Report (M.S.)--Virginia Polytechnic Institute and State University. M.S. 1992. / Vita. Abstract. Includes bibliographical references (leaf 102). Also available via the Internet.

International Telemetering Conference Proceedings / October 23-26, 2000 / Town & Country Hotel and Conference Center, San Diego, California / Spectral sidelobes of QPSK, OQPSK, IJF-OQPSK, and SQAM modulated signals after nonlinear amplification are compared. It is known that OQPSK has lower spectral sidelobes than QPSK. However, in the presence of frequency selective fading, a decision-feedback adaptive equalizer is able to equalize the QPSK signal but not the OQPSK signal. By using phase pre-distortion on the OQPSK waveform before nonlinear amplification, not only is the adaptive equalizer able to equalize this signal, its spectral sidelobes are also reduced. Simulations are presented to confirm these results.

Friction Stir Welding has seen a fast uptake in many industry segments. Mechanical properties superior to fusion welding, the ability to weld "unweldable" aluminium alloys and low distortion are often described as the main reasons for the fast industrial implementation of FSW. Most existing applications consist of long straight welding joints. Applications with complex weld geometries, however, are rarely produced by FSW. These geometries can induce thermal variations during the welding process, thus making it challenging to maintain a consistent weld quality. In-process adaptation of weld parameters to respond to geometrical variations and other environmental variants allow new design opportunities for FSW. Weld quality has been shown to be reliant on the welding temperature. However, the optimal methodology to control the temperature is still under development.The research work presented in this thesis focuses on some steps to take in order to reach the improvement of the FSW temperature controller, thus reach a better and consistent weld quality. In the present work different temperature methods were evaluated. Temperature measurements acquired by the tool-workpiece thermocouple (TWT) method were accurate and fast, and thereby enhanced suitable for the controller. Different environmental conditions influencing the material heat dissipation were imposed in order to verify the controller effect on the joint quality. In comparison with no controlled weld, the use of the controller enabled a fast optimization of welding parameters for the different conditions, leading to an improvement of the mechanical properties of the joint.For short weld lengths, such as stitch welds, the initial plunge and dwell stages occupy a large part of the total process time. In this work temperature control was applied during these stages. This approach makes the plunge and dwell stages more robust by preventing local material overheating, which could lead to a tool meltdown. The TWT method was demonstrated to allow a good process control during plunging and continuous welding. The approach proposed for control offers weld quality consistency and improvement. Also, it allows a reduction of the time required for the development of optimal parameters, providing a fast adaptation to disturbances during welding and, by decreasing the plunge time, provides a significant decrease on the process time for short welds.

Neste trabalho analisa-se o problema do transporte do feixe de elétrons em canhões de elétrons, estruturas periódicas de ímãs permanentes e em coletores de simples e múltiplos estágios. Essa análise é de relevância em projetos de dispositivos de micro-ondas de potência dos tipos amplicador klystron e válvula TWT. Determina-se a dinâmica das partículas a partir da solução da equação da trajetória que é derivada da força de Lorentz e da conservação de energia. A equação da trajetória obtida é diferencial de segunda ordem, não-linear e independentemente do tempo para o potencial generalizado. Utiliza-se o método de Runge-Kutta de 4a Ordem para integrar a equação da trajetória das partículas. Obtém-se o potencial escalar elétrico a partir da solução da equação de Poisson. Numericamente, obtêm-se os po- tenciais escalares elétricos e magnéticos, por meio do Método de Elementos Finitos (MEF). Ao longo do movimento de uma partícula, obtida a partir da solução da equação da trajetória, deposita-se carga elétrica. Utilizam-se macropartículas, uma vez que é praticamente impossível modelar cada partícula do problema, a partir do método Partícula na Célula (Particle in Cell - PIC). Neste caso, tem-se um problema acoplado para o potencial escalar elétrico e as trajetórias das macropartículas, uma vez que, as trajetórias das macropartículas dependem dos potenciais e estes, por sua vez, dependem