Several Langmuir probe voltage sweeps by a model of the ESA spacecraft Rosetta was simulated in a plasma with solar wind parameters using the ESA open source software SPIS 5. The simulations were carried out to in- vestigate the features of the spacecraft’s environment in the solar wind and the effect of photoemission from the spacecraft surface on the measurements made by the Langmuir probes on board Rosetta. We report a best fit to an existing probe sweep result in the solar wind near the Earth at 1 AU from 9 Nov 2009 for a 4 million particle simulation in SPIS of an 8 V positively charged spacecraft with the following parameters: Tph = 2 eV, Te = 12 eV, Ti = 5 eV, ne = 5 cm−3. We also report that the spacecraft is shielding the Langmuir probes on Rosetta from plasma electrons, and particularly low energy electrons. In one instance, this blocking is shown to lead to an over- estimation of solar wind electron temperature by 12% and underestimate the plasma density by 24% by the Langmuir Probe for a +10 V charged spacecraft in ne= 5 cm−3, Te = 12 eV solar wind. Two models used in lit- erature on photoemission, one for isotropical emission from a plane and the other for radial emission from a point, was used and compared. We report a clear preference to the approximation of a Maxwellian energy distribution of photoelectrons emitted radially from a point source model with our sim- ulation result on the Langmuir Probe aboard Rosetta. We also report the solar aspect angle dependence on the plasma potential and plasma density result, which are in overall agreement with Rosetta measurements from the second Earth fly-by.

Polymer and organic electronic memory devices offer the potential for cheap, simple memories that could compete across the whole spectrum of digital memories, from low cost, low performance applications, up to universal memories capable of replacing all current market leading technologies, such as hard disc drives, random access memories and Flash memories. Polymer memory devices (PMDs) are simple, two terminal metal-insulator-metal (MIM) bistable devices that can exist in two distinct conductivity states, with each state being induced by applying different voltages across the device terminals. Currently there are many unknowns and much ambiguity concerning the working mechanisms behind many of these PMDs, which is impeding their development. This research explores some of these many unanswered questions and presents new experimental data concerning their operation. One prevalent theory for the conductivity change is based on charging and charge trapping of nanoparticles and other species contained in the PMD. The work in this research experimentally shows that gold nanoparticle charging is possible in these devices and in certain cases offers an explanation of the working mechanism. However, experimental evidence presented in this research, shows that in many reported devices the switching mechanism is more likely to be related to electrode effects, or a breakdown mechanism in the polymer layer. Gold nanoparticle charging via electrostatic force microscopy (EFM) was demonstrated, using a novel device structure involving depositing gold nanoparticles between lateral electrodes. This allowed the gold nanoparticles themselves to be imaged, rather than the nanoparticle loaded insulating films, which have previously been investigated. This method offers the advantages of being able to see the charging effects of nanoparticles without any influence from the insulating matrix and also allows charging voltages to be applied via the electrodes, permitting EFM images to capture the charging information in near real-time. Device characteristics of gold nanoparticle based PMDs are presented, and assessed for use under different scenarios. Configurations of memory devices based on metal-insulator-semiconductor (MIS) structures have also been demonstrated. Simple interface circuitry is presented which is capable of performing read, write and erase functions to multiple memory cells on a substrate. Electrical properties of polystyrene thin films in the nanometre thickness range are reported for the first time, with insulator trapped charges found to be present in comparable levels to those in silicon dioxide insulating films. The dielectric breakdown strength of the films was found to be significantly higher than bulk material testing would suggest, with a maximum dielectric strength of 4.7 MV•cm-1 found, compared with the manufacturers bulk value of 0.2 – 0.8 MV•cm-1. Conduction mechanisms in polystyrene were investigated with the dominant conduction mechanism found to be Schottky emission.

