Theoretical And Experimental Study On 0.22-THz Folded Waveguide Backward Wave Oscillator

Nowadays, Terahertz backward wave oscillator(THz-BWO) is acting as an important tool for understanding, studying and utilizing terahertz waves. With the development of large-current-emission cathode and micro machining technologies, THz BWO launched with folded waveguide(FW) slow wave structure and round electron beam becomes a feasible technical route. The theoretical analysis, numerical simulation and experimental study of 0.22 THz FW BWO will be described in this dissertation.The one dimensional(1-D) models about the beam-wave interaction in FW-BWO, the effect of initial velocity spread on electron beam optical system, the equivalent circuit models of modified pill-box window and the test result of prototype tube will be highlights in this work.Firstly, the parametric model for the dispersion characteristics of the lossy FW SWS is established and numerically verified. The 1-D linear and nonlinear beam-wave interaction models in frequency domain and in time domain are finally established with the consideration of the lossy wall effect, relativistic effect, end reflection effect and space charge effect. Based on these models, the 1-D models are further improved to be able to discuss the impacts from the non-ideal electron beam and irregular FW SWS for conducting fast error analysis.Secondly, the beam optical system is primarily designed according to the relativistic model on classic convergence Pierce gun and paraxial movement approximation. Nevertheless, the further investigation reveals that the initial velocity spread will induce large influence on the formation of beam envelop in low perveance electron gun. Thus the error and trial method with the combination of the classic perturbation theoretical model and numerical model within consideration of thermal velocity effect is put forward to finish the design of the electron gun, permanent magnetic focusing system and depressed collector. Thirdly, the theoretical models for the power coupling system in THz-BWO are discussed: The modified pill-box window inserted with sapphire slice is adopted, whose equivalent circuit model is therefore developed; The simplified transmission/reflection method is proposed to measure the permittivity and electric loss tangent of attenuating ceramics using the data by vectornetwork analyzer; The scattering matrix of transition section in FW is calculated by mode coupling theory and transverse field matching method to increase the calculation accuracy and optimization efficiency.The theoretical and mechanical design with error analysis of the 0.22 THz FW-BWO is then advanced following series of attempts of structure precision guarantee in key process. Based on the successful development of the beam optical system and power coupling system as components, the prototype tubes based on two different high frequency designing schemes are fabricated, assembled and experimentally studied.Mainly due to the axial phase velocity spread and non-ideal electron beam, the ratio of start oscillation current and operating current becomes much higher than the result predicted by our ideal model, which demonstrate that hundreds of milliwatts and watt scale output power level can be achieved. Finally, in the first-time principle-verification experiment, the 0.22 THz folded waveguide achieved up to 36 mW output power around 0.229 THz, which is stimulated by electron beam with voltage of 17.47 kV and current of 12.22 mA. The repetition frequency is 100 Hz and the duty cycle is 1/10 with the beam transmission rate of 77%.