Synthesis and capture of antihydrogen in controlled laboratory conditions will enable precise studies of neutral antimatter. The work presented deals with some of the physics pertinent to manipulating charged antiparticles in order to create neutral antimatter, and may be applicable to other scenarios of plasma confinement and charged particle interaction. The topics covered include the electrostatic confinement of a reflecting ion beam and the transverse confinement of an ion beam in a purely electrostatic configuration; the charge sign effect on the Coulomb logarithm for a two component (e.g., antihydrogen) plasma in a Penning trap as well as the collisional scattering for binary Coulomb interactions that are cut off at a distance different than the Debye length; and the formation of magnetobound positronium and protonium.

L’expérience GBAR propose de mesurer, au CERN, l’accélération de la pesanteur terrestre sur l’antimatière grâce à des atomes froids (neV) d’antihydrogène soumis à une chute libre. Ceux-ci sont obtenus en refroidissant d’abord des ions positifs d’antihydrogène, obtenus grâce à deux réactions consécutives se produisant lors de la collision d’un faisceau d’antiprotons avec un nuage dense de positronium.Le travail de thèse porte sur l'étude de ces réactions dans le but d’optimiser la production des ions d’antihydrogène. Pour cela, les sections efficaces des deux réactions ont été calculées dans le cadre d’un modèle de théorie des perturbations (Continuum Distorted Wave – Final State) pour des antiprotons ayant une énergie comprise entre 0 et 30 keV ; différents états excités du positronium ont été examinés. Ces sections efficaces ont ensuite été intégrées à une simulation de la zone d’interaction entre positronium et antiprotons afin de définir les paramètres expérimentaux optimaux pour GBAR. Les résultats suggèrent d’utiliser les états 2P, 3D ou, dans une moindre mesure, 1S du positronium, respectivement pour des antiprotons de 2, moins de 1 ou 6 keV. L’importance de compresser les impulsions temporelles d’antiprotons est soulignée ; le positronium devra être confiné dans un tube de 20 mm de long pour 1 mm de diamètre.Un laser en impulsion à 410 nm permettant d’exciter la transition à deux photons vers l’état 3D du positronium avait déjà été proposé. Son principe repose sur le doublage en fréquence d’un laser titane-saphir à 820 nm. Le dernier volet de la thèse fut dédié à la réalisation de ce laser, qui délivre des impulsions courtes (9 ns) de 4 mJ à 820 nm. / The future CERN experiment called GBAR intends to measure the gravitational acceleration of antimatter on Earth using cold (neV) antihydrogen atoms undergoing a free fall. The experiment scheme first needs to cool antihydrogen positive ions, obtained thanks to two consecutive reactions occurring when an antiproton beam collides with a dense positronium cloud.The present thesis studies these two reactions in order to optimise the production of the anti-ions. The total cross sections of both reactions have been computed in the framework of a perturbation theory model (Continuum Distorted Wave – Final State), in the range 0 to 30 keV antiproton kinetic energy; several excited states of positronium have been investigated. These cross sections have then been integrated to a simulation of the interaction zone where antiprotons collide with positronium; the aim is to find the optimal experimental parameters for GBAR. The results suggest that the 2P, 3D or, to a lower extend, 1S states of positronium should be used, respectively with 2, less than 1 or 6 keV antiprotons. The importance of using short pulses of antiprotons has been underlined; the positronium will have to be confined in a tube of 20 mm length and 1 mm diameter.In the prospect of exciting the 1S-3D two-photon transition in positronium at 410 nm, a pulsed laser system had already been designed. It consists in the frequency doubling of an 820 nm pulsed titanium-sapphire laser. The last part of the thesis has been dedicated to the realisation of this laser system, which delivers short pulses (9 ns) of 4 mJ energy at 820 nm.

