In space and astrophysical plasmas, turbulence is responsible for transferring energy from large scales driven by violent events or instabilities, to smaller scales where turbulent energy is ultimately converted into plasma heat by dissipative mechanisms. In the inertial range, the self-similar turbulent energy cascade to smaller spatial scales is driven by the nonlinear interaction between counterpropagating Alfvén waves, denoted Alfvén wave collisions. For the more realistic case of the collision between two initially separated Alfvén wavepackets (rather than previous idealized, periodic cases), we use a nonlinear gyrokinetic simulation code, AstroGK, to demonstrate three key properties of strong Alfvén wave collisions: they (i) facilitate the perpendicular cascade of energy and (ii) generate current sheets self-consistently, and (iii) the modes mediating the nonlinear interaction are simply Alfvén waves. Once the turbulent cascade reaches the ion gyroradius scale, the Alfvén waves become dispersive and the turbulent energy starts to dissipate, energizing the particles via wave-particle interactions with eventual dissipation into plasma heat. The novel Field-Particle Correlation technique determines how turbulent energy dissipates into plasma heat by identifying which particles in velocity-space experience a net gain of energy. By utilizing knowledge of discrete particle arrival times, we devise a new algorithm called PATCH (Particle Arrival Time Correlation for Heliophysics) for implementing a field-particle correlator onboard spacecraft. Using AstroGK, we create synthetic spacecraft data mapped to realistic phase-space resolutions of modern spacecraft instruments. We then utilize Poisson statistics to determine the threshold number of particle counts needed to resolve the velocity-space signature of ion Landau damping using the PATCH algorithm.

Inertial Alfvén waves occur in plasmas where the Alfvén speed is greater than the electron thermal speed and the scale of wave field structure across the background magnetic field is comparable to the electron skin depth. Such waves have an electric field aligned with the background magnetic field that can accelerate electrons. It is likely that electrons are accelerated by inertial Alfvén waves in the auroral magnetosphere and contribute to the generation of auroras. While rocket and satellite measurements show a high level of coincidence between inertial Alfvén waves and auroral activity, definitive measurements of electrons being accelerated by inertial Alfvén waves are lacking. Continued uncertainty stems from the difficulty of making a conclusive interpretation of measurements from spacecraft flying through a complex and transient process. A laboratory experiment can avoid some of the ambiguity contained in spacecraft measurements. Experiments have been performed in the Large Plasma Device (LAPD) at UCLA. Inertial Alfvén waves were produced while simultaneously measuring the suprathermal tails of the electron distribution function. Measurements of the distribution function use resonant absorption of whistler mode waves. During a burst of inertial Alfvén waves, the measured portion of the distribution function oscillates at the Alfvén wave frequency. The phase space response of the electrons is well-described by a linear solution to the Boltzmann equation. Experiments have been repeated using electrostatic and inductive Alfvén wave antennas. The oscillation of the distribution function is described by a purely Alfvénic model when the Alfvén wave is produced by the inductive antenna. However, when the electrostatic antenna is used, measured oscillations of the distribution function are described by a model combining Alfvénic and non-Alfvénic effects. Indications of a nonlinear interaction between electrons and inertial Alfvén waves are present in recent data.