Durant les dernières décennies, la technique Langmuir-Blodgett (LB) s’est beaucoup développée dans l’approche « bottom-up » pour la création de couches ultra minces nanostructurées. Des patrons constitués de stries parallèles d’environ 100 à 200 nm de largeur ont été générés avec la technique de déposition LB de monocouches mixtes de 1,2-dilauroyl-sn-glycéro-3-phosphatidylcholine (DLPC) et de 1,2-dipalmitoyl-sn-glycéro-3-phosphatidylcholine (DPPC) sur des substrats de silicium et de mica. Afin d’amplifier la fonctionnalité de ces patrons, la 1-palmitoyl-2-(16-(S-methyldithio)hexadécanoyl)-sn-glycéro-3-phosphatidylcholine (DSDPPC) et la 1-lauroyl-2-(12-(S-methyldithio)dodédecanoyl)-sn-glycéro-3-phosphatidylcholine (DSDLPC) ont été employées pour la préparation de monocouches chimiquement hétérogènes. Ces analogues de phospholipide possèdent un groupement fonctionnel méthyldisulfide qui est attaché à la fin de l’une des chaînes alkyles. Une étude exhaustive sur la structure de la phase des monocouches Langmuir, Langmuir-Schaefer (LS) et LB de la DSDPPC et de la DSDLPC et leurs différents mélanges avec la DPPC ou la DLPC est présentée dans cette thèse. Tout d’abord, un contrôle limité de la périodicité et de la taille des motifs des stries parallèles de DPPC/DLPC a été obtenu en variant la composition lipidique, la pression de surface et la vitesse de déposition. Dans un mélange binaire de fraction molaire plus grande de lipide condensé que de lipide étendu, une vitesse de déposition plus lente et une plus basse pression de surface ont généré des stries plus continues et larges. L’addition d’un tensioactif, le cholestérol, au mélange binaire équimolaire de la DPPC/DLPC a permis la formation de stries parallèles à de plus hautes pressions de surface. La caractérisation des propriétés physiques des analogues de phospholipides a été nécessaire. La température de transition de phase de la DSDPPC de 44.5 ± 1.5 °C comparativement à 41.5 ± 0.3 °C pour la DPPC. L’isotherme de la DSDPPC est semblable à celui de la DPPC. La monocouche subit une transition de phase liquide-étendue-à-condensée (LE-C) à une pression de surface légèrement supérieure à celle de la DPPC (6 mN m-1 vs. 4 mN m-1) Tout comme la DLPC, la DSDLPC demeure dans la phase LE jusqu’à la rupture de la monocouche. Ces analogues de phospholipide existent dans un état plus étendu tout au long de la compression de la monocouche et montrent des pressions de surface de rupture plus basses que les phospholipides non-modifiés. La morphologie des domaines de monocouches Langmuir de la DPPC et de la DSDPPC à l’interface eau/air a été comparée par la microscopie à angle de Brewster (BAM). La DPPC forme une monocouche homogène à une pression de surface (π) > 10 mN/m, alors que des domaines en forme de fleurs sont formés dans la monocouche de DSDPPC jusqu’à une π ~ 30 mN m-1. La caractérisation de monocouches sur substrat solide a permis de démontrer que le patron de stries parallèles préalablement obtenu avec la DPPC/DLPC était reproduit en utilisant des mélanges de la DSDPPC/DLPC ou de la DPPC/DSDLPC donnant ainsi lieu à des patrons chimiquement hétérogènes. En général, pour obtenir le même état de phase que la DPPC, la monocouche de DSDPPC doit être comprimée à de plus hautes pressions de surface. Le groupement disulfide de ces analogues de phospholipide a été exploité, afin de (i) former des monocouches auto-assemblées sur l’or et de (ii) démontrer la métallisation sélective des terminaisons fonctionnalisées des