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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been developed to adequately amplify current signals in instances where traditional current measuring techniques are no longer valid.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.

The purpose of this thesis is to investigate whether it is feasible to use Langmuir probes on pico-satellites flying in low Earth orbit over mid- to low-latitude geographic regions. Following chapters on the expected ionospheric conditions and an overview of Langmuir probe theory, a chapter addressing the difficulties involved with pico-satellite Langmuir probes is presented. Also, the necessary satellite-to-probe surface area requirements in order to achieve confidence in pico-satellite Langmuir probe data, for the orbital regions of interest to this thesis, are stated.

We present results on the ionospheric structure around Titan observed during numerous deep (<1000 km) flybys by the Cassini spacecraft. Our results are based on measurements by the radio and plasma wave science instrument, in particular the Langmuir probe. In addition, data from the magnetometer and electron spectrometer have contributed. The ionosphere of Titan is created when the atmosphere of the moon becomes ionised. There are several mechanisms that contribute to this, the most important of which are considered to be photoionisation by EUV from the Sun with associated photoelectron ionisation, and particle impact ionisation by electrons and ions from Saturn’s corotating magnetosphere. We investigate the influence of the solar zenith angle on the electron number density at the ionospheric peak. The results show on average four times more plasma on the dayside compared to the nightside, with typical densities of 2500 – 3500 cm-3 and 400 – 1000 cm-3, respectively. In a complementary study, we make a case study of a nightside flyby and show that the altitude structure of the deep ionosphere is reproducible by a simple electron impact ionisation model. Taken together, this leads to the conclusion that solar photons are the main ionisation source of the dayside ionosphere. However, magnetospheric particle precipitation also contributes and can explain the electron densities seen on the nightside. As Titan does not exhibit any large intrinsic magnetic field, the fact that it is embedded in the magnetosphere of Saturn means that the Kronian field drapes around the moon and gives rise to an induced magnetosphere. We show that there are currents of the order of 10 – 100 nA m-2 flowing in the ionosphere of the moon. Associated with the currents are perpendicular electric fields ranging from 0.5 to 3 µV m-1. Finally, we investigate measurements obtained during T70, the deepest Titan flyby performed to date. We show that there is a substantial amount of negative ions present below an altitude of 900 km. This confirms previous result by the electron spectrometer, showing negative ions at higher altitudes in Titan’s ionosphere.

The profile of the poloidal velocity in the edge region of tokamak plasmas has been identified as playing a major role in the confinement of particles and energy. It has been suggested that a strongly sheared poloidal flow can reduce particle and energy losses by the stabilization of unstable modes and decorrelation of turbulence the edge region of the plasma. A Gundestrup probe, a Mach probe array, is used to measure both the parallel and perpendicular flow velocities in the Saskatchewan Torus-Modified (STOR-M) tokamak during several discharge conditions. It is observed that during Ohmic discharges there is no velocity shear and the direction of the parallel flow is independent of the direction of the toroidal magnetic field. During H-mode induced by a turbulent heating current pulse, a region of strong velocity shear develops in the plasma edge and an edge transport barrier develops. This results in a short period of improved particle and energy confinement with reduced fluctuation amplitudes. During electrode biasing experiments, a stainless steel biasing electrode is inserted into the plasma up to r = 82 mm and biased to +500 V relative to the vacuum chamber. It is observed that the particle confinement improves during the biasing phase while the energy confinement is degraded. A region of weak shear in the poloidal flow is observed in the plasma scrapeoff layer (SOL). The results from STOR-M are compared with results from data taken in the Czech Academy of Sciences Torus (CASTOR) tokamak during both Ohmic discharges and discharges with electrode biasing.

The profile of the poloidal velocity in the edge region of tokamak plasmas has been identified as playing a major role in the confinement of particles and energy. It has been suggested that a strongly sheared poloidal flow can reduce particle and energy losses by the stabilization of unstable modes and decorrelation of turbulence the edge region of the plasma. A Gundestrup probe, a Mach probe array, is used to measure both the parallel and perpendicular flow velocities in the Saskatchewan Torus-Modified (STOR-M) tokamak during several discharge conditions. It is observed that during Ohmic discharges there is no velocity shear and the direction of the parallel flow is independent of the direction of the toroidal magnetic field. During H-mode induced by a turbulent heating current pulse, a region of strong velocity shear develops in the plasma edge and an edge transport barrier develops. This results in a short period of improved particle and energy confinement with reduced fluctuation amplitudes. During electrode biasing experiments, a stainless steel biasing electrode is inserted into the plasma up to r = 82 mm and biased to +500 V relative to the vacuum chamber. It is observed that the particle confinement improves during the biasing phase while the energy confinement is degraded. A region of weak shear in the poloidal flow is observed in the plasma scrapeoff layer (SOL). The results from STOR-M are compared with results from data taken in the Czech Academy of Sciences Torus (CASTOR) tokamak during both Ohmic discharges and discharges with electrode biasing.