das trajetórias. À convergência deste problema acoplado utiliza-se o Método das Aproximações Sucessivas (MAS). A plataforma desenvolvida, baseada nos métodos acima, compõe-se de duas ferramentas computacionais. A primeira, XMGUN, dedica-se ao projeto de: canhões de elétrons com grades e grades de sombreamento; e coletores de simples e múltiplos estágios considerando, ainda, a emissão de elétrons secundários. A segunda, XMAGUN, volta-se ao projeto de estruturas periódicas com ímãs permanentes. Afere-se o desempenho da ferramenta computacional XMGUN com o diodo plano de Pierce operando na condição em que a corrente é limitada pelas cargas espaciais. Por sua vez, verica-se o desempenho do XMAGUN por meio de simulações com estruturas do tipo PPM separadas pelo vácuo e na presença de pole pieces. Os resultados obtidos em todas as simulações convergiram satisfatoriamente para as soluções analíticas. Utilizando o XMGUN, projeta-se um canhão de elétrons com 30 kV de tensão de anodo e uma perveância de 1,37 Perv com capacidade de fornecer uma corrente elétrica de 7,1 A. Esse canhão tem uma malha com 2796 elementos e 5057 nós. As principais características geométricas do canhão de elétrons são: raio do catodo rc=14,6 mm; raio do disco do catodo rk =6,2 mm; e ângulo do eletrodo de focalização = 37. Neste caso, a velocidade transversal normalizada e o alcance do feixe zw observados são de 0,068 e 26,88 mm respectivamente. Obtém-se uma concordância superior a 93% em corrente e perveância com o EGUN. Utilizando, ainda, o XMGUN, são simulados coletores de simples e múltiplos estágios. O coletor de simples estágio apresenta 1612 nós e 2969 elementos, e o de 4 (quatro) estágios, 2496 nós e 4257 elementos. As tensões dos eletrodos do 1o, 2o, 3o e 4o estágio são de 9,45 kV, 8,65 kV, 6,45 kV e 3,45 kV respectivamente. Durante as simulações, devido à emissão de elétrons secundários, observa-se, para o coletor de simples estágio, macropartículas penetrando na região de deriva, fenômeno este indesejado, e não observado para o coletor de 4 (quatro) estágios. Considerando o XMAGUN, projeta-se um arranjo periódico com pole pieces e 5 (cinco) ímãs permanentes, capaz de fornecer um campo magnético, no centro da estrutura, de 0,42 T. Neste caso, a geometria do arranjo periódico obtida é: raio interno rm1 e externo rm2 do ímã permanente são iguais a 3,5 mm e 7,5 mm respectivamente; raio externo do pole piece r3 = 7,5 mm ; raio interno rf1 e externo rf2 da ponteira do pole piece são 1,6 mm e 3,05 mm respectivamente; espessura do ímã permanente T=2,95 mm; período magnético L =8,5 mm. A remanência do ímã permanente utilizada é de Br=0,85 T. A malha dessa estrutura periódica magnética apresenta pouco mais de 20.000 nós e 40.000 elementos. / In this paper we analyze the problem of transport of the electron beam in electron guns, periodic arrays of permanent magnets and collectors of simple and multiple stages. This analysis is of relevance in the design of power microwave devices such as klystron amplier and TWT valve. The dynamics of particles is determined from the solution of the equation of the trajectory that is derived from the Lorentz force and energy conservation law. The equation of the trajectory obtained is differential of second-order, non-linear and time independent for the generalized potential. It is used the Runge-Kutta 4th order method to integrate the equation of the trajectory of the particles. The electric scalar potential is obtained from the solution of the Poisson equation. Numerically, we obtain the electric and magnetic scalar potentials, using the Finite Element Method (FEM). Throughout the motion of a particle, obtained from the solution of the equation of the trajectory, electrical charge it is deposited. Macroparticles are used, since it is virtually impossible to model each particle of the problem, based on Particle in Cell scheme (Particle in Cell - PIC). In this case, there is a coupled problem for the electric scalar potential and the trajectories of the macroparticles, since these trajectories depend on the potential and the potential, in turn, depends on the trajectories. In order to abtain the convergence of this coupled problem, it used the Method of Successive Approximations (MSA). The platform developed, based on the above methods, consists of two computational tools. The rst, XMGUN, is dedicated to the project of: electron guns with grids and shadow-grids, and collectors of simple and multiple stages, where secondary electrons emission is considered. The second, XMAGUN, is used to the design of periodic permanent magnets structures. The XMGUN was benchmarked against the plan Pierce diode under space charge limited condiction. In turn, the