L'objectif de cette thèse est d'étudier l'oscillation collective dans un matériau semi-conducteur (InGaAs) dans le but d'élargir les connaissances théoriques et de proposer de nouvelles configurations et des structures pour la conception de détecteurs ou émetteurs THz innovants et efficaces. Pour ce faire, nous développons un modèle théorique permettant l'étude de l'oscillation collective soumis ou non à une excitation externe (battement optique ou rayonnements THz). Une attention particulière est faite pour prendre en compte des phénomènes physiques importants tels que la mobilité différentielle dynamique négative et les oscillations de Gunn.Cette étude est faite à travers le développement d'un outil de simulation numérique basé sur l'approche HD couplé à un solveur de Poisson unidimensionnel. Le modèle HD décrit le temps de vol et le mécanisme de diffusion par l'énergie et la vitesse de relaxation. En outre, on prend en compte les frottements et leur évolution, la variation de l'énergie, la vitesse, et la masse effective. Par conséquent, le modèle HD permet l'observation des régimes transitoires ainsi que d'effectuer des études de fréquence. L'influence des différents paramètres physiques et technologiques sur les oscillations et résonances collectives des électrons sont évalués. Ensuite, le régime de petits signaux est étudié et la réponse de la diode aux perturbations optiques et électriques harmoniques et non harmoniques est évaluée. L'influence du fort biais appliqué à la diode sur les processus d'émission et de détection est ensuite décrit. / The purpose of this thesis is to obtain theoretical results in order to propose new configurations and structures for the conception of innovant and efficient THz detectors or emitters. For this sake, we develop a theoretical model allowing the study of collective oscillation in a semiconductor materials (we choose InGaAs), submitted or not to an external excitation (that is to optical beating or THz radiations). A particular attention is payed also to important physical phenomena such as negative dynamic differential mobility and Gunn oscillations.This study is made through the development of numerical simulation tool, which is based on the HD approach coupled to a one-dimensional Poisson solver. The HD model describes the free-flight and scattering mechanism through energy and velocity relaxation rates. Also it takes into account frictions and their evolution, the variation of energy, velocity and effective mass. Hence, the HD model allows us observing the transient regimes and performing frequency studies. The influence of the different physical and technological parameters on the electron collective oscillations and resonances are evaluated. Then, small-signal regime is studied and the diode response to harmonic and non-harmonic optical and electrical perturbations is evaluated. The influence of the high bias applied to the diode on emission and detection processes is then described.

Le domaine térahertz est une région du spectre électromagnétique comprise entre 300 GHz et 30 THz. Elle représente un fort intérêt pour la communauté scientifique pour plusieurs raisons : la radiation térahertz possède en effet un potentiel de télécommunication à très haut débit important, elle constitue un moyen d'investigation efficace et non destructif pour différents types d'éléments et composés, minéraux ou organiques et elle représente une importance cruciale pour les astronomes qui estime que 98 % des photons émis par le Big Bang se trouvent dans ce domaine de fréquences. Malheureusement, à l'heure actuelle, le manque de sources et détecteurs facilement exploitables, intégrables et fonctionnant à température ambiante ne permet pas l'utilisation du domaine térahertz à grande échelle. Un nouveau phénomène physique exploitable tel que les oscillations d'ondes de plasma dans les nanotransistors représente une piste prometteuse pour combler ce manque. Ce phénomène étudié de manière analytique dans le milieu des années 1990, a donné lieu à un modèle qui reste très loin de la réalité physique et des conditions expérimentales. Des expériences récentes effectuées à température ambiante ont permis de montrer la possibilité d'exciter des oscillations d'ondes de plasma à l'intérieur d'un canal de HEMT par une radiation THz directe. Ce travail se propose de réaliser une étude systématique des transistors sous excitation effectuée par battement optique térahertz. Ceci afin de mieux comprendre et exploiter les ondes de plasma dans les nanotransistors à effet de champ. Cela nous a conduit à étudier l'effet des paramètres géométriques et physiques du transistor comme les longueurs de grille, les longueurs des cap-layers, la tension de drain et la tension de grille. En parallèle à ce travail expérimental un modèle hydrodynamique pseudo-2D était utilisé pour confronter l'ensemble des résultats pour une meilleure compréhension des phénomènes physiques. Ce travail a permis d'accéder à une compréhension et une description fines du phénomène d'excitation des ondes de plasma. ceci a permis d'initier l'étude de nouveaux dispositifs tel que un émetteur à base d'un transistor HEMT assisté par battement optique et la réalisation d'un mélangeur hétérodyne