The rectangular pulsed plasma thruster (PPT) is an electromagnetic thruster that ablates Teflon propellant to produce thrust in a discharge that lasts 5-20 microseconds. In order to integrate PPTs onto spacecraft, it is necessary to investigate possible thruster plume-spacecraft interactions. The PPT plume consists of neutral and charged particles from the ablation of the Teflon fuel bar as well as electrode materials. In this thesis a novel application of quadruple Langmuir probes is implemented in the PPT plume to obtain electron temperature, electron density, and ion speed ratio measurements (ion speed divided by most probable thermal speed). The pulsed plasma thruster used is a NASA Glenn laboratory model based on the LES 8/9 series of PPTs, and is similar in design to the Earth Observing-1 satellite PPT. At the 20 J discharge energy level, the thruster ablates 26.6 mg of Teflon, creating an impulse bit of 256 mN-s with a specific impulse of 986 s. The quadruple probes were operated in the so-called current mode, eliminating the need to make voltage measurements. The current collection to the parallel to the flow electrodes is based on Laframboise's theory for probe to Debye length ratios between 5 and 100, and on the thin-sheath theory for ratios above 100. The ion current to the perpendicular probe is based on a model by Kanal and is a function of the ion speed ratio, the applied non-dimensional potential and the collection area. A formal error analysis is performed using the complete set of nonlinear current collection equations. The quadruple Langmuir probes were mounted on a computer controlled motion system that allowed movement in the radial direction, and the thruster was mounted on a motion system that allowed angular variation. Measurements were taken at 10, 15 and 20 cm form the Teflon fuel bar face, at angles up to 40 degrees off of the centerline axis at discharge energy levels of 5, 20, and 40 J. All data points are based on an average of four PPT pulses. Data analysis shows the temporal and spatial variation in the plume. Electron temperatures show two peaks during the length of the pulse, a trend most evident during the 20 J and 40 J discharge energies at 10 cm from the surface of the Teflon fuel bar. The electron temperatures after the initial high temperature peak are below 2 eV. Electron densities are highest near the thruster exit plane. At 10 cm from the Teflon surface, maximum electron densities are 1.04e20 ± 2.8e19 m-3, 9.8e20 ± 2.3e20 m-3, and 1.38e21 ± 4.05e20 m-3 for the 5 J, 20 J and 40 J discharge energy, respectively. The electrons densities decrease to 2.8x1019 ± 8.9e18 m-3, 1.2e20 ± 4.2e19 m-3, and 4.5e20 ± 1.2e20 m-3 at 20 cm for the 5 J, 20 J, and 40 J cases, respectively. Electron temperature and density decrease with increasing angle away from the centerline, and with increasing downstream distance. The plume is more symmetric in the parallel plane than in the perpendicular plane. Ion speed ratios are lowest near the thruster exit, increase with increasing downstream distance, but do not show any consistent angular variation. Peak speed ratios at a radial distance of 10 cm are 5.9±3.6, 5.3±0.39, and 4.8±0.41 for the 5 J, 20 J and 40 J discharge energies, respectively. The ratios increase to 6.05±5.9, 7.5±1.6, and 6.09±0.72 at a radial distance of 20 cm. Estimates of ion velocities show peak values between 36 km/s to 40 km/s, 26 km/s to 30 km/s, and 26 km/s to 36 km/s for the % J, 20 J, and 40 J discharge energies, respectively.

