The magnetospheric cusps are two funnel-like regions of Earth's magnetosphere where solar wind plasma can have direct access to the ionosphere. The cusps are very dynamic regions where wave-particle interactions continuously take place and redistribute energy among different particle populations. In this thesis, both low and high frequency plasma waves in the cusp have been studied in detail using data from the Cluster spacecraft mission. The waves were studied with respect to frequency, Poynting flux and polarization. Wavelengths have also been estimated using multi-spacecraft techniques. At low frequencies, kinetic Alfvén waves and nonpotential ion cyclotron waves are identified and at high frequencies, electron cyclotron waves, whistler waves, upper-hybrid and RX-waves are observed. A common generation mechanism called the shell-instability is proposed for several of the wave modes present in the cusp, both at low and high frequencies. The plasma in the cusp is shown to be strongly inhomogeneous. In an inhomogeneous low-frequency magnetoplasma, kinetic Alfvén waves couple to drift-waves. Such drift-kinetic Alfvén waves have long been believed to nonlinearly self-interact and form coherent structures in the form of drift-kinetic Alfvén vortices. In this thesis the first unambiguous direct measurements confirming the existence of such vortices in a turbulent space plasma are presented. Some of the crucial parameters such as the vortex radius are determined. Plasma theory is electrodynamics applied to a large collection of charged particles. In this thesis a new way of looking at the fundamental Maxwell tensor is presented. A covariant spectral density tensor containing information on electromagnetic waves is formed. This tensor is then decomposed into irreducible components by using the spinor formalism for an arbitrary metric. The obtained fundamental tensors are shown to correspond both to well known tensors in Maxwell's theory, as well as several physically interesting new tensors.

A space camera is a promising candidate to address the non-stop rising interest for astrophysics research in the Compton regime. The MeVCube mission is intended to be launched in 2022, hosting an on-board Compton Camera. To better support the development of the instrument in this early stage, a series of feasibility studies to assess two potential launch orbits were performed. The studies were composed by a series of mission analysis simulations which permitted the characterisation of the orbital environments for the two orbital options. Several sources of background noise to the instrument were identified. The population of trapped protons and trapped electrons were simulated for the periods of Solar Minimum and Solar Maximum, as well as the levels of Galactic Cosmic Ray (GCR) flux. The performance of trade-off studies concluded that an equatorial orbit is more preferable for reducing the influence of background noise. To better estimate the environment effects at the equatorial orbit, the number of particles which can penetrate the detector shielding were simulated. The next step was to perform a series secondary studies whose aim were to simulate the induced current on the electrodes, produced by the interactions occurring within the detector. The actualisation of these simulations required the study of photon interaction with matter, the various Cadmium-Zink-Telluride (CZT) types and the how they operate, and the use of a sophisticated software to perform the appropriate simulations. COMSOL, which allows the method of FEA, was chosen as the tool to perform the simulations. The geometry of the detector voxel was primarily designed in SIEMENS NX. The geometry was inserted into COMSOL, where a number of iterations were performed to finalise the appropriate mesh size, which ensured an accurate representation of the Electric field and the Weighting potential within the detector voxel. The induced current on the electrodes was decided to be calculated via MATLAB. As a verification step it was thought useful to firstly plot the weighting potential of the three electrodes under test; the chosen anode pixel, the steering grid and the cathode. The process revealed a series of numerical errors, most likely introduced by the type of mesh chosen or by the data manipulation process via MATLAB. Significant reduction of the numerical errors would lead to more accurate values for the induced current. Unfortunately, due to time constraints this was a task that was not completed. Solving this problem would be optimal for future studies with MATLAB, as the induced current on the electrodes can be correctly calculated based on charge transport within the detector bulk. / MeVCube, DESY