stries. La spectrométrie de photoélectrons induits par rayons X (XPS) a confirmé que la monocouche modifiée réagit avec la vapeur d’or pour former des thiolates d’or. L’adsorption de l’Au, de l’Ag et du Cu thermiquement évaporé démontre une adsorption préférentielle de la vapeur de métal sur la phase fonctionnalisée de disulfide seulement à des recouvrements sub-monocouche. / In the past two decades, the Langmuir-Blodgett (LB) technique has emerged as a bottom-up route to create nanostructured ultrathin films. Patterns consisting of parallel stripes, ∼100 to 200 nm in width, were generated via the LB deposition of mixed monolayers of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) onto silicon and mica substrates. To expand the functionality of these patterns, 1-palmitoyl-2-(16-(S-methyldithio)hexadecanoyl)-sn-glycero-3-phosphocholine (DSDPPC) and 1-lauroyl-2-(12-(S-methyldithio)dodecanoyl)-sn-glycero-3-phosphocholine (DSDLPC) were used to prepare chemically heterogeneous films. These phospholipid analogues have a methyldisulfide group attached to one of the alkyl chain ends. An extensive study of the phase structure of Langmuir, Langmuir-Shaefer and LB films of DSDPPC and DSDLPC and their mixtures with DPPC or DLPC is presented in this thesis. Limited control over the regularity and feature size of the DPPC/DLPC stripe pattern was achieved by varying the lipid composition, deposition pressure, and substrate withdrawal speed. A higher percentage of condensed versus fluid lipid, slower deposition speed, and lower surface pressure create more continuous and wider stripes. The addition of a lineactant, cholesterol, to the DPPC/DLPC 1:1 (mol/mol) mixture allowed the formation of parallel stripes at higher surface pressure. The gel-to-liquid crystalline transition temperature of DSDPPC was determined to be 44.5 ± 1.5 °C versus 41.5 ± 0.3 °C for DPPC by DSC and turbidity measurements. The pressure-area isotherm of DSDPPC is similar to that of DPPC. The monolayer undergoes a liquid expanded-to-condensed (LE-C) phase transition at a surface pressure slightly higher than that of DPPC (6 mN m-1 vs. 4 mN m-1). Like DLPC, DSDLPC remains in the LE phase until the film collapse. The disulfide-modified lipids exist in a more expanded state throughout the monolayer compression and exhibit lower collapse pressures than the unmodified phospholipids. The domain morphologies of DPPC and DSDPPC at the air/water interface were compared using Brewster Angle Microscopy. DPPC forms a homogeneous monolayer at a surface pressure (π) > 10 mN m-1, while flower-like domains exist in the DSDPPC monolayers until π ∼ 30 mN m-1. Solid-supported DSDPPC films were prepared and characterized using various surface analysis techniques. The parallel stripe pattern previously obtained with mixtures of DPPC/DLPC was reproduced using DSDPPC/DLPC or DPPC/DSDLPC mixtures resulting in chemically-differentiated patterns. The average stripe width varied from 150 to 500 nm, depending on the lipid composition and deposition pressure. The disulfide group of the analogues was exploited to (i) form self-assembled monolayers of phospholipids on gold and (ii) demonstrate the selective metallization of the disulfide-terminated areas of the stripe patterns. X-ray photoelectron spectroscopy (XPS) confirmed that the monolayer-bound disulfides react with Au vapor to form a gold-thiolate species. Thermally evaporated Au, Ag and Cu exhibit preferential absorption onto the modified lipids only at submonolayer coverages.