XMAGUN was benchmarked against PPM like structures, separated by a vacuum and in the presence of pole pieces. The results, in all simulations, converged satisfactorily to the analytical solu- tions. Using XMGUN, it is designed an electron gun with 30 kV anode voltage, 1.37 Perv capable of supplying an electric current of 7.1 A. This gun has a mesh with 2796 elements and 5057 nodes. The main geometric characteristics of the electron gun are: cathode radius rc = 14.6 mm; cathode disc radius rk = 6.2 mm; and half cone angle = 37. In this case, the normalized transverse velocity and beam-waist distance from anode zw are 0.068 and 26.88 mm respectively. An agreement above 93% in current and perveance is found when compared with EGUN. XMGUN is also used to simulate single and multi stage collectors. The single-stage collector has 1612 nodes and 2969 elements, while the 4 (four) stages collector has 2496 nodes and 4257 we elements. The collector electrode voltages of the 1st, 2nd, 3rd and 4th stage are 9.45 kV 8.65 kV 3.45 kV 6.45 kV, respectively. During the simulations, due to yield of secondary electrons, for the single stage collector, it is observed macroparticles entering into the drift region, a phenomenon unwanted, and not observed for the 4 (four) stage collector. Whereas XMAGUN is projected at a periodic arrangement with pole pieces and 5 (ve) permanent magnets, capable of providing a magnetic eld in the center of the structure was 0.42 T. In this case, the geometry of the periodic arrangement is obtained: inner and outer radius of the permanent magnet rm1 = 3.5 mm and 7.5 mm respectively rm2 =; outer radius of the pole piece r3 = 7.5 mm, internal radius and external tip of the pole piece rf1=rf2 =1.6 mm and 3.05 mm respectively; permanent magnet thickness T = 2.95 mm magnetic period L = 8.5 mm. The remanence of the permanent magnet used is Br = 0.85 T. The net periodic structure of magnetic features little more than 20,000 nodes and 40,000 elements.

Neste trabalho analisa-se o problema do transporte do feixe de elétrons em canhões de elétrons, estruturas periódicas de ímãs permanentes e em coletores de simples e múltiplos estágios. Essa análise é de relevância em projetos de dispositivos de micro-ondas de potência dos tipos amplicador klystron e válvula TWT. Determina-se a dinâmica das partículas a partir da solução da equação da trajetória que é derivada da força de Lorentz e da conservação de energia. A equação da trajetória obtida é diferencial de segunda ordem, não-linear e independentemente do tempo para o potencial generalizado. Utiliza-se o método de Runge-Kutta de 4a Ordem para integrar a equação da trajetória das partículas. Obtém-se o potencial escalar elétrico a partir da solução da equação de Poisson. Numericamente, obtêm-se os po- tenciais escalares elétricos e magnéticos, por meio do Método de Elementos Finitos (MEF). Ao longo do movimento de uma partícula, obtida a partir da solução da equação da trajetória, deposita-se carga elétrica. Utilizam-se macropartículas, uma vez que é praticamente impossível modelar cada partícula do problema, a partir do método Partícula na Célula (Particle in Cell - PIC). Neste caso, tem-se um problema acoplado para o potencial escalar elétrico e as trajetórias das macropartículas, uma vez que, as trajetórias das macropartículas dependem dos potenciais e estes, por sua vez, dependem das trajetórias. À convergência deste problema acoplado utiliza-se o Método das Aproximações Sucessivas (MAS). A plataforma desenvolvida, baseada nos métodos acima, compõe-se de duas ferramentas computacionais. A primeira, XMGUN, dedica-se ao projeto de: canhões de elétrons com grades e grades de sombreamento; e coletores de simples e múltiplos estágios considerando, ainda, a emissão de elétrons secundários. A segunda, XMAGUN, volta-se ao projeto de estruturas periódicas com ímãs permanentes. Afere-se o desempenho da ferramenta computacional XMGUN com o diodo plano de Pierce operando na condição em que a corrente é limitada pelas cargas espaciais. Por sua vez, verica-se o desempenho do XMAGUN por meio de simulações com estruturas do tipo PPM separadas pelo vácuo e na presença de pole pieces. Os resultados obtidos em todas as simulações convergiram satisfatoriamente para as soluções analíticas. Utilizando o XMGUN, projeta-se um canhão de elétrons com 30 kV de tensão de anodo e uma perveância de 1,37 Perv com capacidade de fornecer uma corrente elétrica de 7,1 A. Esse canhão tem uma malha com 2796 elementos e 5057 nós. As principais características geométricas do canhão de elétrons são: raio do catodo rc=14,6 mm; raio do disco do catodo rk =6,2 mm; e ângulo do eletrodo de focalização = 37. Neste caso, a velocidade