Thèses sur le sujet « Electron Cyclotron Waves »

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1992.
Includes bibliographical references (p. 229-235).
by Jeffrey Alan Colborn.
Ph.D.

The work presented in this PhD Thesis has been performed in the framework of research on Thermonuclear Fusion in magnetically confined plasmas of tokamak-like devices. The effort is aimed at a feasibility study of low-field side Electron Cyclotron Resonance Heating (ECRH) in overdense plasmas. The main aim of this Thesis consists in the study of the applicability of the mode conversion scheme, known as ”O-X-B Double Mode Conversion”, to the Italian tokamak FTU (Frascati Tokamak Upgrade), with the use of millimeter-waves at the 140 GHz frequency. This overdense plasma heating technique, not yet demonstrated at electron density of 2.4 ·10^20 m^−3 and consequently at such a high frequency, exploits the conversion of an ordinary polarized wave (O) into the extraordinary (X) one, which can occur only for radiation propagating in a very narrow angular range at the cutoff region, followed by a subsequent conversion to Bernstein (B) waves, which are then absorbed by the plasma. Simulations have been performed, by using a ray tracing code, to find the optimal launching conditions for the O-X coupling in FTU. The assessment of conversion efficiency was carried out first with the use of one-dimensional models, that describe the density and the magnetic field gradients of the plasma. Moreover, the effects predicted by recent bi-dimensional theoretical models available in literature have been evaluated. The inhomogeneities of a toroidal plasma are thus accounted with a more realistic description. The experimental part of the work for the Thesis can be divided into two main activities. The first one has been carried out at the laboratories of the research center ENEA in Frascati (Roma), where the tokamak FTU is operating. In this phase, experiments have been performed, aimed at the detailed study of the density profiles and gradients, which characterize the overdense plasma regimes. Proper experimental procedures have been developed, to prepare with reliability the optimal plasma ’target’. The second experimental activity consists in the contribution given for the design of a new EC millimeter-waves launcher for FTU, whose installation is scheduled for the first months of 2011. The system has been designed to reach the launching angles requested for O-X-B mode conversion, which have been defined in the present work and that are not achievable with the present launching system. The results of the predictive work confirm that the requested precision in the injection of the wave into the plasma is very high. The simulations on conversion efficiency performed with single ray tracing, show that angular deviations of ±1 degree , in either vertical or horizontal direction, with respect to the optimal injection, implies a 50% drop in the couplingThe work presented in this PhD Thesis has been performed in the framework of research on Thermonuclear Fusion in magnetically confined plasmas of tokamak-like devices. The effort is aimed at a feasibility study of low-field side Electron Cyclotron Resonance Heating (ECRH) in overdense plasmas. The main aim of this Thesis consists in the study of the applicability of the mode conversion scheme, known as ”O-X-B Double Mode Conversion”, to the Italian tokamak FTU (Frascati Tokamak Upgrade), with the use of millimeter-waves at the 140 GHz frequency. This overdense plasma heating technique, not yet demonstrated at electron density of 2.4 ·10^20 m^−3 and consequently at such a high frequency, exploits the conversion of an ordinary polarized wave (O) into the extraordinary (X) one, which can occur only for radiation propagating in a very narrow angular range at the cutoff region, followed by a subsequent conversion to Bernstein (B) waves, which are then absorbed by the plasma. Simulations have been performed, by using a ray tracing code, to find the optimal launch- ing conditions for the O-X coupling in FTU. The assessment of conversion efficiency was carried out first with the use of one-dimensional models, that describe the density and the magnetic field gradients of the plasma. Moreover, the effects predicted by recent bi-dimensional theoretical models available in literature have been evaluated. The inho- mogeneities of a toroidal plasma are thus accounted with a more realistic description. The experimental part of the work for the Thesis can be divided into two main activities. The first one has been carried out at the laboratories of the research center ENEA in Frascati (Roma), where the tokamak FTU is operating. In this phase, experiments have been performed, aimed at the detailed study of the density profiles and gradients, which characterize the overdense plasma regimes. Proper experimental procedures have been developed, to prepare with reliability the optimal plasma ’target’. The second experimen- tal activity consists in the contribution given for the design of a new EC millimeter-waves launcher for FTU, whose installation is scheduled for the first months of 2011. The system has been designed to reach the launching angles requested for O-X-B mode conversion, which have been defined in the present work and that are not achievable with the present launching system. The results of the predictive work confirm that the requested precision in the injection of the wave into the plasma is very high. The simulations on conversion efficiency performed with single ray tracing, show that angular deviations of ±1◦ , in either vertical or horizon- tal direction, with respect to the optimal injection, implies a 50% drop in the coupling efficiency. Moreover, models which account for the shape of the incident beam, show that the maximum reachable efficiency under optimal wave injection does not exceed 40-45%, depending on the model considered. Thus, the development of a control system operating in real-time and in feedback on the plasma parameters, turns out to be important, in order to perform an overdense plasma heating with acceptable efficiency. The best reproducible plasma has been experimentally defined in FTU, with 5.2 T of central magnetic field and 500 kA of plasma current. The first tests on the operation of both the new launching system and its steering control show a good agreement with the design specifications, in particular with the ones needed to perform first experiments on mode conversion in FTU.

