Vacuum microwave tubes, such as klystron, traveling-wave tube, gyrotron are high efficiency devices, where the RF interaction structure facilitates efficient energy transfer from the kinetic energy of the high energy electron beam to the electromagnetic wave. Traveling-wave Tube is the most versatile microwave power amplifier widely used for terrestrial communication, radar and aerospace applications. The waveguide based slow-wave structures like Millman, Karp, inter digital, grated waveguide, ring-plane, ring-bar, millitron and folded-waveguide structure gathered importance for application in millimeter-wave traveling-wave tubes. Among these millimeter-wave interaction structures, the folded-waveguide slow-wave structure became the most popular due to its robust structure, high power capability, low RF loss, simpler coupling, reasonably wide bandwidth and ease of fabrication for millimeter-wave to terahertz frequencies. Hence this thesis aims to analyse the folded-waveguide slow-wave structure for broad-banding and efficiency enhancement. The existing approaches for the analysis of cold circuit parameters (dispersion and interaction impedance characteristics) of folded-waveguide slow-wave structure are reinvestigated and found that these have limitation, as the effects of E-plane bend and beam-hole discontinuities are ignored in the parametric analysis. A cascaded matrix equivalent circuit model includes the effect of E-plane and beam-hole discontinuities for the analysis, but reported only for the serpentine folded-waveguide slow-wave structure. The cold test measurement technique was reported only for the dispersion characteristics. Hence the measurement technique has to be extended for the measurement of interaction impedance. The author proposes to orient the present doctoral work to (i) extend the proposed cascaded transmission matrix equivalent model for the analysis of rectangular folded-waveguide slow-wave structure, (ii) develop a non-resonant perturbation technique for the measurement of interaction impedance characteristics of the folded-waveguide slow-wave structure and also to (iii) establish new analysis models for the folded-waveguide slow-wave structure. The effect of E-plane bend and beam-hole discontinuities on the RF characteristics have been considered and simple, yet accurate closed form expressions for the computation of dispersion and interaction impedance characteristics have been established by three different approaches namely: transmission line equivalent circuit model, conformal mapping equivalent circuit model and quasi-TEM approach. The analysis results are benchmarked against 3-D electromagnetic modeling. The non-resonant perturbation theory is developed for the interaction impedance measurement. Typical Ka-band structures are fabricated by wire-EDM process and cold test measurements are carried out to benchmark the analysis approaches. The equivalent circuit models based on lumped circuit model are simpler than the cascaded matrix equivalent circuit model and can give closed form expressions for the prediction of dispersion and interaction impedance characteristics. The quasi-TEM approach can be extended for the complicated structure like ridge-loaded FWG-SWS. Broad-banding of the conventional folded-waveguide slow-wave structure is attempted by ridge-loading on the broad wall of the structure. The ridge-loaded folded-waveguide slow-wave structure is analyzed by parametric approach, cascaded transmission matrix equivalent circuit model and quasi-TEM approach and validated against numerical simulation. The analysis is extended for exploring the efficacy of the ridge-loading on broad-banding of the traveling-wave tube. Finally efficiency enhancement of the folded-waveguide slow-wave structure is attempted by introducing grating on the broad wall of the structure. The analysis is carried out by numerical simulation for exploring the efficacy of the grating on efficiency enhancement of the traveling-wave tube.

Durant ces quarante dernières années, les Tubes à Ondes Progressives (TOP) n’ont cessé de se développer, orienté par la demande croissante des nouvelles applications (Internet Haut débit, TV HD…). Cette demande croissante en fréquence et en puissance se traduit par des problèmes d’échauffement thermique. En effet, l’augmentation de la puissance de sortie augmente la puissance dissipée. De plus, la montée en fréquence nécessite une diminution des dimensions, qui conduit tout logiquement à des densités de puissance plus importantes. Cette chaleur produite doit être évacuée par des petites surfaces de contact qui dépendent fortement du type d’assemblage. Cet échauffement thermique implique également des changements du comportement mécanique. Dans ce travail de thèse, le point principal a été l’étude du comportement des interfaces dans les tubes à ondes progressive. Il est question d’étudier les interfaces thermomécaniques produites lors de l'assemblage (frettage à chaud). L’objectif est de fournir un modèle de détermination de la température d’hélice en fonctionnement. Compte tenu des configurations de fonctionnement (Vide, haute tension, petite dimension…) une mesure directe n’est pas réalisable. Néanmoins plusieurs méthodes de mesure indirectes ont été investiguées afin de trouver la plus appropriée. Cette étude porte dans un premier temps sur les lignes de transmissions puis sur les collecteurs des TOPs. Nous avons réalisé un modèle analytique purement thermique permettant d’identifier rapidement l’impédance thermique des dispositifs. Une mesure de RTC et une coupe métallographique déterminant les surfaces de contact alimente ce modèle afin de lui donner une meilleure précision. Un modèle élément finis 2D nous permet d’identifier une pression moyenne de contact afin d’utiliser la RTC correspondante.L’impédance thermique, nous permet de trouver la température d’hélice en indiquant la puissance dissipée dans la ligne. / During these last forty years traveling Waves tubes did not stop developing directed by the increasing request of the new applications (High-speed Internet, TV HD). This increasing request in frequency and in power is translated by thermal heating problems. Indeed, the more the output power will be high, the more there will be of the dissipated power, with smaller and smaller size. This leads logically to bigger and bigger power densities. This produced heat must be evacuated by small contact areas, which depend strongly on the type of assembly. This thermal heating also involves changes of the mechanical behaviour. The principal point will be the study of the behaviour of the interfaces in traveling waves tubes. Thesis work, we study the thermal and mechanical interfaces produced during a hot shrinking. Goal of this work is to supply a numerical or analytical model of helix temperature determination with functioning. Considering the configurations of functioning (Vacuum, high-voltage, small dimension) a direct measure is not impossible. Nevertheless several indirect measure methods were investigated to find the most appropriate. This study concerns at first the transmissions lines then the collectors of TOPS. We realized an analytical thermal model allowing to identify quickly the thermal impedance of devices. A thermal contact resistance measurement and a metallographic cutting determining the contact areas feeds this model to give it a better precision. A 2D finite element allows us to identify an average pressure of contact to use the corresponding RTC. The thermal resistance, allows us to find the helix temperature by indicating the power dissipated in the line.