Space-charge effects play a dominant role in many areas of physics. In high-power microwave devices using high-current, relativistic electron beams, it places a limit on the amount of radiation a device can produce. Because the beam's space-charge can actually reflect a portion of the beam, the ability to accurately predict the amount of current a device can carry is needed. This current value is known as the space-charge limited current. Because of the mathematical difficulties, this limit is typically estimated from a one-dimensional theory. This work presents a two-dimensional theory for calculating an upper-bound for the space-charge limited current of relativistic electron beams propagating in grounded coaxial drift tubes. Applicable to annular beams of arbitrary radius and thickness, the theory includes the effect introduced by a finite-length drift tube of circular cross-section. Using Green's second identity, the need to solve Poisson's equation is transferred to solving a Sturm-Liouville eigenvalue problem, which is easily solved by elementary methods. In general, the resulting eigenvalue, which is required to estimate the limiting current, must be numerically determined. However, analytic expressions can be found for frequently encountered limiting cases. Space-charge effects also produce the fundamental collective behavior found in plasmas, especially in plasma sheaths. A plasma sheath is the transition region between a bulk plasma and an adjacent plasma-facing surface. The sheath controls the loss of particles from the plasma in order to maintain neutrality. Using a fully kinetic theory, the problem of a planar sheath with a single-minimum electric potential profile is investigated. Appropriate for single charge-state ions of arbitrary temperature, the theory includes the emission of warm electrons from the surface as well as a net current through the sheath and is compared to particle-in-cell simulations. Approximate expressions are developed for estimating the sheath potential as well as the transition to space-charge saturation. The case of a space-charge limited sheath is discussed and compared to the familiar Child-Langmuir law.

Les interactions plasma-paroi représentent l'un des principaux problèmes à résoudre pour avancer dans la recherche sur la fusion contrôlée. Ce travail de thèse a pour objectif de développer une source d'hydrogène atomique à basse pression (< Torr) dans un réacteur à configuration hélicon en mélange H2/Ar pour étudier la gravure chimique du graphite et de composites à fibres de carbone utilisés dans le tokamak Tore Supra. Selon les conditions expérimentales, le réacteur peut générer les modes de couplages capacitif, inductif, Trivelpiece-Gould et hélicon à bas champ. Leur caractérisation a montré que le mode inductif est, dans ce cas, celui présentant le plus grand intérêt pour la source d'hydrogène atomique. Les études en mode inductif ont révélé un phénomène de décroissance de la densité relative de deux niveaux métastables de l'ion Ar+ et d'un niveau métastable de l'argon neutre lors de l'augmentation du champ magnétique de confinement. Un modèle simple a confirmé que ces niveaux métastables sont détruits par collisions électroniques vers des niveaux de plus grande énergie. La gravure du graphite par la source d'hydrogène atomique est relativement efficace (jusqu'à 3 µm/h) à 10 mTorr et diminue avec la pression. Une analyse qualitative de la cinétique de l'hydrogène atomique a permis de conclure que cette baisse de la vitesse de gravure est due au flux d'hydrogène atomique arrivant sur l'échantillon qui décroît lorsque la pression augmente. Les premières observations de la surface après gravure ont mis en évidence la présence de structures carbonées (agglomérats de nanoparticules et dépôts). Ces structures ressemblent à celles observées dans différents tokamaks / Plasma-wall interactions are one of the main issues in fusion research and must be thoroughly studied to progress in this topic. The objective of this work is to develop an atomic hydrogen source at low pressure (< Torr) in a helicon configuration reactor working in H2/Ar gas mixture. This source is then used to study the chemical etching of graphite and carbon fiber composites composing the limiter of the Tore Supra tokamak. Depending on the experimental conditions, the RF power coupling of the reactor can be in capacitive, inductive, Trivelpiece-Gould or low field helicon mode. The characterization of these modes determined that in this case the inductive one presents the greatest interest for the atomic hydrogen source. Further studies in inductive mode showed that increasing the confinement magnetic field leads to a decrease of measured relative densities of two metastable levels of argon ion and one metastable state of neutral argon. A simple model simulating neutral metastable state behavior confirmed that these levels are destroyed by electronic collisions towards upper levels. The chemical etching of graphite exposed to the atomic hydrogen source is relatively efficient (up to 3 µm/h) at 10 mTorr and drops with the pressure. A qualitative analysis of atomic hydrogen kinetics concluded that this behavior is due to the decrease of atomic hydrogen flux on the sample with increasing pressure. Finally, first observations of the etched surface underlined different structures (nanoparticles clusters and deposits). These can be compared to the ones observed in different tokamaks

