Дисертації з теми "Helicon wave"

Thesis (Ph. D.)--West Virginia University, 2002.
Title from document title page. Document formatted into pages; contains viii, 176 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references.

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Dissertação (Mestrado)
IPEN/D
Instituto de Pesquisas Energéticas e Nucleares - IPEN-CNEN/SP
FAPESP:05/03612-0

Thesis (Ph. D.)--West Virginia University, 2000.
Title from document title page. Document formatted into pages; contains v, 127 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 94-98).

Thesis (Ph. D.)--West Virginia University, 2005.
Title from document title page. Document formatted into pages; contains v, 152 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references.

There are investigated issues of modeling techniques improvement and investigation of improved design of helical and meander slow-wave systems with periodical inhomogeneities in the dissertation. Objects of research – models of slow-wave systems with periodical inhomogeneities and their methods of investigation. Aim of the work – improvement of sophisticated helical and meander slow-wave systems with periodical inhomogeneities and investigation of their methods of analysis, in order to reveal general characteristics of inhomogeneous slow-wave systems and capabilities of using computer-aided design.
Disertacijoje sprendžiama spiralinių ir meandrinių lėtinimo sistemų su periodiniais netolygumais modeliavimo metodų tobulinimo ir jų tobulesnių konstrukcijų paieškos problema. Pagrindiniai tyrimo objektai – nevienalytės periodinės lėtinimo sistemos ir jų modeliai bei analizės metodai. Darbo tikslas yra tobulinti sudėtingas turinčias periodinių netolygumų spiralines ir meandrines lėtinimo sistemas ir ištirti jų analizės metodus, siekiant atskleisti nevienalyčių lėtinimo sistemų savybes ir jų automatizuoto projektavimo galimybes.

O objetivo do trabalho é a investigação da excitação de ondas no plasma com o uso de uma antena externa e fazer uma análise das ressonâncias de Alfvén encontradas. O sistema de antenas de Alfvén no tokamak TCABR foi desenhado para aquecimento do plasma por meio de ressonâncias. Al em do aquecimento, é possível usar a detecção de ondas excitadas com o uso da antena para objetivos de diagnóstico do plasma, encontrando o valor do perfil de segurança e massa efetiva dos íons. Por causa de uma falha nos diodos do campo toroidal usamos o regime de disparos de limpeza, com campo magnético toroidal mais fraco que de disparos tópicos do TCABR, para os testes do método de excitação e identificação de ressonâncias no plasma. Com o uso do circuito demodulador foram medidas ondas de helicon excitadas com a antena de Alfvén no plasma de limpeza usando as sondas magnéticas e de Langmuir. Com simulação foi possível idênticas as ondas medidas. Há disponível um gerador de frequência variável que foi utilizado junto desse experimento. Ambos os equipamentos se encontram preparados para uso, sendo a próxima etapa usar o plasma tópico de disparo do TCABR, que tem maior densidade que o plasma de limpeza. As medidas realizadas foram um teste para o circuito demodulador e gerador de frequência variável, que teve seu comportamento comparado com os dados de um osciloscópio de alta frequência de amostragem. Os equipamentos do TCABR usados nos experimentos, as antenas e sondas magnéticas, um gerador de baixa potência com frequência variável, um circuito demodulador, sonda de Langmuir e o reflectômetro, que tem alta taxa de amostragem (200MHz) e varredura de frequência na banda de 18 40GHz. São todos descritos na dissertação. Para modelagem das ressonâncias de Alfvén foi feito o cálculo do tensor dielétrico do plasma para o modelo cinético e para o limite magnetohidrodinâmico. Por meio de simulação computacional e cálculos considerando plasma como um fluido de 2 componentes, no caso prótons e elétrons, é possível determinar alguns tipos de onda que podem ser excitadas no plasma e sua relação de dispersão, foram calculadas a onda magnetossônica rápida e a onda global de Alfvén. Determinamos radialmente a posição dos campos eletromagnéticos no plasma. Usando o reactômetro foram medidas as ressonâncias das ondas de Alfvén na borda do plasma induzidas pelas antenas, com o plasma tópico do tokamak, com densidade mais alta e o gerador de alta potência com frequência fixa. O método para achar as ressonâncias nos dados do reflectômetro foi com o uso de sidebands que aparecem em torno da frequência da ressonância não sinal do reflectômetro, que é a frequência do gerador. As sidebands foram analisadas com um espectrograma dos dados. As ondas excitadas na borda do plasma puderam ser identificadas também nas simulações. Os resultados da análise mostram que foi possível medir as ondas no plasma que foram excitadas com o uso das antenas e tanto o circuito demodulador com o uso de sondas magnéticas como o reflectômetro são adequados para se achar ressonâncias no plasma.