Effects of a magnetic field on the double-probe technique are studied experimentally by means of symmetric floating computer controlled fast double probes in low and intermediate pressure plasmas. In addition, the effects of the magnetic field on the electron temperature, electron density, and electric field have been investigated. As it is expected, when there is no magnetic field, properties of the discharge plasma are best described by Langmuir theory. Whereas, when there&rsquo / s a magnetic field of sufficient strength Schottky&rsquo / s theory of ambipolar diffusion applies.

Orientador: Munemasa Machida / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-19T18:26:47Z (GMT). No. of bitstreams: 1 Ronchi_Gilson_M.pdf: 6202845 bytes, checksum: e9580b7846e57a9eb526cdf45b808564 (MD5) Previous issue date: 2012 / Resumo: O conhecimento das condições para ruptura de gases e a formação de plasma são questões importantes, não apenas de interesse fundamental, mas também para muitas aplicações, tais como em deposição de filmes finos de óxido ou polímeros, limpeza de superfície de materiais ou implantação iônica, por exemplo. A ruptura de gases descrita pela lei de Paschen com a teoria de Townsend, na qual a tensão de ruptura é uma função do produto da pressão p do gás pela distância d entre os eletrodos, proporciona uma adequada descrição em certas condições, em gases sob baixa pressão, quando a distância entre os eletrodos é muito menor que o raio R dos mesmos. A medida que a razão desses valores aumenta, as curvas de Paschen são deslocadas para regiões de tensão de ruptura mais elevadas, bem como para valores pd mais elevados. Investigamos aqui, os motivos e as consequências desse fenômeno, bem como uma proposta de lei de escalas capaz de determinar a tensão de ruptura em condições distintas de descargas. Tal lei de escalas obtida empiricamente faz uso, além da pressão e distância dos eletrodos, da razão R/d, que surge naturalmente ao se considerar o processo de difusão na descarga. Discutimos ainda sobre as principais técnicas de diagnósticos aplicáveis ao plasma, para obtenção, por exemplo, das temperaturas e densidades eletrônicas/iônicas. Em particular, revisamos as técnicas de espectroscopia e sonda de Langmuir, que são técnicas importantes amplamente utilizadas / Abstract: The understanding of the gas breakdown mechanism and plasma formation are important issues not only of fundamental interests, but also for many applications such as oxide thin films or polymers deposition, surface cleaning and ion implantation, for example. The breakdown described by Paschen¿s law with Townsend¿s theory, where the breakdown voltage is a function of the product of the gas pressure p and the gap distance d, provides a very accurate description under certain conditions as in low pressure gases when the distance between the electrodes is much smaller than its radius R. As the ratio d/R increases, the Paschen curves are shifted to regions of higher breakdown voltage, as well as higher pd values. In this work we investigate the reasons and consequences of this phenomenon, as well as a scaling law capable of determining the breakdown voltage of discharges in different conditions. This scalling law, obtained empirically, uses the gas pressure, the electrodes gap distance and the ratio R/d, which arises naturally when one consider the diffusion process in the discharge. We also discuss the main diagnostic techniques applicable to plasma to obtain, for example, temperature and electronic/ionic density. In particular, we review the techniques of spectroscopy and Langmuir probe, which are important techniques widely used / Mestrado / Física / Mestre em Física