The present thesis is the result of a research period at the Institute of Space and Astronautical Science of the Japanese Aerospace Exploration Agency, ISAS/JAXA within Funaki Laboratory of the Department of Space Flight Systems that followed the path of plume plasma diagnostics for space electric propulsion drives. During the experimental studies two high-current hollow cathodes and an innovative prototype of a low-voltage fully external discharge plasma thruster (XPT) had their plasma plumes diagnosed using electrostatic probes and retarding potential analyser (RPA). A Hall thruster and hollow cathode plume is defined as an unmagnetised quasi-neutral plasma which is mainly formed of neutral particles, electrons, singly and doubly charged ions. Plasma diagnostic techniques provide information through practical observations in order to fully understand the dynamics of the aforementioned plume components, the physical processes taking place within the plume and their effects on the spacecraft, for instance. Mastering these aspects of the plasma plume of space electric propulsion drives bolster the design processes, leading to highly efficient devices. Firstly, the introduction provides insights on the fundamental principles of hollow cathodes and Hall thrusters and a brief presentation of the plasma diagnostic techniques used during the research: single and double Langmuir probes, emissive probes and retarding potential analyser. Then, the fundamental plume diagnostics principles are depicted in an exhaustive way, departing from classical plasma kinetic theory, energy distribution functions and ending with an overview on the theory of charge collection by cylindrical probes. Subsequently, peculiarities of various analysis techniques are exposed for the Langmuir probes, emissive probes and RPA, with an emphasis on their strengths and demerits. The experimental setups for the cathodes and XPT plume diagnostic procedures are then outlined. The experimental logic, setup and electrical diagrams as well as a presentation of each probe design and manufacturing details are extensively discussed. The hollow cathodes experimental results are exposed with a discourse that aims of overviewing the difference between the various data analysis methods applied for the raw data. A discussion ensued based on the results in order to effectively identify mechanisms that produced the observed plasma parameters distributions. For the first time, the plume of a fully external discharge plasma thruster was diagnosed utilising double Langmuir probes.  The thesis highlights the main results obtained for the XPT far-field plume plasma diagnostics. The experimental findings for both thruster centreline positions and 2D plume maps for several axial distances away from the anode plate offer a ground basis for future measurements, a comparison term and a database to support ongoing computational codes. The results are discussed and related to the thruster performances data obtained during previous experiments. The thesis includes consistency analyses between the experimental data and the numerical simulation results and the uncertainties in measured plasma parameters associated with each data analysis procedure are evaluated for each data set. Last, the conclusions underline the main aspects of the research and further work on the previously mentioned plasma diagnostic techniques for hollow cathodes and XPT is suggested. / La présente thèse est le résultat d'une période de recherche à l'Institut des Sciences Spatiales et Astronautiques de l'Agence Spatiale Japonaise, ISAS / JAXA qui a suivi la voie des diagnostics du plasma de la plume de propulseurs électriques spatiaux. Au cours des études expérimentales, deux cathodes creuses à fort courant et un prototype innovant d'un propulseur basse tension à décharge externe de plasma (XPT) avaient leurs faisceaux de plasma diagnostiqués en utilisant des sondes électrostatiques et un analyseur à potentiel retardé. La plume d’un propulseur à effet Hall et d’une cathode creuse est définie