<p>  </p> <p>  </p> <p>An approach to modify surface flashover of insulators in vacuum by limiting duration of its high-current stage responsible for the damaging effects of a classic flashover was developed. The flashover assembly was made by TorrSeal-gluing copper electrodes (10 x 10 x 0.5 mm) to both side of an alumina ceramic sheet (0.635 mm thick). The modified flashover, referred to as low energy surface flashover (LESF), was achieved by utilization of a high voltage (HV) nanosecond pulser or addition of a resistor in series with the LESF assembly when HV DC was utilized. The duration of LESF was visualized by ICCD fast photography to be 100 – 200 ns accompanying electrical characteristics measurements, which gave insight of a way to control the flashover duration by inserting additional capacitor in parallel with the LESF assembly to increase the stored energy prior to breakdown. The LESF assembly was tested for > 1.5 million consecutive pulses and remained operational, while operation in high energy regime with parallel capacitor (4nF) lead to significant damage after 200 pulses.</p> <p>The igniting capabilities of LESF assembly was demonstrated via successful triggering of vacuum arc and a prototype pulsed plasma accelerator. The plasma plume propagation speed and angular distribution was measured via Langmuir probes. Efforts were made for temporally resolved spectroscopy measurements. </p> <p>The LESF assembly was improved by replacing TorrSeal-gluing with direct bonding of copper to alumina ceramic and changing the configuration from parallel plate to coaxial. The improved assembly was demonstrated to be operational throughout and after an extended test of 10 million pulses. A higher resolution ICCD photography revealed finer LESF discharge features including initial bright line across the insulator developing into a double-jet plasma plume propagating at around 10⁵m/s and later-on point-like attachment of the discharge column to the electrodes. The composition of the plasma and erosion pattern on the LESF assembly was studied via SEM/EDX analysis, which supported the predominant ceramic erosion over copper electrodes erosion.</p>

Cette thèse est consacrée au développement d'un modèle End-to-End pour le spectrocalorimètre X-IFU qui observera à partir de 2028 l'Univers en rayons X avec une précision jamais atteinte auparavant. Ce travail s'est essentiellement organisé en deux parties. J'ai dans un premier temps étudié la dynamique des parties les plus internes des binaires X de faible masse à l'aide de deux sondes particulières que sont les sursauts X et les oscillations quasi-périodiques au kHz (kHz QPOs). En me basant sur les données d'archive du satellite Rossi X-ray Timing Explorer et sur des méthodes d'analyse spécifiquement développées dans ce but, j'ai notamment pu mettre en évidence pour la première fois une réaction du premier sur le second, confirmant le lien très étroit entre ces oscillations et les parties les plus internes du système. Le temps de rétablissement du système suite aux sursauts entre également en conflit dans la plupart des cas avec l'augmentation supposée du taux d'accrétion suite à ces explosions. Au travers d'une analyse spectro-temporelle complète des deux kHz QPOs de 4U 1728-34, j'ai également pu confirmer l'incompatibilité des spectres de retard des deux QPOs qui suggère une origine différente de ces deux oscillations. L'étude de leurs spectres de covariance, obtenus pour la première fois dans cette thèse, a quant à elle mis en évidence le rôle central de la couche de Comptonisation et potentiellement celui d'une zone particulièrement compacte de la couche limite pour l'émission des QPOs. Dans le second volet de ma thèse, j'ai développé un simulateur End-to-End pour l'instrument X-IFU permettant de représenter l'ensemble du processus menant à une observation scientifique en rayons X, de l'émission des photons par une source jusqu'à leur mesure finale à bord du satellite. J'ai notamment mis en place des outils permettant la comparaison précise de plusieurs matrices de détecteurs en prenant en compte les effets de la reconstruction du signal brut issu des électroniques de lecture. Cette étude a mis en évidence l'intérêt de configurations hybrides, contenant une sous-matrice de petits pixels capables d'améliorer par un ordre de grandeur la capacité de comptage de l'instrument. Une solution alternative consisterait à défocaliser le miroir lors de l'observation de sources ponctuelles brillantes. Situées au coeur de la performance du X-IFU, j'ai également comparé de manière exhaustive différentes méthodes de reconstruction des signaux bruts issus des détecteurs X-IFU. Ceci a permis de montrer qu'à faible coût en termes de puissance de calcul embarquée, une amélioration significative de la résolution en énergie finale de l'instrument pouvait être obtenue à l'aide