The objective of this work is to investigate the excitation of waves in a plasma using an antenna and to analyse the Alfvén resonances found. The Alfvén antenna heating system of the TCABR tokamak was designed to heat the plasma due to resonances. As the diodes of the toroidal field had burned down we used cleaning discharges, with low toroidal magnetic field, to test the excitation method and the identification of plasma resonances. With the demodulator circuit we measured helicon waves excited with the Alfv en antenna in the cleaning plasma using Langmuir and magnetic probes. With computational simulation we found the measured waves. A generator of variable frequency was used in this experiment. Both equipments are prepared for future experiments with the typical plasma of the TCABR, which has higher density than the cleaning plasma. This work was aimed to test to the demodulator circuit and the variable frequency generator, the data obtained were compared to that of a high sampling frequency oscilloscope. It is presented the description of the TCABR equipments used, antenna, magnetic probe, variable frequency generator of low power, demodulator circuit, Langmuir probe and a reflectometer which has a high sampling frequency (200MHZ) and frequency scanning in the range 18 40GHz, and was built in Portugal. In order to have a model of Alfv en resonances we calculated the plasma dieletric tensor both in the kinetic and magnetohydrodynamic limits. With computational simulation and using a two uid model, protons and electrons, it is possible to find some of the excited waves in the plasma and its dispersion relation, we calculated the fast magnetosonic wave and the global Alfvén wave. We found the radial position of the electromagnetic fields in the plasma. With the re ectometer we measured resonances of Alfvén waves induced by the antenna at the plasma border in a typical TCABR tokamak plasma discharge, with higher density and a high power fixed frequency generator. We used sidebands as a method to find out the resonances in the reflectometer data. These sidebands are localized around the resonance frequency, which is the Alfvén wave generator frequency. The sidebands were analysed with spectrograms of the data. The waves excited at the plasma border were also found in the simulation. The analysis results show that we could detect the plasma waves excited with the antennas. The demodulator circuit along with magnetic probes and the reflectometer can be used to find plasma resonances.

This work examines problems in the statics and traveling wave propagation on uniform elastic rods with constant curvature and torsion, i.e. a straight rod and a helical rod. The first set of problems concerns planar traveling loop-like waves on intrinsically straight rods. It is shown that loops with compact support can exist on homogeneous rods with a nonlinear constitutive relation, where the strain-energy density contains a quartic term. Next, the effect of heterogeneity in the material properties on the shape of the loop is examined using a homogenization method. The second set of problems deals with a system consisting of a helical spring with a force and a torque applied along the helix axis. First, an overview is presented of problems of finding the stresses given the strains, or vice-versa, assuming that the elastic parameters of the spring are known. Then, the inverse problem is examined, where both stresses and strains are measured, and optimal elastic parameters within the linear consitutive model are sought. Various forms of measured strains are considered. Finally, the special problem with zero axial torque is considered, and criteria when the spring overwinds with a tensile axial force applied are established.