transversal normalizada e o alcance do feixe zw observados são de 0,068 e 26,88 mm respectivamente. Obtém-se uma concordância superior a 93% em corrente e perveância com o EGUN. Utilizando, ainda, o XMGUN, são simulados coletores de simples e múltiplos estágios. O coletor de simples estágio apresenta 1612 nós e 2969 elementos, e o de 4 (quatro) estágios, 2496 nós e 4257 elementos. As tensões dos eletrodos do 1o, 2o, 3o e 4o estágio são de 9,45 kV, 8,65 kV, 6,45 kV e 3,45 kV respectivamente. Durante as simulações, devido à emissão de elétrons secundários, observa-se, para o coletor de simples estágio, macropartículas penetrando na região de deriva, fenômeno este indesejado, e não observado para o coletor de 4 (quatro) estágios. Considerando o XMAGUN, projeta-se um arranjo periódico com pole pieces e 5 (cinco) ímãs permanentes, capaz de fornecer um campo magnético, no centro da estrutura, de 0,42 T. Neste caso, a geometria do arranjo periódico obtida é: raio interno rm1 e externo rm2 do ímã permanente são iguais a 3,5 mm e 7,5 mm respectivamente; raio externo do pole piece r3 = 7,5 mm ; raio interno rf1 e externo rf2 da ponteira do pole piece são 1,6 mm e 3,05 mm respectivamente; espessura do ímã permanente T=2,95 mm; período magnético L =8,5 mm. A remanência do ímã permanente utilizada é de Br=0,85 T. A malha dessa estrutura periódica magnética apresenta pouco mais de 20.000 nós e 40.000 elementos. / In this paper we analyze the problem of transport of the electron beam in electron guns, periodic arrays of permanent magnets and collectors of simple and multiple stages. This analysis is of relevance in the design of power microwave devices such as klystron amplier and TWT valve. The dynamics of particles is determined from the solution of the equation of the trajectory that is derived from the Lorentz force and energy conservation law. The equation of the trajectory obtained is differential of second-order, non-linear and time independent for the generalized potential. It is used the Runge-Kutta 4th order method to integrate the equation of the trajectory of the particles. The electric scalar potential is obtained from the solution of the Poisson equation. Numerically, we obtain the electric and magnetic scalar potentials, using the Finite Element Method (FEM). Throughout the motion of a particle, obtained from the solution of the equation of the trajectory, electrical charge it is deposited. Macroparticles are used, since it is virtually impossible to model each particle of the problem, based on Particle in Cell scheme (Particle in Cell - PIC). In this case, there is a coupled problem for the electric scalar potential and the trajectories of the macroparticles, since these trajectories depend on the potential and the potential, in turn, depends on the trajectories. In order to abtain the convergence of this coupled problem, it used the Method of Successive Approximations (MSA). The platform developed, based on the above methods, consists of two computational tools. The rst, XMGUN, is dedicated to the project of: electron guns with grids and shadow-grids, and collectors of simple and multiple stages, where secondary electrons emission is considered. The second, XMAGUN, is used to the design of periodic permanent magnets structures. The XMGUN was benchmarked against the plan Pierce diode under space charge limited condiction. In turn, the XMAGUN was benchmarked against PPM like structures, separated by a vacuum and in the presence of pole pieces. The results, in all simulations, converged satisfactorily to the analytical solu- tions. Using XMGUN, it is designed an electron gun with 30 kV anode voltage, 1.37 Perv capable of supplying an electric current of 7.1 A. This gun has a mesh with 2796 elements and 5057 nodes. The main geometric characteristics of the electron gun are: cathode radius rc = 14.6 mm; cathode disc radius rk = 6.2 mm; and half cone angle = 37. In this case, the normalized transverse velocity and beam-waist distance from anode zw are 0.068 and 26.88 mm respectively. An agreement above 93% in current and perveance is found when compared with EGUN. XMGUN is also used to simulate single and multi stage collectors. The single-stage collector has 1612 nodes and 2969 elements, while the 4 (four) stages collector has 2496 nodes and 4257 we elements. The collector electrode voltages of the 1st, 2nd, 3rd and 4th stage are 9.45 kV 8.65 kV 3.45 kV 6.45 kV, respectively. During the simulations, due to yield of secondary electrons, for the single stage collector, it is observed macroparticles entering into the drift region, a phenomenon unwanted, and not observed for the 4 (four) stage collector. Whereas XMAGUN is projected at a periodic arrangement with pole pieces and 5 (ve) permanent magnets, capable of providing a magnetic eld in the center of the structure was 0.42 T. In this case, the geometry of the periodic arrangement is obtained: inner and outer radius of the permanent magnet rm1 = 3.5 mm and 7.5 mm respectively rm2 =; outer radius of the pole piece r3 = 7.5 mm, internal radius and external tip of the pole piece rf1=rf2 =1.6 mm and 3.05 mm respectively; permanent magnet thickness T = 2.95 mm magnetic period L = 8.5 mm. The remanence of the permanent magnet used is Br = 0.85 T. The net periodic structure of magnetic features little more than 20,000 nodes and 40,000 elements.