AEGIS (Antimatter Experiment, Gravity, Interferometry and Spectroscopy) isan experiment under development at CERN which will measure earth's gravitationalforce on antimatter. This will be done by creating a horizontal pulsedbeam of low energy antihydrogen, an atom consisting of an antiproton anda positron. The experiment will measure the vertical de ection of the beamthrough which it is possible to calculate the gravitational constant for antimatter.To characterise the production process in the current state of the experimentit is necessary to develop an imaging detector for single excited hydrogenatoms. This thesis covers the design phase of that detector and includes studiesand tests of detector components. Following literature studies, tests and havingdiscarded several potential designs, a baseline design was chosen. The suggesteddetector will contain a set of ionising rings followed by an electron multiplyingmicrochannel plate, a light emitting phosphor screen, a lens system and nallya CCD camera for readout. The detector will be able to detect single hydrogenatoms, measure their time of ight as well as being able to image electronplasmas and measure the time of ight of the initial particles in such a plasma.Tests were made to determine the behaviour of microchannel plates at the lowtemperatures used in the experiment. Especially, the resistance and multiplicationfactor of the microchannel plates have been measured at temperaturesdown to 14 K. / AEGIS

AEgIS is an experiment at CERN where the goal is to directly measure the gravitational force on antimatter by producing antihydrogen. The antihydrogen will be produced by a charge exchange reaction using laser excited positronium and cold antiprotons. Having a well-characterized positron plasma with at least 108 positrons and knowing how it can be controlled is essential for the positronium production. This thesis is based on the goals of AEgIS experiment and describes the positron plasma manipulations being used in AEgIS in order to achieve the required plasma properties for the experiment. The positron system is made up by a source, a Surko trap and a Penning-Malmberg trap. This system was first optimized to increase the number of positrons. The plasma was then moved to the main traps of the experiment where it was systematically characterized in terms of lifetime, cooling efficiency and compression. Positron plasma compression in time, trapping and cooling was tested for the first time in AEgIS using a buncher and Penning-Malmberg traps respectively. In this thesis, it is shown that a compression of more than 50 % in time of the positron cloud using a buncher can be achieved. It is also shown that trapping and cooling with an efficiency of nearly 100 % in the main traps using a “V” shaped potential trap was successful. On top of that, the lifetime inside this “V” shaped potential trap was observed to be longer than 30 minutes.

L’expérience GBAR repose sur la production d’ions antihydrogène positifs dans le but de mesurer l’accélération gravitationnelle à laquelle est soumise l’antimatière au repos. Le projet ANTION, sous-projet de GBAR, a pour but la production de ces ions d’antimatière. Il vise également à mesurer la section efficace de production d’antihydrogène dans les collisions d’antiprotons sur des atomes de positronium, ainsi que les sections efficaces correspondantes avec la matière, de production d’hydrogène et de l’ion hydrogène négatif. Ces expériences reposent sur la formation d’un nuage très dense de positronium, et nécessitent donc une grande quantité de positons qui seront implantés sur un matériau convertisseur de positons en positronium. Cette thèse décrit la construction d’un piège à “buffer gas” à trois étages, destiné à piéger et accumuler des positons pour le projet ANTION. L’association d’un piège de Penning avec une source basée sur un Linac constitue un montage expérimental unique. Le piège a été construit et optimisé, et est maintenant pleinement opérationnel. Les protocoles de piégeage ont été étudiés et les effets du gaz tampon et du gaz de refroidissement sur le taux de piégeage et la durée de vie des positons ont été quantifiés. Afin de faciliter la mesure de la section efficace de production de l’hydrogène, une simulation avec GEANT4 a été mise au point. Elle décrit l’évolution temporelle et spatiale des atomes d’ortho-positronium dans la cavité où aura lieu la production d’hydrogène. On estime que 2.7 atomes d’hydrogène sont produits pour des proton de 6 keV d’énergie incidente, en utilisant les sections efficaces calculées avec le modèle “Coulomb-Born Approximation”, et 1.6 atomes d’hydrogène pour des protons de 10 keV, si l’on utilise la méthode “two-center convergent close-coupling”. Les simulations permettent également d’estimer le bruit de fond associé aux positons et à l’annihilation du para-positronium. Cette étude amène à proposer une modification permettant d’augmenter le nombre d’atomes de positronium dans la cavité. En parallèle, une étude a porté sur l’efficacité de modération de positons d’une couche épitaxiale de carbure de silicium 4H-SiC. Une efficacité de modération de 65% a été mesurée pour des positons implantés avec une