Non-Destructive Testing of industrial components carries vital importance, both financially and safety-wise. Among all Non-Destructive techniques, Long Range Ultrasonic Testing utilizing the guided wave phenomena is a young technology proven to be commercially valid. Owing to its well-documented analytical models, Ultrasonic Guided Waves has been successfully applied to cylindrical and plate-like structures. Its applications to complex structures such as multi-wire cables are fairly immature, mainly due to the high complexity of wave propagation. Research performed by the author approaches the long range inspection of overhead transmission line cables using ultrasonic guided waves. Existing studies focusing on guided wave application on power cables are extremely limited in inspection range, which dramatically degrades its chances of commercialization. This thesis consists of three main chapters, all of which approaches different problems associated with the inspection of power cables. In the first chapter, a thorough analysis of wave propagation in ACSR (most widely used power cable) cables is conducted. It is shown that high frequency guided waves, by concentrating the energy on the surface layers, can travel much further in the form of fundamental longitudinal wave mode, than previous studies have shown. Defect detection studies proved the system’s capability of detecting defects which introduce either increase or decrease in cross sectional area of the cable. Results of the chapter indicate the detectability of defects as small as 4.5% of the cross sectional area through a 26.5 meter long cable without any post-processing. In the second chapter, several algorithms are proposed to increase the inspection range and signal quality. Well-documented wavelet-denoising algorithm is optimized for power cables and up to 24% signal-to-noise ratio improvement is achieved. By introducing an attenuation correction framework, a theoretical inspection range of 75 meters is presented. A new framework combining dispersion compensation and attenuation correction is proposed and verified, which shows an inspection range of 130 meters and SNR improvement up to 8 dBs. Last chapter addresses the accurate localization of structural defects. Having proven the optimum excitation and related wave propagation in ACSR cables, a system having a more complex wave propagation characteristics is studied. A new algorithm combining pulse compression using Maximal Length Sequences and dispersion compensation is applied to multi-modal signals obtained from a solid aluminum rod. The algorithm proved to be able to improve signal quality and extract an accurate location for defects. Maximal Length Sequences are compared to chirp signals in terms of SNR improvement and localization, which produced favourable results for MLS in terms of localization and for chirp in terms of SNR improvement.

To investigate the device which is capable of generate and amplify a coherent electromagnetic radiation in the millimeter and submillimeter wavelength ranges is extremely important today. In the research under discussion the model of a plasma-beam superheterodyne free electron laser (SFEL) is the following. Plasma is located in the longitudinal focusing magnetic field with strength H0. Relativistic electron beam is injected into this plasma environment at an angle α with respect to the magnetic field strength vector. We chose a circularly polarized intense low-frequency electromagnetic wave as a pump. This wave propagates along the guiding magnetic field and in the opposite direction to the electron beam. Also we feed a weak high-frequency circularly polarized electromagnetic wave (signal wave) into the system. The direction of the wave signal may be different, so the wave number can either positive or negative. The parametric resonance between the signal wave and the pump wave results in excitation of a space-charge wave (SCW).

Ma thèse est consacrée aux nouveaux phénomènes nano-optiques et aux dispositifs basés sur l'interaction spin-orbite de la lumière (SOI). Tout d'abord, il a été démontré un SOI uniquement piloté par le champ magnétique de la lumière permettant de diriger avec précision les ondes de surface de Bloch, offrant ainsi une nouvelle manifestation du champ magnétique optique. Ensuite, nous avons proposé et démontré le concept de nano-antenne plasmonique hélicoïdale à ondes progressives (TW-HPA), c’est-à-dire un fil hélicoïdal en or étroit alimenté optiquement par une nano-antenne dipolaire dans une configuration « end-firing ». Une telle nano-antenne a été démontrée comme la première optique de polarisation sublongueur d’onde. L’agencement de TW-HPAs à l’échelle de quelques microns a permis de convertir « à la carte » un faisceau polarisé linéairement en une distribution de faisceaux directifs présentant des polarisations différentes définies de façon déterministe par la géométrie et les dimensions des nano-antennes. Par le biais d’un couplage en champ proche de quatre nano-antennes à hélicités opposées, nous avons obtenus une optique sublongueur d’onde permettant un degré de liberté dans le contrôle de la polarisation qui est interdit avec les composants et méthodes classiques basées sur l’exploitation de matériaux biréfringents ou dichroïques, ou de métamatériaux imitant ces propriétés
My thesis is devoted to novel nano-optical phenomena and devices based on spin-orbit interaction (SOI) of light. First, magnetic spin-locking, i.e., an SOI solely driven by the magnetic field of light, is demonstrated with Bloch surface waves. It provides a new manifestation of the magnetic light field. Then, we propose and demonstrate the concept of traveling-wave plasmonic helical antenna (TW-HPA), consisting of a narrow helical gold-coated wire non-radiatively fed with a dipolar nano-antenna. By swirling surface plasmons, the TW-HPA combines subwavelength illumination and polarization transformation. The TW-HPA is demonstrated to radiate on the subwavelength scale almost perfectly circularly polarized optical waves upon illumination with linearly polarized light. With this subwavelength plasmonic antenna, we developed strongly integrated arrays of point-light emissions of opposite handedness and tunable intensities. Finally, by coupling two couples of TW-HPAs of opposite handedness, we obtained new polarization properties so far unattainable