comme un plasma quasi-neutre non-magnétisé qui est principalement formé de particules neutres, d’électrons, d’ions monovalents et bivalents. Les techniques de diagnostic du plasma fournissent des informations, via des observations pratiques, afin de bien comprendre la dynamique des composants de la plume mentionnés ci-dessus, les processus physiques qui se déroulent dans la plume et leurs effets sur une sonde spatiale, par exemple. La maîtrise de ces aspects du plasma de la plume généré par les propulseurs électriques spatiaux renforce les processus de conception de ce type de propulsion, ce qui conduit à des dispositifs hautement efficaces. Tout d'abord, l'introduction donne un aperçu sur les principes fondamentaux de cathodes creuses et de propulseurs à effet Hall, et une brève présentation des techniques de diagnostic du plasma utilisées lors de la recherche : sondes de Langmuir simples et doubles, des sondes émissives et d’analyseur à potentiel retardé. Ensuite, les principes fondamentaux de diagnostic de la plume sont représentés de manière exhaustive, d’abord la théorie cinétique classique du plasma, les fonctions de distribution en énergie et pour terminer une vue d'ensemble de la théorie de la collecte de charge par des sondes cylindriques. Par la suite, les particularités des diverses techniques d'analyse sont exposées pour les sondes de Langmuir, les sondes émissives et RPA, en mettant l'accent sur leurs avantages et leurs inconvénients. Les montages expérimentaux pour les procédures de diagnostic de la plume-plasma de cathodes et du XPT sont ensuite décrits. La logique expérimentale, les schémas électriques ainsi qu'une présentation de la conception et de la fabrication de chaque sonde sont largement discutés. Les résultats expérimentaux pour les cathodes creuses sont exposés de façon à présenter la différence entre plusieurs méthodes d'analyse de données appliquées aux données brutes. Une discussion s’ensuit, basée sur les résultats afin d'identifier efficacement les mécanismes qui ont produits les propriétés électroniques observées. Pour la première fois, la plume d'un propulseur à décharge externe de plasma a été diagnostiquée en utilisant des sondes de Langmuir doubles. La thèse met en évidence les principaux résultats obtenus pour le diagnostic en champ lointain de la plume-plasma du XPT. Les résultats expérimentaux pour les positions sur l'axe du propulseur et le cartes 2D de la plume pour plusieurs distances axiales loin de l’anode offrent une base pour de futures mesures, un terme de comparaison et une base de données pour appuyer les codes numériques. Les résultats sont discutés et sont rapportés aux données de performances du propulseur obtenus lors des essais précédents. La thèse comprend des analyses de la cohérence entre les données expérimentales et les résultats de simulation numérique, et les incertitudes des paramètres mesurés du plasma associées à chaque procédure d'analyse des données sont évaluées pour chaque ensemble de données. Enfin, les conclusions soulignent les principaux aspects de la recherche et une poursuite des travaux sur les techniques de diagnostic de plasma pour les cathodes creuses et le XPT est suggérée.

O nitreto de alumínio (AlN) tem atraído muito interesse pelas suas notáveis propriedades físicas: dureza, alta condutividade térmica, resistência para altas temperaturas. Uma fonte de catodo oco DC foi adicionada a um sistema de sputtering magnetron. Isto leva a uma ionização na região do catodo e habilita a descarga a operar a pressões significativamente mais baixas do que um catodo magnetron plano típico. Como uma conseqüência, átomos ejetados do catodo podem alcançar um substrato com uma perda de energia mínima devido às colisões com os átomos do gás. O catodo oco magnetron consiste em dois eletrodos planos e paralelos de alumínio como alvos e uma mistura de gases argônio e nitrogênio variando a pressão de (0,05 a 0,5) Pa para uma variação de fluxo de (1 a 12)sccm. A tensão de descarga variou de (250 a 400)V correspondendo a uma corrente de descarga de (10 a 500) mA. As propriedades de plasma foram obtidas através da espectroscopia de massa e da técnica das Sondas de Langmuir para diferentes distâncias inter-catódicas. A densidade de elétrons do plasma é da ordem de 1018 m-3 e a temperatura de elétrons varia de (3 a 6) eV, medidos ao longo de uma linha reta ao longo da região de evaporação. Devido ao bombardeamento de íons e uma temperatura realçada do catodo, átomos de alumínio são arrancados do catodo e reagem com o radical nitrogênio para formar AlN+ no plasma. Filmes finos de AlN crescem em substratos de silício a temperatura ambiente; foram caracterizados com respeito a estrutura e morfologia por análises de DRX e AFM, respectivamente.