Este trabalho apresenta o desenvolvimento de uma plataforma para o estudo teórico e experimental de dispositivos amplificadores de micro-ondas do tipo válvula de ondas progressivas (TWT). A plataforma é composta por um modelo matemático e uma bancada de testes. O modelo matemático descreve a TWT como uma linha de transmissão acoplada a um feixe eletrônico unidimensional, onde as forças de carga espacial AC e DC são calculadas auto consistentemente, eliminando-se a necessidade de um cálculo separado para o fator de redução de carga espacial. O modelo matemático deu origem a dois códigos para a simulação da TWT. Ambos foram comparados com resultados experimentais e teóricos disponíveis na literatura especializada para uma pré-validação. O nível de concordância entre os presentes resultados e aqueles de referência foi acima de 90%, o que atendeu as expectativas de exatidão do modelo, tendo em vista que nem todos os parâmetros de entrada estavam disponíveis na referência. A bancada de testes construída é composta por uma TWT com banda de operação de 6,0 a 18 GHz e potência saturada máxima em torno de 55 dBm (316 W) em 13 GHz, um circuito de polarização para a mesma e a instrumentação necessária para a realização das medidas pertinentes aos amplificadores de potência. A TWT em questão foi caracterizada segundo seu comportamento mono-sinal e multi-sinal. As curvas de ganho e potência foram obtidas em função da frequência utilizando a voltagem de aceleração do feixe eletrônico e a potência de entrada como parâmetros. As curvas de transferência de potência, de fase e compressão de ganho foram obtidas para frequências escolhidas ao longo da banda, tendo novamente a voltagem de aceleração como parâmetro. Adicionalmente, a produção de produtos de intermodulação de terceira ordem foi caracterizada no ponto de 1 dB de compressão de ganho ao longo da banda analisada. Um teste de linearização por injeção de sinais, que estava previsto no plano de trabalho, não apresentou o desempenho esperado devido a problemas no funcionamento do circuito linearizador. Esses problemas foram analisados e listou-se uma série de passos para saná-los. / This work deals with the development of a platform for theoretical and experimental investigations of microwave amplifiers devices of the type traveling-wave tube (TWT). The platform consists of a mathematical model and a test bench. The mathematical model describes the TWT as a transmission line coupled to a onedimensional electron beam, in which the AC and DC space charge forces are calculated self-consistently, eliminating the need for a separate calculation for the space charge reduction factor. The mathematical model gave rise to two codes for the simulation of TWTs. Both codes were validated against experimental and theoretical results available in the literature. The overall level of agreement between the present results and those from the reference was above 90%, which was considered satisfactory since not all input parameters were available in the reference. The test bench consists of a wideband TWT operating from 6.0 to 18 GHz and maximum saturated power around 55 dBm (316 W) at 13 GHz, a biasing circuit, and the instrumentation needed to perform the relevant measurements to the power amplifier. The TWT in question was characterized according to its mono-signal and multi-signal behavior. The gain and power curves were obtained as a function of the frequency using the beam voltage and the input power as parameters. The curves of power transfer, phase transfer and gain compression were obtained for selected frequencies along the operating band, again, using the beam voltage as a parameter. Furthermore, the production of third-order intermodulation products was measured at the 1 dB gain compression point over the band analyzed. A linearization test applying the signal injection technique, which was part of the initial work plan, presented inadequate performance due to problems in the linearizer circuit operation. These problems were analyzed and a guide to solve them was provided.