Ce travail de thèse porte sur les propriétés de stabilité des tourbillons hélicoïdaux, structures que l'on retrouve notamment dans le sillage des rotors d'hélicoptères et d'éoliennes.Dans une première partie, le développement spatio-temporel de l'instabilité d'appariement est caractérisé à l'aide d'un code numérique pseudo-spectral pour une allée infinie d'anneaux tourbillonnaires. On montre que ce modèle axisymétrique d'écoulement est en effet une bonne approximation du système hélicoïdal dans la limite des grands rayons et petits pas d’hélice. Dans ces conditions, et en utilisant un adimensionnement judicieux, on obtient également que le résultat théorique pour le taux de croissance spatio-temporel obtenu pour une double allée de tourbillons ponctuels s’avère être une bonne prédiction pour le cas hélicoïdal.Dans une seconde partie, on décrit comment un ou plusieurs tourbillons hélicoïdaux ont pu être générés de façon très peu perturbée à l’aide de modèles réduits de rotors dans le canal hydrodynamique du laboratoire. Grâce à l’introduction de perturbations d’amplitudes et de fréquences soigneusement contrôlées, le taux de croissance de l’instabilité d’appariement a pu être mesuré et comparé aux résultats théoriques. L’évolution non linéaire de ces perturbations ainsi que d’autres modes instables, à plus petites longueurs d’onde, ont également pu être observés expérimentalement pour la première fois.Enfin, ces résultats ont été appliqués au cas des rotors d’hélicoptères pour la prédiction du régime de Vortex Ring State (VRS) et à la transition vers la turbulence du sillage des éoliennes
This thesis is devoted to the stability properties of helical vortices, which are of interest for applications such as helicopter and wind turbine wakes.In a first part, the spatio-temporal development of the pairing instability is characterised for an infinite array of vortex rings, using a pseudo-spectral numerical code. We show that this axisymmetric flow model is indeed a good approximation of the helical system in the limit of large helix radius and small pitch. Under these assumptions, and by using appropriate dimensionless variables, we also show that the theoretical result concerning the spatio-temporal growth rate for a double row of point vortices represents a good prediction for the helical case.In a second part, we describe how one or several helical vortices were generated in a carefully controlled way using small-scale rotor models in the water channel of the laboratory. Introducing perturbations with well-defined amplitudes and frequencies, the growth rate of the pairing instability could be measured experimentally and compared to theoretical predictions. The non-linear evolution of these perturbations, as well as other unstable modes of smaller wavelengths, were also observed experimentally for the first time.Finally, these results were applied to helicopter wakes for the prediction of the Vortex Ring State (VRS) regime and to the transition to turbulence in wind turbine wakes

Дисертаційна робота присвячена теоретичному дослідженню динаміки електромагнітних хвиль у мультигармонічних двопотокових супергетеродинних лазерах на вільних електронах Н-убітронного типу з гвинтовими електронними пучками з урахуванням множинних трихвильових параметричних взаємодій різного типу. Запропоновано використовувати гвинтові електронні пучки у мультигармонічних двопотокових супергетеродинних лазерах на вільних електронах (ДСЛВЕ), призначених для одержання потужного електромагнітного сигналу з широким частотним спектром. З’ясовано, що такі пристрої мають менші поздовжні габарити та здатні формувати мультигармонічні електромагнітні хвилі з більш широким частотним спектром порівняно з ДСЛВЕ, які використовують прямолінійні електронні пучки. У кубічному нелінійному наближенні одержано самоузгоджену систему диференціальних рівнянь для амплітуд гармонік електромагнітних хвиль у мультигармонічних двопотокових супергетеродинних лазерах на вільних електронах Н-убітронного типу з гвинтовими релятивістськими електронними пучками (РЕП). Виявлені та враховані трихвильові резонансні взаємодії різного типу, зокрема й пов’язані з обертальним рухом електронів у фокусувальному магнітному полі. Показано, що множинні взаємодії хвиль у таких системах є визначальними для формування електромагнітного сигналу з широким частотним спектром. З’ясовано, що завдяки використанню гвинтових двопотокових РЕП у ДСЛВЕ вдається збільшити інкременти зростання хвиль за умови збільшення кута вльоту електронного пучка відносно фокусувального магнітного поля. Показано, що у гвинтових двопотокових РЕП критична частота двопотокової нестійкості також зростає зі збільшенням кута вльоту. Це дозволяє збільшити ширину частотного спектра електромагнітних хвиль, що формується мультигармонічними ДСЛВЕ. Завдяки використанню гвинтових двопотокових РЕП у ДСЛВЕ вдається сформувати мультигармонічну електромагнітну хвилю з більш широким частотним спектром та на менших відстанях порівняно з ДСЛВЕ, що використовують прямолінійні РЕП. Продемонстровано, що в разі використання мультигармонічного вхідного сигналу з вузьким частотним спектром у мультигармонічних ДСЛВЕ-підсилювачах з гвинтовими РЕП формування потужної мультигармонічної електромагнітної хвилі відбувається на довжинах, у два і більше разів менших порівняно із системами, на вхід яких подається монохроматичний вхідний сигнал.