Com o uso de uma sonda de Langmuir e modelos numéricos se investigou uma descarga de oxigênio num reator acoplado capacitivamente a uma fonte de radiofrequência, com o propósito de se estudar os parâmetros intrínsecos do plasma, como densidade eletrônica e iônica, potencial flutuante, potencial do plasma, temperatura eletrônica, potencial da bainha catódica - VDC, em relação aos parâmetros de processo: potência e pressão. Dentre os resultados apresentados neste trabalho destacamos a boa concordância da densidade eletrônica calculada pelo Modelo Global - MG, e medida pela sonda de Langmuir. Além disso, tanto o método partícula-na-célula - PIC quanto o MG apresentaram uma boa concordância em relação a temperatura eletrônica simulada e medida experimentalmente. Foi observado experimentalmente que a temperatura eletrônica - Te, é intensificada para pressões abaixo de 30 mTorr, embora a densidade eletrônica seja baixa, como é característico do reator de corrosão a plasma por íons reativos - RIE. Aumentando-se a potência consegue-se um aumento da densidade eletrônica e iônica, embora a temperatura tenda a se manter em torno de 2 eV, para potências acima de 50 W. Um aumento da temperatura eletrônica apenas é substancial para potências menores que 30 W, para qualquer valor de pressão, embora para essa faixa de potência a densidade eletrônica seja baixa. Uma anomalia no perfil da densidade eletrônica em função da pressão foi encontrada para a região em torno de 85 mTorr. Estudos complementares serão conduzidos com o objetivo de melhor esclarecer essa anomalia. Por último, a FDEE se mostrou bi-Maxwelliana para pressões abaixo de 20 mTorr para todos os valores de potência investigados.

Neste trabalho são apresentados os resultados de investigações realizadas na coluna positiva de uma descarga de alta densidade de corrente (3,0 . 103 - 8,0 . 103 A/m2), operando em baixa pressão de vapor de mercúrio (2,0 . 10-2 - 1,4 . 10-1 Pa) e livre de campo magnético. Nestas condições a velocidade de deriva dos elétrons na direção axial, é o principal parâmetro responsável pelo transporte de corrente da descarga e pode alcançar uma apreciável fração da velocidade térmica dos elétrons. Esta velocidade de deriva influenciará na distribuição de velocidades dos elétrons, modificando-a para uma chamada maxwelliana com deriva. Os efeitos desta modificação são introduzidos na teoria radial da coluna positiva de Tonks e Langmuir para avaliar o comportamento do potencial de plasma e da densidade de elétrons com a distância radial e da temperatura de elétrons com a pressão. Estes parâmetros foram inferidos de características de sondas eletrostáticas (Sondas de Langmuir) cujas posições radiais e longitudinais podem ser alteradas, o que permitiu, também, verificar a uniformidade axial do plasma quando a velocidade de deriva dos elétrons é conduzida a valores críticos de auto-excitação de instabilidades eletrostáticas e formação de camada dupla. Teoricamente este efeito é estudado analisando-se as características de instabilidade de uma perturbação eletrostática axial do regime permanente. Assim, procedeu-se uma análise da estabilidade do plasma constituído de três fluidos: elétrons, íons e partículas neutras, considerando-se efeitos de colisões elásticas e inelásticas. Como resultado, algumas instabilidades eletrostáticas são previstas, identificadas pela relação entre a velocidade de deriva e a velocidade térmica dos elétrons. Na prática, ocorre uma faixa de valores desta relação que distingue os regimes de operação da descarga, desde o regime permanente (estável) para o qual esta faixa de valores é constante (0,4) até o regime de fortes oscilações de corrente da descarga quando assume valores em torno da unidade. Neste regime é conduzido um estudo sobre formação e manutenção de camadas eletrostáticas que se formam na coluna positiva do arco. A camada dupla (CD) é induzida a se formar num local desejado, pela constrição da seção de área transversal da descarga (constrição magnética da coluna de plasma) por um campo magnético axial produzido por uma bobina simples ou por uma bobina de Helmholtz. Este campo atua apenas localmente