhance destructive plasma-wall interactions. Plasma ions accelerated by the electric field in the radio-frequency (RF) sheath have been found to be the main cause of these interactions. The ICRF antenna design could be optimized to reduce the observed effects. The physics of these effects can be studied on a simple specially designed experiment. Aline (A LINear Experiment) is a linear low-temperature plasma device. The machine is focused on plasma characterization with the Langmuir probe diagnostic. The presence of magnetic field changes completely the particle transport in plasma, therefore conventional methods of data analysis are not applicable. Especially it is true for a small cylindrical Langmuir probe parallel to the magnetic field or at a small angle to it. In this thesis theories are presented which were developed for Langmuir probe data processing for magnetized plasma. The first results are also presented, as well as a comparison to line-averaged densities by interferometry. Presented data analysis techniques are not only important for application on Aline but can be used on any machine with magnetized plasma. IShTAR (Ion cyclotron Sheath Test Arrangement) is closer to tokamak conditions than Aline because it has an ICRF antenna which mimics tokamak antennas. In the framework of this thesis the objective is to study comprehensively the ICRF wave propagation in IShTAR configuration. Probe diagnostics were employed to quantify the relevant plasma parameters and the relevant ICRF wave fields. Numerical simulations of the ICRF slow wave were done in COMSOL. Plasma was implemented as a material with manually assigned physical properties. Field structures obtained for the slow wave differ significantly from the other mode, fast wave, and exhibit strong dependence on the density profile on the plasma edge. The results of this thesis work contribute to the studies of the RF sheath physics on dedicated linear devices, as well as the physics of ICRF waves on the tokamak plasma edge in general. In ICRF simulations for tokamak devices the slow wave propagation on the edge is avoided. Results of this thesis can be used to improve the complex tokamak ICRF simulations.
Author: Mariia Usoltceva
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Coupling power to the plasma with ion cyclotron range of frequencies (ICRF) waves is a promising method for heating tokamak plasmas to fusion relevant temperatures. For high efficiency, the ICRF antenna must be placed close to the plasma, but they enhance destructive plasma-wall interactions. Plasma ions accelerated by the electric field in the radio-frequency (RF) sheath have been found to be the main cause of these interactions. The ICRF antenna design could be optimized to reduce the observed effects. The physics of these effects can be studied on a simple specially designed experiment. Aline (A LINear Experiment) is a linear low-temperature plasma device. The machine is focused on plasma characterization with the Langmuir probe diagnostic. The presence of magnetic field changes completely the particle transport in plasma, therefore conventional methods of data analysis are not applicable. Especially it is true for a small cylindrical Langmuir probe parallel to the magnetic field or at a small angle to it. In this thesis theories are presented which were developed for Langmuir probe data processing for magnetized plasma. The first results are also presented, as well as a comparison to line-averaged densities by interferometry. Presented data analysis techniques are not only important for application on Aline but can be used on any machine with magnetized plasma. IShTAR (Ion cyclotron Sheath Test Arrangement) is closer to tokamak conditions than Aline because it has an ICRF antenna which mimics tokamak antennas. In the framework of this thesis the objective is to study comprehensively the ICRF wave propagation in IShTAR configuration. Probe diagnostics were employed to quantify the relevant plasma parameters and the relevant ICRF wave fields. Numerical simulations of the ICRF slow wave were done in COMSOL. Plasma was implemented as a material with manually assigned physical properties. Field structures obtained for the slow wave differ significantly from the other mode, fast wave, and exhibit strong dependence on the density profile on the plasma edge. The results of this thesis work contribute to the studies of the RF sheath physics on dedicated linear devices, as well as the physics of ICRF waves on the tokamak plasma edge in general. In ICRF simulations for tokamak devices the slow wave propagation on the edge is avoided. Results of this thesis can be used to improve the complex tokamak ICRF simulations.