Диссертация посвящена теоретическому исследованию динамики электромагнитных волн в мультигармонических двухпотоковых супергетеродинных лазерах на свободных электронах Н-убитронного типа с винтовыми электронными пучками с учетом множественных трёхволновых параметрических взаимодействий разного типа. Предложено использовать винтовые электронные пучки в мультигармонических двухпотоковых супергетеродинных лазерах на свободных электронах (ДСЛСЭ), предназначенных для получения мощного электромагнитного сигнала с широким частотным спектром. Установлено, что такие устройства имеют меньшие продольные габариты и способны формировать мультигармонические электромагнитные волны с более широким частотным спектром по сравнению с ДСЛСЭ, использующими прямолинейные электронные пучки. В кубическом нелинейном приближении получена самосогласованная система дифференциальных уравнений для амплитуд гармоник электромагнитных волн в мультигармонических двухпотоковых супергетеродинных лазерах на свободных электронах Н-убитронного типа с винтовыми релятивистскими электронными пучками (РЭП). Выявлены и учтены трёхволновые резонансные взаимодействия различного типа, в том числе и связанные с вращательным движением электронов в фокусирующем магнитном поле. Показано, что множественные взаимодействия волн в таких системах являются определяющими для формирования электромагнитного сигнала с широким частотным спектром. Выяснено, что благодаря использованию винтовых двухпотоковых РЭП в ДСЛСЭ удается увеличить инкременты нарастания волн при условии увеличения угла влета электронного пучка относительно фокусирующего магнитного поля. Показано, что в винтовых двухпотоковых РЭП критическая частота двухпотоковой неустойчивости также вырастает с увеличением угла влета. Это позволяет увеличить ширину частотного спектра электромагнитных волн, формирующегося мультигармоническими ДСЛСЭ. Благодаря использованию винтовых двухпотоковых РЭП в ДСЛСЭ удается сформировать мультигармоническую электромагнитную волну с более широким частотным спектром и на меньших расстояниях по сравнению с ДСЛСЭ, использующими прямолинейные РЭП. Продемонстрировано, что в случае использования мультигармонического входного сигнала с узким частотным спектром в мультигармонических ДСЛСЭ-усилителях с винтовыми РЭП формирование мощной мультигармонической электромагнитной волны происходит на длинах, в два и более раз меньших по сравнению с системами, на вход которых подается монохроматический входной сигнал.
The thesis is devoted to the theoretical study of the electromagnetic waves dynamics in multiharmonic two-stream superheterodyne free-electron lasers of the H-ubitron type with helical electron beams, considering multiple three-wave parametric interactions of different types. It has been proposed to use helical electron beams in multiharmonic two-stream superheterodyne free-electron lasers (TSFELs), which are designed to produce a powerful electromagnetic signal with a wide frequency spectrum. It has been found that such devices have smaller longitudinal dimensions and are capable of forming multiharmonic electromagnetic waves with wider frequency spectrum compared to TSFELs that use straight electron beams. A self-consistent system of differential equations has been obtained in the cubic nonlinear approximation for the harmonics amplitudes of electromagnetic waves in multiharmonic two-stream superheterodyne free-electron lasers of H-ubitron type with helical relativistic electron beams (REBs). Three-wave resonant interactions of various types, including those associated with the rotational motion of electrons in a focusing magnetic field, have been identified and considered. It has been shown that multiple interactions of waves in such systems are decisive for the formation of an electromagnetic signal with wide frequency spectrum. It has been found that due to the use of two-stream helical REBs in TSFEL, it is possible to increase the wave growth rates under the condition of increasing the electron beam input angle relative to the focusing magnetic field. It has been also shown that in helical two-stream REBs, the critical frequency of two-stream instability also increases with an angle. This allows to increase the frequency spectrum width of electromagnetic waves formed by multiharmonic TSFEL. Using two-stream helical REB in TSFEL, it is possible to form a multiharmonic electromagnetic wave with wide frequency spectrum and at smaller distances compared to TSFELs that use straight REBs. It has been demonstrated that in the case of multiharmonic input signal with a narrow frequency spectrum in multiharmonic TSFEL amplifiers with helical REB, the formation of powerful multiharmonic electromagnetic wave occurs at lengths two and more times smaller than the systems using monochromatic input signal.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1998.
