<p>Epidural cortical stimulation (ECS) is a developing therapy for many neurological disorders. However, the mechanisms by which ECS has its effects are unknown, and this lack of understanding has limited the development and optimization of this promising therapy. This dissertation investigates the effects of ECS on the neurons in the cortex and how these effects vary with electrode geometry and location as well as the electrical and geometrical properties of the anatomy.</p><p>The effects of ECS on cortical neurons were investigated using a three dimensional computational model of the human precentral gyrus and surrounding anatomy. An epidural electrode was placed above the gyrus, and the model was solved using the finite element method. The outputs of the model included distributions of electric potential, the second spatial derivative of potential (activating function), and current density. The distributions of electric potential were coupled to compartmental models of cortical neurons to quantify the effects of ECS on cortical neurons. A sensitivity analysis was performed to assess how thresholds and distributions of activating function were impacted by changes in the geometrical and electrical properties of the model. In vivo experiments of epidural electrical stimulation of cat motor cortex were performed to measure the effects of stimulation parameters and electrode location on thresholds for evoking motor responses.</p><p>During ECS, the region of cortex directly underneath the electrode was activated at the lowest thresholds, and neurons deep in the sulcus could not be directly activated without coactivation of neurons located on the crowns or lips of the gyri. The thresholds for excitation of cortical neurons depended on stimulation polarity as well as the orientation and position of the neurons with respect to the electrode. In addition, the patterns and spatial extent of activation were influenced by the geometry of the cortex and surrounding layers, the dimensions of the electrodes, and the positioning of the lead. In vivo thresholds for evoking motor responses were dependent on electrode location and stimulation polarity, and bipolar thresholds were often different from monopolar thresholds through the respective anode and cathode individually. The effects of stimulation polarity and electrode location on thresholds for evoking motor responses paralleled results of the computational model. Experimental evidence of indirect effects of ECS, mediated by synaptic interactions between neural elements, revealed an opportunity for further development of the computational model. The outcome of this dissertation is an improved understanding of the factors that influence the effects of ECS on cortical neurons, and this knowledge will help facilitate the rational implantation and programming of ECS systems.</p> / Dissertation

This work is concerned with the response of a Blasius boundary layer to dielectric-barrier-discharge (DBD) plasma actuators for the purpose of using these devices in bypass transition control. The plasma actuators consist of a spanwise-periodic array of high voltage electrodes, which are oriented to produce streamwise vortex pairs. The structure of actuator-induced streaks is measured using hot-wire anemometry over a streamwise distance of approximately 100 boundary layer thicknesses, and is decomposed into 4 spanwise Fourier modes. The modal content and corresponding streamwise growth characteristics are discussed for ten plasma actuator geometries over multiple excitation voltages and freestream velocities. Actuator power consumption was found to control the streak amplitude, whereas freestream velocity affected both amplitude and streamwise extent of the streaks. A common relationship between disturbance energy and power consumption was found among actuators of different dielectric thickness and similar electrode geometry.

This work is concerned with the response of a Blasius boundary layer to dielectric-barrier-discharge (DBD) plasma actuators for the purpose of using these devices in bypass transition control. The plasma actuators consist of a spanwise-periodic array of high voltage electrodes, which are oriented to produce streamwise vortex pairs. The structure of actuator-induced streaks is measured using hot-wire anemometry over a streamwise distance of approximately 100 boundary layer thicknesses, and is decomposed into 4 spanwise Fourier modes. The modal content and corresponding streamwise growth characteristics are discussed for ten plasma actuator geometries over multiple excitation voltages and freestream velocities. Actuator power consumption was found to control the streak amplitude, whereas freestream velocity affected both amplitude and streamwise extent of the streaks. A common relationship between disturbance energy and power consumption was found among actuators of different dielectric thickness and similar electrode geometry.