Titan, the largest moon of Saturn, is the only known moon with a fully developed nitrogen-rich atmosphere, its ionosphere is detectable as high as 2200 km above its surface and hosts complex organic chemistry. Titan’s atmosphere and ionosphere has striking similarities to current theories of these regions around Earth 3.5 billion years ago. The Cassini spacecraft has been in orbit around Saturn since 2004 and carries a wide range of instruments for investigating Titan’s ionosphere, among them the Langmuir probe, a “space weather station”, manufactured and operated by the Swedish Institute of Space Physics, Uppsala. This thesis presents studies of positive ions, negative ions and negatively charged dust grains (also called aerosols) in Titan’s ionosphere using the in-situ measurements by the Cassini Langmuir probe, supplemented by the data from particle mass spectrometers. One of the main results is the detection of significant (up to about 4000 cm-3) charge densities of heavy (up to about 13800 amu/charge) negative ions and dust grains in Titan’s ionosphere below 1400 km altitude. The dust is found to be the main negative charge carrier below about 1100 km on the nightside/terminator ionosphere, forming a dusty plasma (also called “ion-ion” plasma). A new analysis method is developed using a combination of simultaneous observations by multiple instruments for a case study of four flybys of Titan’s ionosphere, further constraining the ionospheric plasma charge densities. This allows to predict a dusty plasma in the dayside ionosphere below 900 km altitude (thus declaring it a global phenomenon), as well as to empirically estimate the average charge of the negative ions and dust grains to between -2.5 and -1.5 elementary charges. The complete Cassini dataset spans just above 13 years, allowing to study effects of the solar activity on Titan’s ionosphere. From solar minimum to maximum, the increase in the solar EUV flux increases the densities by a factor of ~2 in the dayside ionosphere and, surprisingly, decreases by a factor of ~3-4 in the nightside ionosphere. The latter is proposed to be an effect of the ionospheric photochemistry modified by higher solar EUV flux. Modelling photoionization also reveals an EUV trend (as well as solar zenith angle and corotational plasma ram dependencies) in the loss rate coefficient.

La possibilité de produire de l'énergie en utilisant la fusion par confinement magnétique est un défi scientifique et technologique. La perspective d'ITER transmet des signaux forts afin d'intensifier les efforts de modélisation pour les plasmas de fusion. Le succès de la fusion est conditionnée par la qualité du confinement du plasma dans le cœur du réacteur et par le contrôle des flux de particules et de chaleur qui arrivent sur la paroi. Les deux phénomènes sont liés au transport turbulent. L'étude de ces phénomènes est d'autant plus compliquée que la géométrie magnétique est complexe. Cela nécessite une amélioration de notre capacité à développer des outils numériques capables de reproduire les propriétés du transport turbulent fiables.Cette thèse présente le modèle fluide du code TOKAM3X pour simuler plasma de bord turbulent. Une attention particulière a été portée sur la vérification et la validation de ce code, ce qui est une étape nécessaire avant d'utiliser un code comme un outil prédictif. Ensuite, de nouvelles études sur les propriétés physiques de la turbulence bord du plasma sont examinées. En particulier, les asymétries poloïdales induites par la turbulence et observées expérimentalement côté faible champ sont étudiées en détail. Un grand soin est dédié à la reproduction du scénario MISTRAL, qui consiste à changer la configuration magnétique et à en observer l'impact sur les flux parallèles dans le plan poloïdal. Les simulations reproduisent les mesures expérimentales et fournissent de nouvelles informations sur l'effet du point de contact plasma-paroi sur les caractéristiques de la turbulence, qui ne sont pas accessibles dans les expériences. / The possibility to produce power by using magnetically confined fusion is a scientific and technological challenge. The perspective of ITER conveys strong signals to intensify modeling effort on magnetized fusion plasmas. The success of the fusion operation is conditioned by the quality of plasma confinement in the core of the reactor and by the control of plasma exhaust on the wall. Both phenomena are related to turbulent cross-field transport that is at the heart of the notion of magnetic confinement studies, particle and heat losses. The study of edge phenomena is therefore complicated by a particularly complex magnetic geometry.This calls for an improvement of our capacity to develop numerical tools able to reproduce turbulent transport properties reliable to predict particle and energy fluxes on the plasma facing components. This thesis introduces the TOKAM3X fluid model to simulate edge plasma turbulence. A special focus is made on the code Verification and the Validation. It is a necessary step before using a code as a predictive tool. Then new insights on physical properties of the edge plasma turbulence are explored. In particular, the poloidal asymmetries induced by turbulence and observed experimentally in the Low-Field-Side of the devices are investigated in details. Great care is dedicated to the reproduction of the MISTRAL base case which consists in changing the magnetic configuration and observing the impact on parallel flows in the poloidal plane. The simulations recover experimental measurements and provide new insights on the effect of the plasma-wall contact position location on the turbulent features, which were not accessible in experiments.