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This thesis details an experimental, theoretical, and numerical investigation into helicon wave propagation in the presence of low diverging magnetic fields (< 5 m T). Experiments are performed in the Piglet helicon reactor, which consists of a Pyrex source tube connected to a larger aluminium diffusion chamber. A double-saddle field antenna (operated at 13.56 MHz), is used to create both the plasma and launch helicon waves, while the diverging magnetic field is produced by a number of solenoids that surround both the antenna and source tube. Experiments are conducted with argon gas in the pressure range 0.04-0.4 Pa, and for rf input powers below 400 W. As the magnetic field is increased (using single solenoid), the plasma density is observed to increase rapidly over a narrow range of magnetic values (between about 1 mT < Bo < 5 mT), where a distinct density peak is formed. The density at the maximum of the peak (>1017 m-3 ) is more than an order of magnitude larger than that before or after, and is associated with a corresponding peak in the measured antenna resistance; showing that a larger percentage of the input power is deposited within the plasma. In the presence of the diverging magnetic field an ion beam is observed to form simultaneously with the low –field helicon mode. The ion beam, which is present for argon gas pressures below around 0.3 Pa, is produced by upstream ions accelerated by a decreasing plasma potential set up by the spatially decaying plasma density profile. An analytical model, based on simple flux conservation, is developed to describe the general features and behaviour of the observed ion energy distribution functions (IEDFs), which are found to be strong functions of the plasma potential profile and neutral gas pressure. During the low-field mode, m = 1 helicon waves was observed with B-dot probes in the source region of Piglet. With just a single solenoid producing the magnetic field, waves are prevented reaching the downstream region (that is, the waves appear “trapped”), but slight modifications to the magnetic field geometry allows the axial distance over which waves can propagate to be controlled. Critical to the modification of the wave propagation behaviour is the magnetic field strength ( and geometry) near the exit of the plasma source region, which gives electron cyclotron frequencies close to the wave frequency of 13.56MHz. By solving the wave equation using cold plasma approximation, and separately by making use of a 1D electromagnetic particle-in-cell (PIC) simulation, wave propagation and absorption are investigated in the presence of a low diverging magnetic field. The numerical results from both studies are in good qualitative agreement with the experimental measurements, and provide strong evidence to suggest that the observed wave “trapping” is due to electron cyclotron damping of helicon waves in the spatially decaying magnetic field; an electron heating process not usually dominant in conventional helicon discharges, thus opening up additional possibilities for the use and optimization of helicon systems in processing and propulsion applications.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1996.