Author: Gurleen Kaur Bal
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
Ion cyclotron range of frequencies (ICRF) heating in fusion plasmas is significantly hampered by the phenomenon of RF sheath rectification. Addressing RF sheaths and their related effects, such as impurity generation and convective cell formation, is important to make ICRF an effective heating option for future fusion devices. Experiments were performed on the Large Plasma Device (LAPD) using a single strap ICRF antenna to better understand how to mitigate RF sheath formation and its subsequent effects. The initial set of experiments explored the effects of electrically insulating antenna enclosures on RF-rectified sheaths. A single-strap RF antenna was powered using a high-power amplifier and matching network. Although the high-power amplifier and matching network were constructed during prior work, some improvements and changes were incorporated for this thesis work. For example, the amplifier, the matching network, and the antenna were modeled using the software LT-spice and simulations helped guide the changes made to the matching network. Additionally, the antenna design was updated to better shield against RF noise otherwise broadcasted into the lab, contaminating several data signals and electronics. Data from three experiments were compared where the enclosure material was made of copper, MACOR (electrically insulating), and MACOR over copper, respectively. The non-conductive MACOR material was exposed to the bulk plasma in the case of the MACOR-copper side walls, but a layer of copper was placed below to let image currents flow. All three experiments were carried out in a helium plasma with a background magnetic field of 1kG. In each of these three experiments, a single-strap, high-power (100kW) RF (2.4MHz) antenna was used to launch fast waves into the dense core of the magnetized helium plasma. The core density of the plasma was $n_e \approx 5 \times 10^{12} \ \mbox{cm}^{-3}$ to $8 \times 10^{12} \ \mbox{cm}^{-3}$ during each experiment. No Faraday screens were used on the front face of the antenna enclosure for all three experiments. In the case of the copper enclosure, RF rectified potentials, many times the local electron temperature, and associated formation of convective cells were observed and reported \cite{Martin2017}. The experiments with MACOR and MACOR-copper enclosures showed a considerable reduction in RF rectification. Furthermore, neither of these last two experiments indicated convective cell development. Although the results from the MACOR experiment are reminiscent of the results obtained in ASDEX-U with a 3-strap antenna optimized to reduce image currents on the antenna limiters \cite{bobkov2016first}, the MACOR-copper experiment seems to suggest that insulating plasma-facing materials have at least an equally strong impact on reducing potential rectification. To further explore the DC RF sheath mitigation seen in MACOR and MACOR-copper experiments, another set of experiments were executed with different thickness MACOR enclosures. A 1D voltage divider model by Myra and others has been presented to predict mitigation behavior depending on the insulator material and plasma properties. A series of experiments were conducted to investigate the effect of insulator material qualities and plasma properties on the degree of sheath mitigation. These experiments were conducted using enclosure walls made of copper, 1mm, 2mm, and 5mm MACOR. Also, each experiment was carried out under various plasma conditions by varying the time during discharge when the experiment was performed. RF rectified potentials in the copper enclosure experiment were used as a benchmark to determine the degree of mitigation in the MACOR studies. Findings from the various MACOR thickness and plasma parameters demonstrate that, with a few exceptions, sheath mitigation often follows the indicated trend of the voltage divider model. Moreover, the model's projected mitigation quantities and the measured sheath potentials do not agree well. To more accurately predict sheath mitigation in these experiments, the voltage divider sheath model will need to consider the 2D impacts of evolving density and plasma potential. In addition to the sheath mitigation work outlined above, additional work was done to document the parasitic lower hybrid (slow) wave in the LAPD edge. Most fusion experiments where coupling to the slow wave is a concern often have plasma densities and temperatures that are far too harsh for in-vessel diagnostics to be placed in the plasma volume. This work is unique because a fast-wave RF antenna was used to launch fast-wave, typically used for heating, in the core while simultaneously launching the unwanted slow-wave in the edge. Furthermore, this simultaneous coupling of waves was documented using electric dipole probes. Two new dipole probes were developed for this work to allow for better wave propagation mapping along the LAPD's length. One of the big challenges with this work has been achieving a range of densities in the LAPD that span the propagation region of the slow wave and fast wave. With the newly upgraded large $LaB_6$ source, several different plasma configurations were explored using annular limiters, different species plasmas, and a range of accessible frequencies. After the work done for documenting slow-wave propagation with the new $LaB_6$ source, we are better equipped to run high-power slow-wave experiments for future work.