A pulsed Electron Cyclotron Maser (E. C. M.) was developed and used to generate high power mm-waves in the W-band (75-110GHz) and the G-band (150-220GHz) frequency ranges. The relativistic electron beam (R. E. B.) was produced from a field-immersed, field-emission, cold cathode. A shaped anode cavity was designed for the optimum cavity Q, resonant frequencies, relative mode density, reflection coefficients and mode conversion in the output coupler. Two pulsed conventional field coils were used; coil#1 (maximum B-field : 9T) produced the uniform intra-cavity magnetic field and coil#2 (maximum B-field : 1T) acted as a cathode field tuning coil. The addition of the cathode tuning coil increased the useful output energy in any pulse by a factor of =400. Four diagnostics were used to determine the characteristics of the maser; 1) direct uncalibrated power monitoring, 2) calibrated frequency measurements (made using a quasi-optical diffraction grating spectrometer), 3) near field radiation pattern measurements and 4) calibrated absolute power measurements (made using a thermopile calorimeter). The following characteristics of the maser oscillation were identified: in the W-band, single mode oscillation in the TE03 mode was observed, centred at 95.2GHz, with an output power of =50kW. The cavity was crudely step-tunable with the excitation of the TE13 mode at 81.4GHz and the TE12 mode at 88.OGHz. In the G-band, multi-mode oscillation was observed at all values of the intra-cavity magnetic field. With the increased mode density at these frequencies, the maser was quasi-continuously tunable and 200GHz oscillation was observed. These results proved to be self-consistent with the device-dependent calculations used to design the system and the general E. C. M. theory developed previously.

In this thesis some theoretical problems related to the propagation and absorption of Electron Cyclotron Gaussian beams in tokamak plasmas of interest for nuclear fusion applications are investigated. To account for diffraction effects, beam propagation is analyzed in the framework of the complex eikonal method, a generalization of geometrical optics in which the phase function is assumed to be complex valued, with the non-negative imaginary part accounting for the finite width of the beam cross section. Within this framework, the solution at the dominant order in the expansion parameter is well-known, and the wave beam is modeled as a bundle of “extended rays”. The derivation of the transport equation for the field amplitude is much more complicated with respect to the standard geometrical optics one, hampering the derivation of the wave energy flux. In this work, an argument is proposed that greatly simplifies the analysis of the transport equation allowing us to derive the wave energy flux. This result, not available in the literature in the case of beam propagation in anisotropic media like magnetized plasmas, has been obtained in collaboration with O. Maj (IPP, Garching, Germany), and published on Physics of Plasmas. The effects of the finite beam width on the Electron Cyclotron resonant interaction have been described with a model that takes into account the transverse wave vector spectrum width and the non-uniformity of the equilibrium magnetic field. The model has been implemented in a modified version of the GRAY code [D. Farina, Fusion Sci. Technol. 52, 154 (2007)]. The differences between the power absorption profi les obtained using this model and the “plane wave” one are illustrated numerically in ITER conditions and are found to be small for realistic cases, thus justifying the use of the usual model for practical purposes.

Possibilities of generating plasma turbulence to provide control of space weather processes have been of particular interest in recent years. Such turbulence can be created by chemical released into a magnetized background plasma. The released plasma clouds are heavy ions which have ring velocity distribution and large free energy to drive the turbulence. An electromagnetic hybrid (fluid electrons and particle ions) model incorporating electron inertia is developed to study the generation and nonlinear evolution of this turbulence. Fourier pseudo-spectral methods are combined with finite difference methods to solve the electron momentum equations. Time integration is accomplished by a 4th-order Runge-Kutta scheme or predicator-corrector method. The numerical results show good agreement with theoretical prediction as well as provide further insights on the nonlinear turbulence evolution. Initially the turbulence lies near harmonics of the ring plasma ion cyclotron frequency and propagates nearly perpendicular to the background magnetic field as predicted by the linear theory. If the amplitude of the turbulence is sufficiently large, the quasi-electrostatic short wavelength ion cyclotron waves evolve nonlinearly into electromagnetic obliquely propagating shear Alfven waves with much longer wavelength. The results indicate that ring densities above a few percent of the background plasma density may produce wave amplitudes large enough for such an evolution to occur. The extraction of energy from the ring plasma may be in the range of 10-15% with a generally slight decrease in the magnitude as the ring density is increased from a few percent to several 10's of percent of the background plasma density. Possibilities to model the effects of nonlinear processes on energy extraction by introducing electron anomalous resistivity are also addressed. Suitability of the nonlinearly generated shear Alfven waves for applications to scattering radiation belt particles is discussed.
Master of Science

L'équation de dispersion est résolue en y injectant un modèle particulier de fonction de distribution des quantités de mouvement d'un plasma de tokamak. Les résultats obtenus sont en complet désaccord avec le modèle maxwellien, en particulier pour la disposition de l'énergie dans le cas de l'absorption, la température radiative dans le cas de l'émission et la fréquence d'intéraction onde-particule, qui est inférieure à la fréquence cyclotron électronique, ce qui présente plusieurs avantages techniques et une voie prometteuse pour le chauffage des plasmas de fusion