d'algorithmes plus sophistiqués. En tenant compte des contraintes de calibration, le candidat le plus prometteur apparaît aujourd'hui être l'analyse dans l'espace de résistance. En me servant de la caractérisation des performances des différents types de pixels, j'ai également mis en place une méthode de simulation rapide et modulable de l'ensemble de l'instrument permettant d'obtenir des observations synthétiques à long temps d'exposition de sources X très complexes, représentatives des futures capacités du X-IFU. Cet outil m'a notamment permis d'étudier la sensibilité de cet instrument aux effets de temps mort et de confusion, mais également d'estimer sa future capacité à distinguer différents régimes de turbulence dans les amas de galaxies et de mesurer leur profil d'abondance et de température. A plus long terme ce simulateur pourra servir à l'étude d'autres cas scientifiques, ainsi qu'à l'analyse d'effets à l'échelle de l'ensemble du plan de détection tels que la diaphonie entre pixels. / This thesis is dedicated to the development of an End-ta-End model for the X-IFU spectrocalorimeter scheduled for launch in 2028 on board the Athena mission and which will observe the X-ray universe with unprecedented precision. This work has been mainly organized in two parts. I studied first the dynamics of the innermost parts of low mass X-ray binaries using two specific probes of the accretion flow: type I X-ray bursts and kHz quasi-periodic oscillations (kHz QPOs). Starting from the archivai data of the Rossi X-ray Timing Explorer mission and using specific data analysis techniques, I notably highlighted for the first time a reaction of the latter to the former, confirming the tight link between this oscillation and the inner parts of the system. The measured recovery time was also found in conflict with recent claims of an enhancement of the accretion rate following these thermonuclear explosions. From the exhaustive spectral timing analysis of both kHz QPOs in 4U 1728-34, I further confirmed the inconsistancy of their lag energy spectra, pointing towards a different origin for these two oscillations. The study of their covariance spectra, obtained here for the first time, has revealed the key role of the Comptonization layer, and potentially of a more compact part of it, in the emission of the QPOs. In the second part of my thesis, I focused on the development of an End-to-:End simulator for the X-IFU capable of depicting the full process leading to an X-ray observation, from the photon emission by the astrophysical source to their on-board detection. I notably implemented tools allowing the precise comparison of different potential pixel array configurations taking into account the effects of the event reconstruction from the raw data coming from the readout electronics. This study highlighted the advantage of using hybrid arrays containing a small pixel sub-array capable of improving by an order of magnitude the count rate capability of the instrument. An alternative solution would consist in defocusing the mirror during the observation of bright point sources. Being a key component of the overall X-IFU performance, I also thoroughly compared different reconstruction methods of the pixel raw signal. This showed that with a minimal impact on the required on-board processing power, a significant improvement of the final energy resolution could be obtained from more sophisticated reconstruction methods. Taking into account the calibration constraints, the most promising candidate currently appears to be the so-called "resistance space analysis". Taking advantage of the obtained performance characterization of the different foreseen pixel types, I also developed a fast and modular simulation method of the complete instrument providing representative synthetic observations with long exposure times of complex astrophysical sources suffinguish different turbulence regimes in galaxy clusters and to measure abundance and temperature profiles. In the longer run, this simulator will be useful for the study of other scientific cases as well as the analysis of instrumental effects at the full detection plane level such as pixel crosstalk.

With the increase in speculation about the nature of our universe, there has been a growing need to find the truth about Dark Matter. Recent research shows that the Planck-Mass range could be a well-motivated space to probe for the detection of Dark Matter through gravitational coupling. This thesis dives into the possibility of doing the same in two parts. The first part lays out the analysis framework that would sense such an interaction, while the second part outlines a prototype experiment that when scaled up using quantum optomechanical sensors would serve as the skeleton to perform the analysis with.