sobre os elétrons tal que seus efeitos nas regiões adjacentes das camadas eletrostáticas não são significativos. O deslocamento da camada dupla ao longo do eixo da coluna de plasma é controlado pelo movimento da bobina nesta direção o que permitiu não somente a caracterização do plasma nas regiões adjacentes à camada dupla como também obter vários perfis de potenciais das camadas eletrostáticas utilizando-se uma sonda auto-emissiva fixa no eixo da coluna. Neste aspecto, foi dado ênfase ao estudo experimental sobre a variação do perfil de potencial de camadas eletrostáticas da forma de perfeitos buracos de elétrons com relação aos parâmetros operacionais: a corrente da descarga, a pressão de vapor de mercúrio e a indução magnética local. Finalmente, foram efetuadas medidas em ambos os lados da CD das energias dos íons dirigidas radialmente, do potencial flutuante da parede e da dispersão de energia dos íons usando-se um analisador eletrostático de energia posicionado rente a parede do tubo de descarga. Os resultados mostram que os íons são fortemente aquecidos, especialmente no lado anódico da camada dupla.

This research seeks to increase the applicable range and sensitivity of Triple Langmuir Probes (TLPs) and Retarding Potential Analyzers (RPAs) in the characterization of sub-centimeter scale, unsteady plasmas found in micropropulsion and other non-propulsive applications. The validation of these plasma diagnostics is accomplished by their implementation in the plume of a Micro Liquid-fed Pulsed Thruster (MiLiPulT) prototype developed and MEMS fabricated by the Johns Hopkins University Applied Physics Laboratory. A current-mode TLP (CM-TLP) theory of operation for the thin-sheath and the transitional regimes is expanded to include the Orbital Motion Limited regime applicable to low density plasmas. An optimized CM-TLP bias circuit employing operational amplifiers in both a differential amplifier configuration as well as a voltage follower configuration has been developed to adequately amplify current signals in instances where traditional current measuring techniques are no longer valid. This research also encompasses novel sub-microampere signal amplification in the presence of substantial common-mode noise as well as several a priori electromagnetic interference elimination and filtering techniques. The CM-TLP wires used in the experiments were designed with a radius of 37.5 micron and a length of 5 mm. Measurements were taken in the plume of the MiLiPulT at 2.0 cm, 6.0 cm and 10.0 cm downstream of the exit using a linear translation stage. Reduced electron temperature and electron number density profiles for a set of filtered CM-TLP raw currents are presented. The results indicate increased accuracy due to successful amplification of CM-TLP current signals at the risk of op-amp saturation due to inherent electrical noise of the plasma source. This research also includes the experimental validation of two new and distinct collimating RPA design types. Specifically, these design improvements include a 406 micron diameter single channel bore and a multi-channel plate (MCP) consisting of sixty-four 2 micron diameter bores, respectively. Both of these collimators relax the Debye length constraints within the electrode series and increase the instrument's range while minimizing the presence of space charge limitations. The single channel needle also has the added advantage of providing a relatively small cross-section to the incident plasma, thus minimizing pressure gradients and shock effects inherent to bulkier instrumentation. Experimental results obtained in the plume of the MiLiPulT are benchmarked against those of a traditional gridded RPA (having a 650 micron grid wire gap) and are reduced using an iterative fuzzy logic algorithm. Modifications to the classical RPA current collection theory include a thorough treatment of geometrical flux limitations due to an electrically floating cylindrical channel of high diameter to length aspect ratio. The differences between true and effective RPA collimating channel transparencies in the presence of a Maxwellian plasma are also addressed.