Author: Mueller, Daniel
Publisher: KIT Scientific Publishing
ISBN: 3731508222
Category :
Languages : en
Pages : 214
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Author: Haruhiko Kohno
Publisher:
ISBN:
Category :
Languages : en
Pages : 191
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Book Description
Electromagnetic plasma waves in the ion cyclotron range of frequencies (ICRF) are routinely used in magnetic fusion experiments to heat plasmas and drive currents. However, many experiments have revealed that wave energy losses in the plasma edge and at the wall are significant, and detected that the acceleration of ions into the walls due to the formation of radio-frequency (RF) sheaths is one of the root causes of this problem. Since the RF-enhanced sheaths have many undesirable effects, such as impurity production and hot spot generation, a predictive numerical tool is required to quantitatively evaluate these effects with complicated boundary shapes of tokamaks taken into account. In this thesis the numerical code that solves self-consistent RF sheath-plasma interactions in the scrape-off layer for ICRF heating is developed based on a nonlinear finite element technique and is applied to various problems in the one-dimensional (1D) and two-dimensional (2D) domains corresponding to simplified models for the poloidal plane of a tokamak. The present code solves for plasma waves based on the cold plasma model subject to the sheath boundary condition, in which the most important physics that happens in the sheath is captured without using the field quantities in the sheath. Using the developed finite element code, several new properties of the RF sheath plasma interactions are discovered. First, it is found in the 1D domain that multiple roots can be present due to the resonance of the propagating slow wave and its nonlinear interaction with the sheath. Second, sheath-plasma waves are identified in a 2D slab geometry, and it is proved in conjunction with an electrostatic 2D sheath mode analysis that the sheath-plasma wave only appears in the vicinity of the sheath surface if the plasma density is greater than the lower hybrid density, and its wavelength depends on various parameters. Third, as a consequence of the self-consistent interaction between the propagating slow wave and the sheath, it is shown that the electric field distribution pattern in the plasma smoothly varies along the magnetic field lines between the conducting-wall and quasi-insulating limits. In the numerical analysis employing the 2D domain whose scale is equivalent to the Alcator C-Mod device, it is demonstrated that the calculated sheath potential can reach the order of kV, which is sufficient to yield enhanced sputtering at the wall. In addition, it is shown that the sheath potential in the close vicinity of the antenna current strap can be insensitive to the direction of the background magnetic field in the RF sheath dominated regime. Further, it is found from a series of nonlinear calculations that the sheath potential sensitively varies depending on the plasma density and electron temperature, which is consistent with the scaling derived from the Child-Langmuir law and the definition of the RF sheath potential. Lastly, a new finite element approach, which is named the finite element wave-packet method, is developed for the purpose of solving for multiscale plasma waves in the tokamak poloidal plane accurately with reasonable computational cost. This method is established by combining the advantages of the finite element and spectral methods, so that important properties in the finite element method, such as the sparsity of the global matrix and the ease in satisfying the boundary conditions, are retained. The present scheme is applied to some illustrative 1D multiscale problems, and its accuracy improvement is demonstrated through comparisons with the conventional finite element method.
Author: U. Flender
Publisher:
ISBN:
Category :
Languages : en
Pages : 48
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Author: Roger Leroy Kramer
Publisher:
ISBN:
Category : Electric resonators
Languages : en
Pages : 138
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Author: Andres Edelmiro De La Garza
Publisher:
ISBN:
Category : Ball lightning
Languages : en
Pages : 124
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Book Description
A particular interest of the principal investigator regarding a rare atmospheric phenomenon called ball lightning influenced the design and construction of a custom RF plasma chamber. However, the primary focus of this experiment is not on the ball lightning but instead on the interesting phenomenon that occurs when RF plasma is near its plasma frequency. At its plasma frequency, it behaves as a metamaterial which are said to exhibit interesting microwave properties. A Langmuir probe was needed to be built to collect data on the plasma density and find relationships between plasma density and how well it acts as a waveguide. A custom plasma chamber was then designed and built for the purpose of testing the microwave attenuation/dispersion characteristics (S-parameters) of a cylindrically symmetric RF plasma (13.56 MHz). The principal investigator was interested in studying how plasma electron density affects waveguide dispersion characteristics. Since commercially available Langmuir probes were beyond the project budget a Langmuir probe as well as the associated sensing circuitry was designed, built, tested and calibrated in-house. The necessary theoretical background needed to understand the probe and sensing circuitry is presented as well as construction of the probe and sense circuitry.
Author: Thomas J. Dolan
Publisher: Springer Science & Business Media
ISBN: 1447155564
Category : Technology & Engineering
Languages : en
Pages : 816
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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: R. A. Cairns
Publisher: CRC Press
ISBN:
Category : Art
Languages : en
Pages : 180
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Book Description
Cross and Manheimer/Lashmore-Davies sell steadily in the export market - potential for Cairns here. This book provides a concise introduction to the basic physics of radiofrequency heating. Most existing literature on the subject is at the research level, aimed at specialists in the field. It provides a survey of theoretical and experimental results with a large number of references to help the reader wishing for more detail. Provides a concise and readable account/ Basic physical principles are emphasised and more complicated mathematical ideas are explained in outline. Radiofrequency current drive is discussed in detail. "One criticsm may be lack of detail. I have deliberatly avoided including a lot of mathematical and experimental detail in order to keep the book short and to concentrate on the essential ideas. Enough references are given to lead anyone wishing more detail to the appropriate literature" - comment from the author? Export Market: Japan, USA, CERN, all important centres for fusion research Tokamaks - supercolliders for particle acceleration - particles are bombarded by extremely powerful lasers the beams of which are split many times through kms of piping - massive structurees largest currently in Japan but also important structure in US.
Author: I. H. Hutchinson
Publisher: Cambridge University Press
ISBN: 9780521675741
Category : Science
Languages : en
Pages : 460
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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.