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Both space and laboratory plasmas can be associated with static magnetic field, and the field geometry varies from uniform to non-uniform. This thesis investigates the impact of magnetic geometry on wave modes in cylindrical plasmas. The cylindrical configuration is chosen so as to explore this impact in a tractable but experimentally realisable configuration. Three magnetic geometries are considered: uniform, focused and rippled. For a uniform magnetic field, wave oscillations in a plasma cylinder with axial flow and azimuthal rotation are modelled through a two-fluid flowing plasma model. The model provides a qualitatively consistent description of the plasma configuration on a Radio Frequency (RF) generated linear magnetised plasma (WOMBAT, Waves On Magnetised Beams And Turbulence [Boswell and Porteous, Appl. Phys. Lett. 50, 1130 (1987)]), and yields agreement between measured and predicted dependences of the wave oscillation frequency with axial field strength. The radial profile of the density perturbation predicted by this model is consistent with the data. Parameter scans show that the dispersion curve is sensitive to the axial field strength and the electron temperature, and the dependence of the oscillation frequency with electron temperature matches the experiment. These results consolidate earlier claims that the density and floating potential oscillations are a resistive drift mode, driven! by the density gradient. This, to our knowledge, is the first detailed physics modelling of plasma flows in the diffusion region away from the RF source. For a focused magnetic field, wave propagations in a pinched plasma (MAGPIE, MAGnetised Plasma Interaction Experiment [Blackwell et al., Plasma Sources Sci. Technol. 21, 055033 (2012)]) are modelled through an ElectroMagnetic Solver (EMS) based on Maxwell's equations and a cold plasma dielectric tensor. [Chen et. al., Phys. Plasmas 13, 123507 (2006)] The solver produces axial and radial profiles of wave magnitude and phase that are consistent with measurements, for an enhancement factor of 9.5 to the electron-ion Coulomb collision frequency and a 12% reduction in the antenna radius. It is found that helicon waves have weaker attenuation away from the antenna in a focused field compared to a uniform field. This may be consistent with observations of increased ionisation efficiency and plasma production in a non-uniform field. The relationship between plasma density, static magnetic field strength and axial wavelength agrees well with a simple theory developed previously. More! over, the wave amplitude is lowered and the power deposited into the core plasma decreases as the enhancement factor to the electron-ion Coulomb collision frequency increases, possibly due to the stronger edge heating for higher collision frequencies. For a rippled magnetic field, the spectra of radially localised helicon (RLH) waves [Breizman and Arefiev, Phys. Rev. Lett. 84, 3863 (2000)] and shear Alfvén waves (SAW) in a cold plasma cylinder are investigated. A gap-mode analysis of the RLH waves is first derived and then generalised to ion cyclotron range of frequencies for SAW. The EMS is employed to model the spectral gap and gap eigenmode. For both the RLH waves and SAW, it is demonstrated that the computed gap frequency and gap width agree well with the theoretical analysis, and a discrete eigenmode is formed inside the gap by introducing a defect to the system's periodicity. The axial wavelength of the gap eigenmode is close to twice the system's periodicity, which is consistent with Bragg's law, and the decay length agrees well with the analytical estimate. Experimental realisation of a gap eigenmode on a linear plasma device such as the LArge Plasma Device (LAPD) [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] may be possible by introducing a symmetry-breaking defect to the system's periodicity. Such basic science studies could provide the possibility to accelerate the science of gap mode formation and mode drive in toroidal fusion plasmas, where gap modes are introduced by symmetry-breaking due to toroidicity, plasma ellipticity and higher order shaping effects. These studies suggest suppressing drift waves in a uniformly magnetised plasma by increasing the field strength, enhancing the efficiency of helicon wave production of plasma by using a focused magnetic field, and forming a gap eigenmode on a linear plasma device by introducing a local defect to the system's periodicity, which is useful for understanding the gap-mode formation and interaction with energetic particles in fusion plasmas.