énergie de l’ordre du kilo- électronvolt. Ce résultat intéresse les expériences de physique utilisant des positons lents, car il permet d’améliorer la luminosité de faisceaux de positons; dans le cas de GBAR cela permettrait d’augmenter l’efficacité de piégeage des positons. / The GBAR experiment relies on the production of antihydrogen positive ions to achieve its goal of measuring the gravitational acceleration of antimatter at rest. The ANTION project, included in the GBAR enterprise, is responsible for the production of these antimatter ions. Moreover, it also aims to measure the cross section of antihydrogen production throughout the collision of antiprotons and positronium atoms, as well as the matter cross sections of hydrogen and the hydrogen negative ion. These experiments imply the formation of a very dense positronium cloud, thus a large amount of positrons will be implanted on a positron/positronium converter material. This thesis reports the construction of a three stage buffer gas trap with the goal of trapping and accumulating positrons for the ANTION project. The combination of the Penning-type trap with a LINAC source constitutes a unique experimental setup. The trap was commissioned and optimized and is now fully operational. Trapping protocols were studied and the effect of the buffer and cooling gases on the positron trapping rate and lifetime was assessed. In order to assist the cross section measurement of hydrogen, a GEANT4 simulation was developed. It evaluates the time and spatial evolution of the ortho-positronium atoms in a cavity, where hydrogen production will take place. It was estimated that 2.7 hydrogen atoms are produced for proton impact energy of ∼ 6 keV, according to the cross sections computed with the Coulomb-Born Approximation model, and 1.6 hydrogen atoms for a proton impact energy of ∼ 10 keV, according to the two-center convergent close-coupling method. The simulations also allow the estimation of the background associated with the positron and para-positronium decay. In addition, a suggestion is proposed to increase the number of positronium atoms in the cavity. In parallel, the positron moderation efficiency of a commercially available 4H-SiC epitaxial layer was studied. A 65% moderation efficiency was observed for kiloelectronvolt implanted positrons. This result can be of interest to slow positron physics experiments by improving the brightness of positron beams, and in particular to GBAR as it can potentially increase the efficiency of positron trapping.

Depuis les années 1975, les atomes de Rydberg sont étudiés et maintenant utilisés en information quantique pour leurs propriétés particulières d’interaction. Cependant, ces objets physiques peuvent se retrouver impliqués dans différentes autres applications, où leurs caractéristiques remarquables en font de parfaits outils. Dans ce mémoire, nous nous intéresserons à deux applications distinctes faisant intervenir des atomes de Rydberg de césium. Tout d’abord, nous verrons comment utiliser de tels atomes pour produire une source d’électrons monocinétiques, grâce au mécanisme d’ionisation singulier de ce type d’atomes à une valeur précise de champ électrique dépendante du niveau d’excitation. Les électrons ainsi produits sont ensuite extraits et leur dispersion en énergie mesurée. On montrera notamment de façon théorique et d’après les premières mesures expérimentales réalisées pendant la thèse, que l’on peut espérer obtenir une dispersion en énergie des électrons produits par cette technique de l’ordre du meV, résolution jamais atteinte à ce jour. Ce type de source devient aujourd’hui un outil indispensable pour accéder à la mise au point et l’étude de nouveaux matériaux par contrôle de réactions chimiques à l’échelle moléculaire, et à la cartographie des phonons. Dans un second temps, nous verrons qu’il est possible de désexciter un nuage d’atomes de Rydberg de niveaux variés grâce à une source externe dans le domaine térahertz. Ce projet s’inscrit dans le cadre des expériences d’étude de l’antimatière menées actuellement au CERN, qui visent à élucider le mystère de l’asymétrie matière/antimatière. Les méthodes actuelles de production de l’antihydrogène, forment des nuages de ces anti-atomes dans différents états de Rydberg. Pour les étudier, il est alors nécessaire de désexciter le plus d’atomes d’antihydrogène possible vers le niveau fondamental. Nous présenterons la méthode envisagée, ainsi que les résultats obtenus expérimentalement sur un dispositif créé pendant la thèse pour montrer la faisabilité de la technique. Ces premiers résultats montrent qu’il est possible d’accélérer la désexcitation d’un atome de Rydberg sur un état très élevé grâce à une lampe se comportant comme un corps noir. Nous détaillerons les améliorations envisagées, en particulier pour adapter le spectre des fréquences THz à utiliser et empêcher la photoionisation des atomes, par des filtres ou par le façonnage spectral via l’utilisation d’un photomixer. / Since 1975, Rydberg atoms have been studied and now used in quantum information for their particular interaction properties. However, these physical objects can be involved in various other applications, where