d'une radiation térahertz transposé par un battement optique en une fréquence intermédiaire plus basse et facilement exploitable. / The terahertz range covers the electromagnetic spectrum for frequencies between 300 GHz and 30 THz. It presents a strong interest in the scientific community for several reasons: Terahertz carriers allow for high-speed free-space telecommunications; Terahertz radiations can be used for efficient and non-destructive characterization of various components and materials (minerals or organic); Terahertz detection is of major interest for astronomers as 98 % of photons emitted since the Big Bang are in this frequency domain. Unfortunately, the lack of adequate sources and detectors, i.e. room-temperature-operating, low-cost and integrated, strongly limits the use of terahertz radiations for the above-mentioned applications. A new physical phenomenon called plasma waves in nanotransistors is very promising for the realization of terahertz sources and detectors. This new phenomenon was proposed in the mid-1990s on the basis of analytical calculations, although the model was rather simplified and it did not take into account the actual experimental conditions. Recent experiments performed at low and room temperature demonstrated the feasibility to excite plasma waves in the channel of a high-electron-mobility transistor (HEMT), using a THz-radiation excitation.This work presents a different way to excite this plasma wave by using an optical beating excitation. A systematic study of nanometric transistors under optical excitation to better understand and exploit plasma waves is carried on. The effects of geometrical parameters such as transistor gate length or cap-layer length are investigated. The dependence of the plasma waves on different electrical parameters such as drain voltage and gate voltage is also presented. Along with this experimental work, a pseudo-two-dimensional hydrodynamic simulator was developed to analyze the physical processes in the transistors on a more rigorous theoretical basis than the simplified analytical model. As a result of this joint experimental and theoretical investigation, we achieved a better understanding and an accurate description of the complex mechanism of plasma waves excited in field-effect transistors. Finally, we propose new structures to be used, from one hand, as a monochromatic terahertz source based on a HEMT excited by an optical beating, and,from the other hand, a spectrally-resolved heterodyne detector based on the mixing between the terahertz radiation to be analyzed and an optical beating used as a tunable local oscillator.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20184号 / 理博第4269号 / 新制||理||1613(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 田口 聡, 教授 家森 俊彦, 教授 塩谷 雅人 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21769号 / 工博第4586号 / 新制||工||1715(附属図書館) / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 小嶋 浩嗣, 准教授 海老原 祐輔, 准教授 三谷 友彦 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Charged particles constantly stream outward from the Sun to fill the solar system. These particles, consisting mainly of protons and electrons, form a plasma called the solar wind. The solar wind interacts with every celestial body in the solar system, giving rise to different phenomena, such as the auro- ras observed at high latitudes on Earth or disruption of the systems onboard artificial satellites.  The general structure of the solar wind has been established several decades ago, however we still do not fully understand how the solar wind properties, like temperature and velocity distribution, evolve as it propagates outward in the solar system. Observations of these properties cannot be explained from a conventional fluid description. In a system approximated as a fluid, particle collisions dictate its thermodynamic state. However, the solar wind is a weakly collisional plasma that deviates from thermodynamic equilibrium. Therefore, the radial evolution of the solar wind properties must be driven by different processes. In particular, wave-particle interactions are an important regulator of the solar wind properties, because of the strong connection between the electromagnetic fields and the charged particles.  In this thesis, we probe how the velocity distribution of solar wind par- ticles evolves as it travels from the Sun to the Earth. Specifically, we study the contribution of waves on the observed solar wind properties at different distances and how these waves can affect the interplanetary environment. We focus on two types of plasma waves frequently observed in the solar wind, Langmuir and ion-acoustic waves. We present their occurrence rates at differ- ent heliocentric distances and suggest wave generation mechanisms based on Solar Orbiter observations. We show that Langmuir waves in the unperturbed solar wind are more commonly observed in regions where the magnetic field magnitude is lower than the background value. Furthermore, we also find that the occurrence rate of ion-acoustic waves is increased in the ramp regions of interplanetary shocks observed at different heliocentric distances, compared to the ion-acoustic wave occurrence rate in the unperturbed solar wind.