De nos jours, pour accroître le potentiel applicatif des machines de hautes puissances pulsées, il est nécessaire de développer des modulateurs compacts capables de délivrer des impulsions de l’ordre de plusieurs Mégawatts de durée pouvant atteindre plusieurs centaines de microsecondes. Cette amélioration requiert le développement de structures innovantes dont le but est de produire aussi bien des puissances moyennes que des puissances crêtes importantes. Les modulateurs étudiés dans ce mémoire sont basés sur l’utilisation de divers transformateurs pour la génération d’impulsions de très forte puissance. Le projet AGIR (acronyme de « Architecture pour la Génération d’Impulsions Rectangulaires de forte de puissance ») est réalisé dans le cadre d’un RAPID (Régime d’Appui Pour l’Innovation Duale) financé par la Direction Générale de l’Armement (DGA). Le projet est une collaboration avec EFFITECH, une entreprise spécialisée dans les puissances pulsées. L’objectif est de développer deux générateurs pour deux gammes de puissance crête (jusqu’à 10MW pour l’un et 1GW pour l’autre). Le premier modulateur « AGIR1 » repose sur l’association d’un convertisseur AC-DC et de 12 convertisseurs résonants DC-DC qui permettent la génération de plusieurs types d’impulsions (fort courant ou forte tension) en fonction de la configuration choisie. Le second modulateur repose sur le développement d’un transformateur impulsionnel à quatre primaires synchronisés. Chaque primaire est relié à un système de mise en forme de type Blumlein dont le déclenchement est assuré par un éclateur pressurisé à trois électrodes. La synchronisation des quatre éclateurs est assurée par un générateur impulsionnel innovant à faible gigue. La principale difficulté du travail effectué au laboratoire réside dans l’étude des différents transformateurs haute-tension utilisés (résonant ou impulsionnel) et du système de synchronisation des éclateurs. Chaque élément constituant le système est étudié et simulé de manière électrostatique, électromagnétique ou électrique avant d’être réalisé et assemblé. Des essais ponctue l’étude afin de valider le fonctionnement en récurrent avec un système de dissipation thermique adapté. / Nowadays, to increase the application potential of high power pulsed machines, it is necessary to develop compact modulators able to deliver pulses in the range of several megawatts with duration of up to several hundred microseconds. This improvement requires the development of innovative structures whose purpose is to produce both average power and large peak power. Modulators studied in this thesis are based on the use of various transformers for the generation of very high power pulses. The AGIR project (French acronym for "Architecture for Rectangular High Pulse power generation") is achieved within the framework of a RAPID (Dual Innovation Support Regime) funded by the French Defense (DGA). The project is carried on by a collaboration with EFFITECH, a company specialized in pulsed powers. The goal is to develop two generators for two peak power ranges (up to 10MW for one and 1GW for the other). The first modulator "AGIR1" is based on the association of an AC-DC converter and 12 DC-DC resonant converters allowing the generation of several types of pulses (high current or high voltage) depending on the chosen configuration. The second modulator is based on the development of a four synchronized primary pulse transformer. Each primary is connected to a Blumlein pulse forming line triggered by a three-electrode pressurized spark gap. The synchronization of the four spark gaps is ensured by an innovative pulse generator with low jitter. The main difficulty of the work which was completed in the laboratory relies in the study of the different high-voltage transformers used (resonant or pulse) and the spark gap synchronization system. Each element constituting the system is studied and simulated electrostatically, electromagnetically or electrically before being realized and assembled. Trials punctuate the study to validate the recurrent operation with a suitable heat dissipation system.

Thesis (M.S.)--Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.

This thesis presents work performed on explosively driven magnetic flux compression generators. This kind of devices converts the chemically stored energy in a high explosive into electromagnetic energy in the form of a powerful current pulse. The high energy density of the high explosives makes flux compression generators attractive as compact power sources. In order to study these devices a generator was designed at FOI in the mid-90ies. Two generators remained unused and became available for this licentiate work. The thesis reports experiments with, and simulations of, the operation of the two remaining generators. The aim was to fully understand the performance of the generator design and be able to accurately simulate its behaviour. The generators were improved and fitted with various types of diagnostics to monitor the generator operation. Two experiments were performed of which the first generator was operated well below its current capability limits while the second was stressed far above its limits. Since the generator generates a rapidly increasing current, a current measurement is the most important diagnostic revealing the current amplification of the generator and its overall performance. Further it is important to measure the timing of various events in the generator. With a common time reference it is possible to combine data from different probes and extract interesting information which cannot be directly obtained with a single measurement. Two types of numerical simulations have been performed: Hydrodynamic simulations of the high explosive interaction with the armature were used to verify the measured armature dynamics. A zero-dimensional code was used to perform circuit simulations of the generator. The model takes into account the inductance reduction due to the compression of the generator as well as the change in conductivity due to heating of the conductors in the generators. / QC 20101103