their remarkable characteristics make them perfect tools. In this paper, we will focus on two distinct applications involving cesium Rydberg atoms. First, we will see how to use such atoms to produce a source of monocinetic electrons, thanks to the singular ionization mechanism of this type of atoms at a precise value of electric field dependent on the excitation level. The electrons thus produced are then extracted and their energy dispersion measured. Theoretically and according to the first experimental measurements made during the thesis, we will show that we can hope an energy dispersion of the electrons produced by this meV technique, a resolution never reached before. Today, this type of source is becoming an indispensable tool for the development and study of new materials by molecular scale chemical reaction control and for phonon mapping. In a second step, we will see that it is possible to de-energize a cloud of Rydberg atoms of various levels thanks to an external source in the tera-hertz domain. This project is part of the ongoing anti-matter experiments at CERN, which aim to unravel the mystery of the matter/anti-matter asymmetry. The current methods of production of antihydrogen, forms clouds of these anti-atoms in different Rydberg states. To study them, it is then necessary to de-energize as many antihydrogen atoms as possible to the fundamental level. We will present the method envisaged, as well as the results obtained experimentally on a device created during the thesis to show the feasibility of the technique. These first results show that it is possible to accelerate the deenergization of a Rydberg atom on a very high state thanks to a lamp behaving like a black body. We will detail the improvements envisaged, in particular to adapt the spectrum of the THz frequencies to use and prevent the photoionization of atoms, by filters or by spectral shaping via the use of a photomixer.

The production and control of antihydrogen at very low temperatures provided a key tool to test the validity for the antimaterial of the fundamental principles of the interactions of nature such as the weak principle of equivalence (WEP), and CPT symmetry (Charge, Parity, and Time reversal). The work presented in this dissertation studies the collisions of electrons and positrons in strong magnetic fields that generate magnetobound positronium (positron-electron system temporarily bound due to the presence of a magnetic field) and its possible role in the generation of antihydrogen.

The ALPHA experiment at CERN is an ongoing project which tests fundamental symmetries between matter and antimatter by producing and trapping antihydrogen atoms in order to perform precision spectroscopic measurements. A logical next step is to form the antihydrogen molecular ion (consisting of one positron and two antiprotons). This system possesses net charge, and can therefore be trapped electrostatically, making repeated measurements possible. Moreover it has been suggested that the molecule has the potential to allow for higher-precision comparisons with ordinary matter than have been attained with the atom. Since both momentum and energy have to be conserved in a collision, a simple collision process between an antihydrogen atom (“Hbar”) and an antiproton (“pbar”) does not suffice in order to form the molecular ion. However it is possible, upon mixing of the two species, for a pbar colliding with an Hbar in the ground electronic state to form a metastable molecular state (i.e., a resonance) which is weakly coupled to a stable molecular state (i.e., a bound state) via spontaneous quadrupole transition. During the time a metastable ion exists, a second pbar can happen to undergo a Coulomb collision with the metastable molecular ion. The quadrupole electrostatic interaction with this secondary antiproton acts as a time-dependent perturbation on the molecular system which can strengthen the coupling between resonance and bound state. Hence a collision with a secondary pbar can induce a transition to a bound state whereby the excess energy is carried off by the secondary pbar. This work aims to determine the efficiency of the process just described. On the theoretical side, the following is done: a study is conducted on the topic of resonance scattering as it relates to the problem in consideration; building on this study a generalized time-dependent perturbation theory is constructed which is valid for transitions to and from resonant states as well as bound states. On the numerical side: the effective potential for pbar-Hbar scattering in the ground electronic state is obtained numerically within the adiabatic approximation; the energies and lifetimes of the resonant states of the molecular ion are estimated; a temperature-dependent rate coefficient is obtained for the process described which, in order to obtain a proper rate, needs to be multiplied by the square of the density of the antiproton plasma and by the number of antihydrogen atoms. It is concluded that at current capacity for trapping and storage of pbar and Hbar the process examined is not competitive with respect to other formation routes which have been proposed for the molecular ion.

