Studies On U-Band Gyro-TWT With Distributed-loss

The gyrotron traveling-wave tube amplifier （gyro-TWT） featuring high power and broad bandwidth is an ideal source for advanced radar, communication and radio-electronic warfare applications. Many institutions have been paid much attention to it. However, an actual gyro-TWT interaction system is highly susceptible to potential absolute instabilities and velocity spread of electron beam. The performance of early gyro-TWT experiments was severely limited by the oscillation problems for a long time. Therefore, kinds of high frequency structures come forth to suppress oscillations. Of these novel structures, distributed-loss loaded scheme has been successfully applied to beam-wave interaction system. The emphasis on the fundamental issues of stability in gyro-TWTs has lead to the recent series of experimental demonstrations of distributed wall loss gyro-TWTs which have efficiently suppressed oscillations of opration mode and parasitic modes and obtained exciting results. The distributed-loss scheme employs lossy material in considerable part of interaction section of gyro-TWT to suppress the absolute instability induced by opration mode and parasitic modes at cutoff frequency region which lead to enhancing of output power and widen of bandwidth.U band, which also known as Extremely High Frequency Band, is defined as 30GHz～300GHz and corresponding wavelength is 1～10mm. Comparing with C, Ku and Ka band, which is under applying, U band satellite communications could attain considerable band-width（more than 3GHz）, applicable for high data rate and large capacity satellite communications.Taking advantage of frequency-expanding technology, high disposing gain could be achieved. With certain beam, U band antenna size could be greatly decreased and interference could be prevented by means of applying antenna with zero beam function and phased array antenna or spot beam. At U band, frequency is high enough and antenna caliber size is small, therefore, very narrow beam could be produced. Such narrow beam is hard to block and suit for military communication for safety purpose. Meanwhile, high reception and radiation gain could also be met. Due to its good directional property, narrow beam, as well as interference is less for few countries using it, operation crowd could be avoided if satellite communications applying at U band. Due to advantages such as wide band-width, less interference, small equipment volume etc. as mentioned above, communications at this band has large capacity, properties of anti-disturbing and anti-capture techniques such as frequency-expand, frequency-leap, can also be greatly enhanced, therefore, it’s very suitable for strategic, tactic and high data rate, large capacity satellite communications. U band military satellite communication system provides tactic user’s real time, flexible, safe and anti-interference communication support. We must carry out studies on oppositional U band military satellite communication system as soon as possible and develop corresponding equipment to take up dominance of special electromagnetism communication in possible local war in the future. At present, electro-vacuum instruments are mostly used to produce U band high output power to interfere satellite communication uplink in ground interference station, as the most potential evolution microwave source, Gyro-TWT is the preferred instruments.Supporting by the“Momentous Technic Item”and“863 Plan”, this thesis has analyzed and simulated a u-band, TE₀₁ mode gyro-TWT distributed-loss loaded interaction section and input/output coupler. Calculation programs and design criterion of a stable and high efficiency gyro-TWT are presented. The main works of this dissertation are listed as following:1. The linear small signal theory of TE_mn mode gyro-TWT is expatiated. With the help of Vlasov-Maxwell equation and taking the dielectric loss into consideration, the small signal dispersion equation of gyro-TWT is modified by perturbation-method.2. Based on analysis of the linear small signal theory, self-consistent nonlinear simulation code of gyro-TWT beam-wave interaction is obtained by solving Maxwell equation taking source into consideration, effection on nonlinear self-consistent equations caused by velocity spread and dielectric-loading are taken into consideration also. Combine the small signal theory with the equation, optimum parameters of stable working gyro-TWT, and effective analyzing and simulation method to improve output power, widen banwidth and increase efficiency of gyro-TWT are present.3. The linear theory is used to analyze stability of gyro-TWT. Absolute instability simulation code and small signal gyro-BWO code are programmed to calculate and analyse oscillations of operation mode and parasitic modes in distributed-loss loading structure. By numerically calculating distributed-losss gyro-TWT’s dispersion equation and with the help of Briggs-Bers absolute instability criterion, propagation loss of operation and parasitic modes, start current of operation mode under different propagation loss are calculated. Star length of the three main parasitic modes under different loss-loading conditions and operation current are given. Dielectric loss parameters including thickness of loss layer and relative permittivity are decided. The small signal gain of gyro-TWT with dielectric loading is analyzed, and relationship of the small signal gain response to bandwidth is obtained. Based on integrative small signal gain analysis, initial parameters of gyro-TWT are determined.4. The performance of gyro-TWT nonlinear characteristics has been evaluated using a self-consistent nonlinear particle-tracing code to meet requirements of oscillation suppressing and choice of operation parameters. The 3 dimension PIC software Magic is used to build distributed-loss interaction structure of gyro-TWT and to validate the calculation results. Based on theoretical analysis and a great deal of simulations, a series of parameters including electron beam, power and frequency of RF input, magnetic field, interaction circuit and dielectric loading, etc. are analyzed about their relationships with interaction efficiency, output power and bandwidth. The optimum procedure of RF circuit together with electro-optic system is presented.5. With the help of field match method and HFSS, an input coupler with high mode purity is simulated and optimized. Effects on mode purity of the parameters of input coupler structure are compared and a wide-band, high mode purity input coupler is designed. The output taper and the output window are theoretically analized and numerically calculated, an output taper matched with RF circuit and the collector with a low reflectivity, low power absorbtion output window are designed.6. According to the status of our high power Gyrotron hot test Lab, essential parameters of distributed-loss loading circuit and input/output coupler of U-band gyro-TWT are determined and proved by experiments carried out on the Gyrotron Lab. This thesis proffers theoretical analysis and simulation tools for high gain U-band TE₀₁ mode gyro-TWT, producing 150kW saturated output power at 34 dB stable gain with a 3-dB bandwidth of 2GHz.