"The ablative Teflon pulsed plasma thruster (PPT) is an onboard electromagnetic propulsion enabling technology for small spacecraft missions. The integration of PPTs onboard spacecraft requires the understanding and evaluation of possible thruster/spacecraft interactions. To aid in this effort the work presented in this thesis is directed towards the development and application of Langmuir probe techniques for use in the plume of PPTs. Double and triple Langmuir probes were developed and used to measure electron temperature and density of the PPT plume. The PPT used in this thesis was a laboratory model parallel plate ablative Teflon® PPT similar in size to the Earth Observing (EO-1) PPT operating in discharge energies between 5 and 40 Joules. The triple Langmuir probe was operated in the current-mode technique that requires biasing all three electrodes and measuring the resulting probe currents. This new implementation differs from the traditional voltage-mode technique that keeps one probe floating and requires a voltage measurement that is often susceptible to noise in the fluctuating PPT plume environment. The triple Langmuir probe theory developed in this work incorporates Laframboise’s current collection model for Debye length to probe radius ratios less than 100 in order to account for sheath expansion effects on ion collection, and incorporates the thin-sheath current collection model for Debye length to probe radius ratios greater than 100. Error analysis of the non-linear system of current collection equations that describe the operation of the current-mode triple Langmuir probe is performed as well. Measurements were taken at three radial locations, 5, 10, and 15 cm from the Teflon® surface of the PPT and at angles of 20 and 40 degrees to either side of the thruster centerline as well as at the centerline. These measurements were taken on two orthogonal planes, parallel and perpendicular to the PPT electrodes. A data-processing software was developed and implements the current-mode triple Langmuir probe theory and associated error analysis. Results show the time evolution of the electron temperature and density. Characteristic to all the data is the presence of hot electrons of approximately 5 to 10 eV at the beginning of the pulse, occurring near the peak of the discharge current. The electron temperature quickly drops off from its peak values to 1-2 eV for the remainder of the pulse. Peak electron densities occur after the peak temperatures. The maximum electron density values on the centerline of the plume of a laboratory PPT 10 cm from the Teflon® surface are 6.6x10^19 +/- 1.3x10^19 m^-3 for the 5 J PPT, 7.2x10^20 +/- 1.4x10^20 m^-3 for the 20 J PPT, and 1.2x10^21 +/- 2.7x10^20 m^-3 for the 40 J PPT. Results from the double Langmuir probe taken at r=10 cm, theta perpendicular=70 degrees and 90 degrees of a laboratory PPT showed good agreement with the triple probe method."

De nos jours, un large éventail d’applications de défense nécessite des générateurs de puissances pulsées pour produire des rayonnements électromagnétiques de fortes puissances. Dans les systèmes conventionnels, le générateur est composé d’une source d’énergie primaire et d’une antenne, séparé par un système d’amplification de la puissance, généralement un générateur de Marx ou un transformateur Tesla, qui transmet l’énergie vers l’antenne. Le système présenté dans ce mémoire, utilise un concept innovant basée sur un transformateur résonant impulsionnel compact pour alimenter l’antenne. La source développée, appelée MOUNA (acronyme de « Module Oscillant Utilisant une Nouvelle Architecture ») est composé d’une batterie, d’un convertisseur DC/DC permettant de charger quatre condensateurs, de quatre éclateurs à gaz synchronisés, d’un transformateur résonant impulsionnel qui génère des impulsions de 600kV en 265ns, d’un éclateur de mise en forme à huile, et d’une antenne dipôle. / Nowadays, a broad range of modern defense applications requires compact pulsed power generators to produce high-power electromagnetic waves. In a conventional design, such generators consist of a primary energy source and an antenna, separated by a power-amplification system, such as a Marx generator or a Tesla transformer, which forwards the energy from the source to the antenna. The present system, however, uses a novel and very compact high-voltage resonant pulsed transformer to drive a dipole antenna. The complete pulsed power source, termed MOUNA (French acronym for “Module Oscillant Utilisant une Nouvelle Architecture”), is composed of a set of batteries, a dc/dc converter for charging four capacitors, four synchronized spark gap switches, a resonant pulsed transformer that can generate 600 kV in 265 ns pulses, an oil peaking switch and, a dipole antenna.