Comparisons between measurements of the ground-state hyperfine structure and gravitational acceleration of hydrogen and antihydrogen could provide a test of fundamental physical theories such as CPT (charge conjugation, parity, time-reversal) and gravitational symmetries. Currently, antihydrogen traps are based on Malmberg-Penning traps. The number of antiprotons in Malmberg-Penning traps with sufficiently low energy to be suitable for trappable antihydrogen production may be reduced by the electrostatic space charge of the positrons and/or collisions among antiprotons. Alternative trap designs may be needed for future antihydrogen experiments. A computational tool is developed to simulate charged particle motion in customizable magnetic fields generated by combinations of current loops and current lines. The tool is used to examine charged particle confinement in two systems consisting of dual, levitated current loops. The loops are coaxial and arranged to produce a magnetic null curve. Conditions leading to confinement in the system are quantified and confinement modes near the null curve and encircling one or both loops are identified. Furthermore, the tool is used to examine and quantify charged particle motion parallel to the null curve in the large radius limit of the dual, levitated current loops. An alternative to new trap designs is to identify the effects of the positron space in existing traps and to find modes of operation where the space charge is beneficial. Techniques are developed to apply the Boltzmann density relation along curved magnetic field lines. Equilibrium electrostatic potential profiles for a positron plasma are computed by solving Poisson's equation using a finite-difference method. Equilibria are computed in a model Penning trap with an axially varying magnetic field. Also, equilibria are computed for a positron plasma in a model of the ALPHA trap. Electric potential wells are found to form self-consistently. The technique is expanded to compute equilibria for a two-species plasma with an antiproton plasma confined by the positron space charge. The two-species equilibria are used to estimate timescales associated with three-body recombination, losses due to collisions between antiprotons, and temperature equilibration. An equilibrium where the three-body recombination rate is the smallest is identified.

Ma thèse s’est déroulée dans le cadre de la collaboration AEgIS, une des expériences étudiant l’antimatière au CERN. L’objectif final est de mesurer l’effet de la gravité sur un faisceau froid d’antihydrogène (Hbar). AEgIS se propose de créer les Hbar froids par échange de charges entre un atome de Positronium (Ps) excité (état de Rydberg) et un antiproton piégé : 〖Ps〗^*+ pbar → (H^*)⁻ + e⁻. L’étude de la physique du Ps est cruciale pour AEgIS, et demande des systèmes lasers adaptés. Pendant ma thèse, ma première tâche a été de veiller au bon fonctionnement des systèmes lasers de l’expérience. Afin d’exciter le positronium jusqu’à ses états de Rydberg (≃20) en présence d’un fort champ magnétique (1 T), deux lasers pulsés spectralement larges ont été spécialement conçu. Nous avons réalisé la première excitation par laser du Ps dans son niveau n=3, et prouvé une excitation efficace du nuage de Ps vers les niveaux de Rydberg n=16-17. Ces mesures, réalisées dans la chambre à vide de test d’AEgIS, à température ambiance et pour un faible champ magnétique environnant, sont la première étape vers la formation d’antihydrogène. Le prochain objectif est de répéter ces résultats dans l’enceinte du piège à 1 T, où les antihydrogènes seront formés. Pour autant, malgré l’excitation Rydberg des Ps pour accroître la section efficace de collision, la production d’antihydrogène restera faible, et la température des H bar formés sera trop élevée pour toute mesure de gravité. Pendant ma thèse, j’ai installé au CERN un autre système laser prévu pour pratiquer une spectroscopie précise des niveaux de Rydberg du Ps. Ce système excite des transitions optiques qui pourraient convenir à un refroidissement Doppler : la transition n=1 ↔ n=2. J’ai étudié la possibilité d’un tel refroidissement, en procédant à des simulations poussées pour