Solar wind shocks and the intracluster medium comprise hot, low-density plasmas with few Coulomb collisions. Electrons there are not fluid and so gain and exchange energy by interaction with a variety of plasma waves. We explore two mechanisms for electron energization in such plasmas. In 2D kinetic simulations of solar wind shocks with low beta (magnetic pressure greater than thermal pressure), fast-mode / oblique-whistler waves accelerate electrons in bulk via proton-scale parallel electric fields; electrons’ bulk kinetic energy then converts to heat via magnetic field-aligned electrostatic wave scattering. We show and measure the heating for 2D shocks of varying magnetic obliquity and Mach number, and we qualitatively map the mechanism’s shock parameter regime. Next, consider the intracluster medium: a high-beta plasma (thermal pressure greater than magnetic pressure) in which Megaparsec-scale motions promptly trigger nanoparsec-scale plasma waves, which in turn can scatter 1–100 MeV cosmic ray electrons. Small-scale scattering combined with large-scale motion can heat electrons, and this process is called magnetic pumping. We use 1D simulations of plasma subjected to continuous bulk compression to measure the efficiency of magnetic pumping upon cosmic ray electrons. It is speculated that magnetic pumping may help re-accelerate MeV electrons to radio-emitting energies and so help explain the origin of diffuse, MHz–GHz radio halos enshrouding some clusters of galaxies.

This study considers the effect, on plasma ion oscillations, of various lengths of discharge tubes as well as various cross sections of discharge tubes. Four different gases were used in generating the plasma. Gas pressure and discharge voltage and current were varied to obtain a large number of signals. A historical survey is given to familiarize the reader with the field. The experimental equipment and procedure used in obtaining data is given. An analysis of the data obtained is presented along with possible explanations for the observed phenomena. Suggestions for future study are made.

L'objectif de ce travail de thèse est l'exploitation des oscillations de plasma tridimensionnelles dans des diodes à base d'InGaAs et de GaN, matériaux de grand intérêt pour les applications térahertz à cause de la haute mobilité électronique du premier et des fortes interactions électrons-phonons optiques dans le second. Ce travail s'insère dans le contexte d'études récentes dans lesquelles l'utilisation de dispositifs basés sur l'excitation d'ondes de plasma tridimensionnelles a été proposée pour des applications térahertz, à l'heure où les ondes de plasma bidimensionnelles demeurent très limitées en puissance. Cette étude est menée à travers le développement d'un outil numérique de simulation basé sur le modèle hydrodynamique couplé à un solveur de Poisson unidimensionnel. La réponse des diodes à différentes perturbations optiques et électriques est alors évaluée à travers la description du régime petit-signal, et l'influence sur les résonances de plasma des différents paramètres des diodes est mise en évidence pour l'InGaAs et pour le GaN. Une résolution matricielle de l'équation de Poisson à deux dimensions est également présentée en vue d'un couplage ultérieur avec le modèle hydrodynamique à deux dimensions, ce qui permettrait éventuellement une étude plus approfondie des ondes de plasma dans les transistors. En outre, vu que les paramètres d'entrée du modèle hydrodynamique sont tirés d'un simulateur Monte Carlo dont les paramètres d'entrée sont directement calculés à partir de la structure de bandes du matériau, une partie préliminaire à la simulation des dispositifs, et qui implique le calcul de la structure de bande des matériaux par la méthode semi-empirique du pseudopotentiel, est aussi traitée. / The objective of this thesis is the analysis of three-dimensional plasma oscillations in diodes based on InGaAs and GaN, materials of great interest for terahertz applications because of the high electron mobility of the first and the strong electron-optical phonons interactions in the second. This work falls within the context of recent studies in which the use of devices based on the excitation of three-dimensional plasma waves has been proposed for terahertz applications, at a time when two-dimensional plasma waves remain very limited in emission power. This study is conducted through the development of a numerical simulation based on the hydrodynamic model coupled to a one-dimensional Poisson solver. The response of diodes at different optical and electrical excitations is then evaluated through the description of small-signal regime, and the influence on plasma resonances of the various parameters of the diodes is demonstrated for InGaAs and GaN. A matrix resolution of the two-dimensional Poisson equation is also presented for a subsequent coupling with the two-dimensional hydrodynamic model, which would eventually allow a more thorough study of plasma waves in transistors. In addition, since the input parameters of the hydrodynamic model are derived from a Monte Carlo simulator whose input parameters are directly calculated from the band structure of the material, a preliminary study to devices simulation, which involves the calculation of the materials band structure by the semi-empirical pseudopotential method, is also considered.