Este trabalho apresenta o desenvolvimento de uma plataforma para o estudo teórico e experimental de dispositivos amplificadores de micro-ondas do tipo válvula de ondas progressivas (TWT). A plataforma é composta por um modelo matemático e uma bancada de testes. O modelo matemático descreve a TWT como uma linha de transmissão acoplada a um feixe eletrônico unidimensional, onde as forças de carga espacial AC e DC são calculadas auto consistentemente, eliminando-se a necessidade de um cálculo separado para o fator de redução de carga espacial. O modelo matemático deu origem a dois códigos para a simulação da TWT. Ambos foram comparados com resultados experimentais e teóricos disponíveis na literatura especializada para uma pré-validação. O nível de concordância entre os presentes resultados e aqueles de referência foi acima de 90%, o que atendeu as expectativas de exatidão do modelo, tendo em vista que nem todos os parâmetros de entrada estavam disponíveis na referência. A bancada de testes construída é composta por uma TWT com banda de operação de 6,0 a 18 GHz e potência saturada máxima em torno de 55 dBm (316 W) em 13 GHz, um circuito de polarização para a mesma e a instrumentação necessária para a realização das medidas pertinentes aos amplificadores de potência. A TWT em questão foi caracterizada segundo seu comportamento mono-sinal e multi-sinal. As curvas de ganho e potência foram obtidas em função da frequência utilizando a voltagem de aceleração do feixe eletrônico e a potência de entrada como parâmetros. As curvas de transferência de potência, de fase e compressão de ganho foram obtidas para frequências escolhidas ao longo da banda, tendo novamente a voltagem de aceleração como parâmetro. Adicionalmente, a produção de produtos de intermodulação de terceira ordem foi caracterizada no ponto de 1 dB de compressão de ganho ao longo da banda analisada. Um teste de linearização por injeção de sinais, que estava previsto no plano de trabalho, não apresentou o desempenho esperado devido a problemas no funcionamento do circuito linearizador. Esses problemas foram analisados e listou-se uma série de passos para saná-los. / This work deals with the development of a platform for theoretical and experimental investigations of microwave amplifiers devices of the type traveling-wave tube (TWT). The platform consists of a mathematical model and a test bench. The mathematical model describes the TWT as a transmission line coupled to a onedimensional electron beam, in which the AC and DC space charge forces are calculated self-consistently, eliminating the need for a separate calculation for the space charge reduction factor. The mathematical model gave rise to two codes for the simulation of TWTs. Both codes were validated against experimental and theoretical results available in the literature. The overall level of agreement between the present results and those from the reference was above 90%, which was considered satisfactory since not all input parameters were available in the reference. The test bench consists of a wideband TWT operating from 6.0 to 18 GHz and maximum saturated power around 55 dBm (316 W) at 13 GHz, a biasing circuit, and the instrumentation needed to perform the relevant measurements to the power amplifier. The TWT in question was characterized according to its mono-signal and multi-signal behavior. The gain and power curves were obtained as a function of the frequency using the beam voltage and the input power as parameters. The curves of power transfer, phase transfer and gain compression were obtained for selected frequencies along the operating band, again, using the beam voltage as a parameter. Furthermore, the production of third-order intermodulation products was measured at the 1 dB gain compression point over the band analyzed. A linearization test applying the signal injection technique, which was part of the initial work plan, presented inadequate performance due to problems in the linearizer circuit operation. These problems were analyzed and a guide to solve them was provided.

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.