Depuis plusieurs décennies, les générateurs forts courants sont utilisés dans différents domaines comme l’étude des matériaux, la radiographie ou la fusion par confinement inertiel. Ces générateurs sont capables de délivrer des impulsions de courant de plusieurs millions d’ampères avec des fronts de montée inférieurs à la microseconde. Plusieurs projets à travers le monde ont, aujourd’hui, pour but d’améliorer encore et encore le gradient de courant des impulsions transmises à la charge. De nombreux schémas d’amplificateurs de puissance, dont le rôle est de jouer à la fois sur l’amplitude du courant de charge et sur son temps de montée, ont ainsi été testés. Le multiplicateur de courant dynamique (DLCM pour Dynamic Load Current Multiplier) fait partie de ces concepts novateurs permettant de contourner les limitations des générateurs de puissances pulsées actuels. Il est composé d’un réseau d’électrodes (servant d’autotransformateur), d’un extrudeur de flux dynamique (basé sur l’implosion d’un réseau de fils cylindrique) et d’un commutateur à fermeture sous vide. Dans la thèse, le principe de fonctionnement du DLCM est analysé d’un point de vue théorique par le biais de simulations de type circuits électriques et magnétohydrodynamiques. Une étude spécifique portant sur l’organe principal du DLCM est réalisée. Il s‘agit du commutateur à fermeture sous vide. Ainsi, après une phase de dimensionnement à l’aide d’outils de simulations électrostatiques, deux versions de commutateurs sont validées expérimentalement dans des conditions proches de celles d’un tir très fort courant. Enfin, des tirs sur le générateur SPHINX du CEA Gramat, capable de délivrer une impulsion de courant de 6MA en 800ns (sur charge Z-pinch), sont exposés pour retracer l’évolution du dispositif. Les résultats probants obtenus permettent, au final, de valider le concept DLCM connecté à une charge de type compression isentropique. / For several decades, high power generators are used in various fields such as materials research, radiography or inertial confinement fusion. These generators are capable of delivering current pulses of several millions of amperes with rise times below 1 microsecond. Several projects around the world are, today, trying to improve again and again the current gradient of pulses delivered to the load. Many concepts of power amplifiers, whose role is to optimize both the amplitude of the load current and its rise time, were tested. The Dynamic Load Current Multiplier (DLCM) is one of those innovating concepts used to overcome the existing pulsed power generators limitations. It is made up of concentric electrodes (for autotransformer), a dynamic flux extruder (based on the implosion of cylindrical wire array) and a vacuum closing switch. In this these, the operating principle of the DLCM is theoretically analyzed through electrical and magneto hydrodynamic simulations. A specific study of the DLCM key component is performed. This is the vacuum closing switch. Thus, after a design phase using electrostatic simulation tools, two versions of switches are experimentally validated in conditions similar to those of a very high current shot. Finally, shots on the SPHINX facility located at the CEA Gramat, capable of delivering a current pulse of 6MA in 800ns (on Z-pinch load), are exposed to trace the evolution of this device. The convincing results are used, ultimately, to validate the DLCM concept connected to an isentropic compression experiment load.

This thesis presents experimental and modelling research on atmospheric pressure hollow cathodes and hollow electrodes. Experiments with the hybrid hollow electrode activated discharge (H-HEAD), which is a combination of a hollow cathode and a microwave plasma source, is presented. The experiments show that this source is able to produce long plasma columns in air and nitrogen at atmospheric pressure and at very low gas flow rates. Measurements of the vibrational temperature of the nitrogen molecules are also presented in this thesis. The vibrational temperature is an indication of the electron temperature in the plasma, an important characteristic of the plasma. Modelling work on the hollow cathode at atmospheric pressure with fluid equations is also presented. It is shown that the inclusion of fast and secondary electrons, characteristic of the hollow cathode plasmas, increases the sheath width. The sheath width was found to be of the order of 100 μm. By modelling the plasma as highly collisional by using the drift-diffusion approximation, it was shown that the increase in sheath thickness was larger at lower pressures than at higher pressures. Still, the sheath width can be of the order of 100 μm. A pulsed atmospheric plasma in a hollow electrode geometry was also modelled by the drift-diffusion fluid equations, with the addition of the energy equation for electrons. Rate and transport coefficients for the electrons were calculated from the solution to the Boltzmann equation as functions of mean electron energy. The dynamics of the plasma at pulse rise time showed large electron density and mean energy peaks at the cathode ends, but also that these quantities were enhanced at the centre of the discharge, between the cathode plates.