Theorie lineaire de l'interaction onde-particules. On montre les possibilites de diagnostic d'un moment d'ordre deux : l'emission cyclotronique electronique. On se sert d'un interferometre de michelson qui permet de selectionner a volonte le domaine spectral utile. Le rayonnement cyclotron permet de caracteriser l'action d'une onde de pompage en etudiant la partie du spectre ou le plasma est optiquement mince, on a acces a l'emissivite cyclotronique qui est directement proportionnelle a la puissance absorbee par le plasma. Ceci a permis de lever un grand nombre d'ambiguites toutes liees a un effet jusqu'alors mal compris : la saturation de l'absorption de puissance par le plasma

Thesis (M.S.)--University of Wisconsin--Madison, 1994.
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Energetic electrons undergo significant flux variations in the Earth’s outer radiation belt, where magnetospheric waves play an important role in changing the energetic electron dynamics. In particular, electromagnetic ion cyclotron (EMIC) waves are suggested to drive efficient pitch angle scattering of relativistic electrons, which results in relativistic electron precipitation into the upper atmosphere. Such precipitation provides an important source of energy input into the upper atmosphere, where precipitating electrons can affect atmospheric chemistry and ionization. However, the quantitative role of EMIC waves in energetic electron precipitation in various regions of the magnetosphere is not fully understood. This dissertation aims to answer outstanding open questions on the characteristics and quantification of EMIC-driven precipitation, such as the spatial extent and the energy range of electron precipitation. The relationship between EMIC waves and electron precipitation is evaluated by analyzing magnetic conjunction events when EMIC waves are detected in the magnetosphere by near-equatorial satellites (Van Allen Probes, GOES) and precipitating electrons are measured by Low-Earth-Orbiting satellites (POES, FIREBIRD). Quasi-linear theory is used to quantify the role of various observed magnetospheric waves (e.g., EMIC waves, plasmaspheric hiss, magnetosonic waves) in the electron precipitation. Several in-depth case analyses show that EMIC waves are the main driver of the observed relativistic electron precipitation, while other waves play a minor role. The precipitation events were clearly identified within L shell of ~7.5, favorably near the dusk and night sectors. The analysis shows that each precipitation event was localized on average spatial scales of ~0.3 L, suggesting that the resonance conditions are satisfied in a very localized region of the magnetosphere. The electron precipitation was observed at the expected relativistic (> ~MeV) energies; however, the minimum energy of efficient electron precipitation was newly found to extend down to at least ~200–300 keV. The quantitative analysis using multi-point measurements combined with theoretical calculations in this dissertation provides a more comprehensive understanding of EMIC-driven precipitation, which is a critical electron loss process in the magnetosphere. Moreover, the results are helpful to improve currently existing models of radiation belt, ring current and atmosphere dynamics, as well as theories of wave-particle interactions.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1996.
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博士
國立中央大學
太空科學研究所
102
The cyclotron maser instability (CMI) is an important mechanism for radio emissions from the sun, astrophysical shocks and planets, such as solar radio bursts, auroral kilometric radiation (AKR) and Jovian decametric radiation (DAM). The key ingredients for CMI are (a) the relativistic effect in the resonance condition and (b) a population-inversion distribution providing free energy. The relativistic resonance condition yields an ellipse or hyperbola in the particle momentum space rather than a straight line with constant parallel momentum. A population inversion requires a positive gradient along the perpendicular momentum in the distribution function. According to these characteristics, there are several kinds of distribution that can support CMI, such as loss-cone, ring-beam and horse-shoe distributions. In this thesis, we carry out a series of simulations to study CMI with an initial condition that a population of tenuous energetic electrons with a ring-beam distribution is present in a magnetized background plasma. The simulation results show that the beam component of the ring-beam distribution leads to the two-stream instability at an earlier stage, and the beam mode is coupled to the Langmuir and the whistler modes, leading to excitation of the beam-Langmuir and the beam-whistler waves, respectively. When the beam velocity is large and with a strong two-stream instability, the initial ring-beam distribution is diffused in the parallel direction rapidly, and the wave excitation associated with CMI at a later stage would become weak. On the contrary, when the beam velocity is small and the two-stream instability is weak, CMI can amplify the Z mode, the whistler mode or the X mode effectively while the O mode is relatively weak. In the cases with a pure ring distribution, we further find strong acceleration of energetic electrons by the parallel Z-mode and the parallel whistler-mode waves generated by CMI. The electron acceleration is mainly determined by the wave amplitude and phase velocity, which in turn is affected by the ratio of electron plasma to cyclotron frequencies. For the initial kinetic energy ranging from 100 to 500 keV, the peak energy of the accelerated electrons is found to reach 2~8 times of the initial kinetic energy. We then study the acceleration process via test-particle calculations in which electrons interact with one, two or four waves. The electron trajectories in the one-wave case are simple diffusion curves. In the multi-wave cases, electrons are accelerated simultaneously by counter-propagating waves and can have a higher final energy.