Dans l’état actuel des techniques utilisées pour l’isolation gazeuse des systèmes d’énergie électrique à haute tension, l’hexafluorure de soufre (SF6) tient une place prépondérante en raison de ses performances diélectriques et chimiques (bonne tenue diélectrique, point d'ébullition bas, stabilité chimique, non toxicité, etc.) ; il est l’un des meilleurs isolants gazeux connus à ce jour. Il est principalement utilisé dans les appareils de coupure pour l’extinction de l’arc électrique, les lignes de transmission à isolation gazeuse et autres équipements de puissance. Cependant, de par sa taille excessive, sa durée de vie trop importante et son grand effet radiatif, la molécule de SF6 constitue un agent aggravant de l’effet de serre avec un potentiel de réchauffement global (PRG ou GWP en anglais) très élevé (23900 fois supérieur à celui du CO2). Ainsi, les recommandations internationales (COP3) et européennes (règlement (UE) n°517/2014) tendent à restreindre très fortement, voire interdire son utilisation pour préserver l’environnement. Depuis, une tâche importante a été engagée par les industriels pour trouver d’autres gaz ou mélanges de substitution avec moins d’impact sur l’environnement et des exigences diélectriques comparables ou supérieures à celles du SF6. Les candidats les plus prometteurs appartiennent tous au groupe des molécules poly-fluorées (CF3I, Perfluorinated Ketones, Octafluorotetra-hydrofuran, Hydrofluoroolefin (HFOs) ou heptafluoro-iso-butyronitrile(Fluoronitriles)), Ils offrent tous une tenue diélectrique entre 1.2 et 2.71 par rapport au SF6. Ces molécules candidates ouvrent des perspectives intéressantes pour la substitution de SF6 dans l’appareillage sous enveloppe métallique destiné pour le réseau T&D haute tension. Leur principal inconvénient réside dans leur masse moléculaire élevée, ce qui implique une température de fonctionnement plus élevée par rapport au SF6. Ce travail porte sur l’étude expérimentale des performances diélectriques des mélanges de Fluoronitriles-CO2. Les paramètres intrinsèques de l’avalanche électronique sont identifiés à partir des courants de la décharge stationnaire de Townsend. L’évaluation de ces courants a permis d’extraire le coefficient d’ionisation effectif pour différents mélanges de Fluoronitriles. Une comparaison du potentiel d’isolation de ces mélanges avec celui du SF6 pur, dans différentes géométries électrodes (sphère-plan, pointe-plan, sphère-sphère, système avec le profil de Bruce), sur une large gamme de pression et pour toutes les formes d'ondes de tension normalisées pour d'isolation est présentée. Les résultats de mesures de tensions de claquage de ces mélanges et du CO2 pur, pour comparaison, dans un système coaxial cylindrique de taille réelle, similaire à celui utilisé dans les postes à enveloppe métallique (GIS - Gas Insulated Switchgear), sont également présentés ; une étude de l’influence de la surface et de l’état de surface des électrodes sur la tenue diélectrique en fonction de différents paramètres (pression, rugosité, surface effective, forme de tension et polarité, température de gaz …) est faite. / SULPHUR hexafluoride, SF6, is the most common compressed gas used in high voltage power equipment since the 1950s and in HV transmissions and substations applications (GIS, GCB, GIL …), SF6 holds a prominent place because of its dielectric and chemical performance (high dielectric strength, non-toxicity, low condensation temperature, thermal stability, non-flammability, chemical inactivity with the other constituent materials of the apparatus, availability and moderate cost). It is one of the best insulators gas known today. Despite its excellent properties in both electrical insulation and current interruption performance, the excessive size, the radiative effect and atmospheric lifetime of SF6 molecule makes this gas an aggravating agent for the greenhouse gas effect, with a very high global warming potential (GWP) (23900 times higher than CO2). Thus, the international and European recommendations (COP 3) and (Regulation (EU) No 517/2014) respectively tend to strongly restrict or prohibit its use to preserve the environment. Since then, an important task was undertaken by manufacturers to find other gases or mixtures of substitution with less impact on the environment and dielectric performances comparable or superior to those of SF6. Naturally, the investigations were oriented towards halogenated products that have reduced GWP such as CF3I, Perfluorinated Ketones, Octafluorotetra-hydrofuran, Hydrofluoroolefin (HFOs) or heptafluoro-iso-butyronitrile (Fluoronitriles)), all offer a dielectric strength between 1.2 and 2.71 relative to SF6. These candidates open interesting perspectives for the substitution of SF6 in the GIS applications designed for high voltage T & D network. Their main disadvantage is their high molecular weight, which implies a higher operating temperature compared to SF6. This work deals with the experimental study of dielectric performance of Fluoronitriles CO2 mixtures. Intrinsic parameters of the measured steady state Townsend swarm currents are identified. The evaluation of the currents produce the effective ionization rate constant in different Fluoronitriles-CO2 mixtures. In addition, this work provides a conventional assessing of insulating performance with typical breakdown experiments, conducted for different field configurations over a wide pressure range and for all standard voltage waveforms. The experiments are conducted with different electrodes geometries namely plane-to-plane (Bruce profile), sphere-to-sphere, sphere-to-plane and rod-to-plane. AC and LI breakdown characteristics of CO2 and different Fluoronitriles /CO2 mixtures gas were experimentally analyzed in a real scale GIS coaxial test system. The main investigated parameters are the effect of roughness and effective surface area HV inner conductor on insulation performance, depending on various parameters (pressure, form and voltage polarity, gas temperature…).