déterminer les caractéristiques d’un système laser adapté La focalisation du nuage de Ps grâce au refroidissement des vitesses transverses devrait accroitre le recouvrement des positroniums avec les antiprotons piégés, et ainsi augmenter grandement la production d’Hbar. Le contrôle du refroidissement et de la compression du plasma d’antiprotons est aussi essentiel pour la formation des antihydrogènes. Pendant les temps de faisceaux d’antiprotons de 2014 et 2015, j’ai contribué à la caractérisation et l’optimisation des procédures pour attraper et manipuler les antiprotons, afin d’atteindre des plasmas très denses, et ce, de façon reproductible. Enfin, j’ai participé activement à l’élaboration d’autre projet à l’étude AEgIS, qui vise aussi à augmenter la production d’antihydrogène : le projet d’un refroidissement sympathique des antiprotons, en utilisant un plasma d’anions refroidis par laser. J’ai étudié la possibilité de refroidir l’ion moléculaire C₂⁻, et les résultats de simulations sont encourageants. Nous sommes actuellement en train de développer au CERN le système expérimental qui nous permettra de faire les premiers tests de refroidissement sur le C₂⁻. Si couronné de succès, ce projet ne sera pas seulement le premier résultat de refroidissement par laser d’anions, mais ouvrira aussi les portes à une production efficace d’antihydrogènes froids. / My Ph.D project took place within the AEgIS collaboration, one of the antimatter experiments at the CERN. The final goal of the experiment is to perform a gravity test on a cold antihydrogen (Hbar) beam. AEgIS proposes to create such a cold Hbar beam based on a charge exchange reaction between excited Rydberg Positronium (Ps) and cold trapped antiprotons: 〖Ps〗^* + pbar → (H^*)⁻ + e⁻. Studying the Ps physics is crucial for the experiment, and requires adapted lasers systems. During this Ph.D, my primary undertaking was the responsibility for the laser systems in AEgIS. To excite Ps atom up to its Rydberg states (≃20) in presence of a high magnetic field (1 T), two broadband pulsed lasers have been developed. We realized the first laser excitation of the Ps into the n=3 level, and demonstrated an efficient optical path to reach the Rydberg state n=16-17. These results, obtained in the vacuum test chamber and in absence of strong magnetic field, reach a milestone toward the formation of antihydrogen in AEgIS, and the immediate next step for us is to excite Ps atoms inside our 1 T trapping apparatus, where the formation of antihydrogen will take place. However, even once this next step will be successful, the production rate of antihydrogen atoms will nevertheless be very low, and their temperature much higher than could be wished. During my Ph.D, I have installed further excitation lasers, foreseen to perform fine spectroscopy on Ps atoms and that excite optical transitions suitable for a possible Doppler cooling. I have carried out theoretical studies and simulations to determine the proper characteristics required for a cooling laser system. The transverse laser cooling of the Ps beam will enhance the overlap between the trapped antiprotons plasma and the Ps beam during the charge-exchange process, and therefore drastically improve the production rate of antihydrogen. The control of the compression and cooling of the antiproton plasma is also crucial for the antihydrogen formation. During the beam-times of 2014 and 2015, I participated in the characterization and optimization our catching and manipulation procedures to reach highly compressed antiproton plasma, in repeatable conditions. Another project in AEgIS I took part aims to improve the formation rate of ultracold antihydrogen, by studying the possibility of a sympathetically cooling of the antiprotons using a laser-cooled anion plasma. I investigated some laser cooling schemes on the C₂⁻ molecular anions, and the simulations are promising. I actively contribute to the commissioning of the test apparatus at CERN to carry on the trials of laser cooling on the C₂⁻ species. If successful, this result will not only be the first cooling of anions by laser, but will open the way to a highly efficient production of ultracold antihydrogen atoms.