In this thesis, we investigate wave propagation and plasma equilibrium in MAGPIE, a helicon based linear plasma device constructed at the Australian National University, to study plasma-material interactions under divertor-relevant plasma conditions. We show that MAGPIE is capable of producing low temperature (1–8 eV) high density hydrogen plasma (2–3×10^19 m-3) with 20 kW of RF power when the confining magnetic field is converging. The original research herein described comprises: (1) Characterization of hydrogen plasma in MAGPIE, (2) Analysis of the RF compensation of double Langmuir probes, (3) Excitation, propagation and damping of helicon waves in uniform and non-uniform magnetic fields and (4) Steady-state force balance and equilibrium profiles in MAGPIE. We develop an analytical model of the physics of floating probes to describe and quantify the RF compensation of the DLP technique. Experimental validation for the model is provided. We show that (1) whenever finite sheath effects are important, overestimation of the ion density is proportional to the level of RF rectification and suggest that (2) electron temperature measurements are weakly affected. We develop a uniform plasma full wave code to describe wave propagation in MAGPIE. We show that under typical MAGPIE operating conditions, the helical antenna is not optimized to couple waves in the plasma; instead, the antenna’s azimuthal current rings excites helicon waves which propagate approximately along the whistler wave ray direction, constructively interfere on-axis and lead to the formation of an axial interference pattern. We show that helicon wave attenuation can be explained entirely through electron-ion and electron-neutral collisions. Results from a two-dimensional full wave code reveal that RF power deposition is axially non-uniform with both edge and on-axis components associated with the TG and helicon wave respectively. Finally, force balance analysis in MAGPIE using a two-fluid “Braginskii” type formalism shows that the electron fluid exists in a state of dynamic (flowing) equilibrium between the electric, pressure and thermal forces. The pressure gradient, driven by the non-uniform RF heating, accelerates the plasma into the target region to velocities close to the ion sound speed. From the measured axial plasma flux we find that the plasma column in MAGPIE can be divided into an ionizing and a recombining region. For the conditions investigated, a large fraction of the plasma created in the ionizing region is lost in the recombining region and only a small fraction reaches the end of the device. The equilibrium plasma density along the length of MAGPIE can be quantitatively explained using a 1D transport calculation which includes volumetric particle sources and magnetic compression. We show that the plasma is transported, by the electron pressure gradient, from under the antenna (0.5×10^19 m-3) into the target region where it reaches maximum density (2-3×10^19 m-3). Using the results herein presented, this thesis explores the relationship between the RF power deposition in MAGPIE, parallel plasma transport and the production of high density plasma in the target region.

The goal of this dissertation is to provide experimental insight into the mechanism behind the efficient power absorption of helicon plasmas. This work presents evidence which is consistent with the Radially Localized Helicon (RLH) theory put forth by Breizman and Arefiev. Helicon discharges produce peaked density profiles with radial density gradients creating a potential well that sets up the RLH waves, which we believe is the major power absorption mechanism in the plasma. The experimental data presented in this dissertation must be taken in totality along with parallel efforts in theory and computer simulation. We show photographic evidence along with Langmuir probe measurements of the axial density that shows an asymmetric, right-hand circularly polarized wave being launched in a direction consistent with RLH theory. Additionally, we are able to show, through Langmuir probe measurements, that significant radial density gradients exist in the plasma which is required by the RLH dispersion but contrary to the uniform density assumption of current theory. Furthermore, using the two-dimensional density profile obtained from experiment, we are able to use that data as input into a model which confirms key features of the power absorption in terms of location and magnitude. The time-varying magnetic field is measured and analyzed against the RLH dispersion relation. Using a Fourier decomposition technique, the analysis indicates the proper scaling of the wavenumbers with the RLH dispersion. Finally, using the experimental density as input to a computer model, simulations show very good agreement with the amplitude and phase of the experimentally measured RF magnetic fields.
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We developed a self-consistent model of helicon discharges, motivated by a number of applications. One example is a plasma-based space propulsion system that employs a helicon discharge as its plasma source. Our study of helicon discharges involves two steps. An electro-magnetic wave solver is first developed to study wave phenomena and power deposition. In this work, we model a resonant response of the discharge observed in a recent experiment. The radially localized helicon (RLH) wave is identified as the primary mechanism of rf-power deposition into the plasma. The second step is to take into account electron heat transfer and ion transport so that a self-consistent simulation can be performed. As a case study of validating the model, we simulated one of Boswell’s early experiment in which a jump of plasma density in a scan of external magnetic field is observed. Calculation shows that a classical heat transport is unable to sustain the plasma density profile observed in the experiment. Solutions comparable to the experiment are obtained only when extra heat conductivity is used. The density profiles and excited wave-lengths are in good agreement with the experiment. Especially, the dual-stable solution of the simulation supports the observed plasma density jump.
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Thesis (M.S.)--University of Wisconsin--Madison, 2002.
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The interaction between the solar wind and a rotating planet causes the field lines in the planetary magnetotail to twist into a helix. Using a simplified magnetotail model, we examine hydromagnetic waves propagating down the magnetopause for such a field configuration and derive the dispersion relation of the waves. It turns out that only under certain special circumstances can the hydromagnetic waves be stable. In a thin magnetopause boundary layer, the helical wave is found to be always stable and its wave frequency depends weakly on the plasma and the field within the layer. The current system of the boundary layer is found to be modulated by the wave and the modulation is proportional to the velocity perturbation of the plasma. The wave influence on the spiral angle is examined briefly for some special cases for which we find the variation of the angle increases monotonically with increasing radial distance.