Pulsed Detonation Engines (PDEs) have the potential to revolutionize fight by better utilizing the chemical energy content of reactive fuel/air mixtures over conventional combustion processes. Combustion by a super-sonic detonation wave results in a significant increase in pressure in addition to an increase in temperature. In order to harness this pressure increase and achieve a high power density, it is desirable to operate PDEs at high frequency. The process of detonation initiation impacts operating frequency by dictating the length of the chamber and contributing to the overall cycle time. Therefore a key challenge in the development of a practical PDEs is the requirement to rapidly initiate a detonation in hydrocarbon-air mixtures. This thesis evaluates the influence of spark energy and airflow velocity on this challenging initiation process. The influence of spark energy, number of sparks and airflow velocity on Deflagration-to-Detonation Transition (DDT) was studied during cyclic operation of a small-scale PDE at the General Electric Global Research Center. Experiments were conducted in a 50 mm square transitioning to cylindrical channel PDE with optical access operating with stoichiometric ethylene-air mixture. Total spark energy was varied from 250 mJ to 4 J and was distributed between one and four spark plugs located in the same axial location. Initial flame acceleration was imaged using high-speed shadowgraph and was characterized by the time to reach 20 cm from the spark plug. Measurements of detonation wave velocity and emergence time, the time it takes the detonation wave to exit the tube, was measured using dynamic pressure transducers and ionization probes. It was found that the flame front spread was faster at higher spark energies and with more spark locations. Initial flame acceleration was 16% faster for the 4-spark, 4 J case when compared to the baseline 1-spark, 1 J case. When looking at the effect of airflow on the influence of spark energy, it was found that airflow had a larger effect on emergence time at high energies, versus energies less than 1 J. Finally, for a selected case of 0.25 J spark energy and 4 sparks, the velocity of the fuel-air mixture during fill was found to have a varying influence on detonation initiation and emergence time.

<p>Multiple industry applications, including combustion, flow control, and medicine, have leveraged nanosecond pulsed plasma (NPP) discharges to create plasma generated reactive species (PGRS). The PGRS are essential to induce plasma-assisted mechanisms, but the rate of generation and permanence of these species remains complex. Many of the mechanisms surrounding plasma discharge have been discovered through experiments, but a consistent challenge of time scales limits the plasma measurements. Thus, a well-constructed model with experimental research will help elucidate complex plasma physics. The motivation of this work is to construct a feasible physical model within the additional numerical times scale limitations and computational resources. This thesis summarizes the development of a one-moment fluid model for NPP discharges, which are applied due to their efficacy in generating ionized and excited species from vacuum to atmospheric pressure. </p><p>From a pulsed power perspective, the influence of pulse parameters, such as electric field intensity, pulse shape and repetition rate, are critical; however, the effects of these parameters on PGRS remain incompletely characterized. Here, we assess the influence of pulse conditions on the electric field and PGRS computationally by coupling a quasi-one-dimensional model for a parallel plate geometry, with a Boltzmann solver (BOLSIG+) used to improve plasma species characterization. We first consider a low-pressure gas discharge (3 Torr) using a five-species model for argon. <a>We then extend to a 23 species model with a reduced set of reactions for air chemistry remaining at low pressure.</a> The foundations of a single NPP is first discussed to build upon the analysis of repeating pulses. Because many applications use multiple electric pulses (EPs) the need to examine EP parameters is necessary to optimize ionization and PGRS formation. </p><p>The major goal of this study is to understand how the delivered EP parameters scale with the generated species in the plasma. Beginning with a similar scaling study done by Paschen we examine the effects of scaling pressure and gap length when the product remains constant for the two models. This then leads to our study on the relationship of pulsed power for different voltages and pulse widths of EPs. By fixing the energy delivered to the gap for a single pulse we determine that the electron and ion number densities both increased with decreasing pulse duration; however, the rate of this increase of number densities appeared to reach a limit for 3 ns. These results suggest the feasibly of achieving comparable outputs using less expensive pulse generators with higher pulse duration and lower peak voltage. Lastly, we study these outcomes when increasing the number of pulses and discuss the effects of pulse repetition and the electron temperature.</p><p>Future work will extend this parametric study to different geometries (i.e. pin-to-plate, and pin-to pin) and ultimately incorporate this model into a high-fidelity computational fluid dynamics (CFD) model that may be compared to spectroscopic results under quiescent and flowing conditions will be discussed.<br></p>