| Millimeter wave is between microwave and light wave, and it is of great potential application in radars, science and communications. Klystron is a important microwave source in Millimeter wave, which has many advantages such as high-gain, high-efficiency, high-power and so on. However, many problems will appear when klystron is applied in millimeter wave band, such as increasing high-frequency loss and decreasing power capability, which arise from wavelength-size-matching effect. To solve these problems, extended interaction klystron, which with multiple interaction gaps in every cavity, is applied. Theoretical analysis, particle-in-cell(PIC) simulation and test of some module of extended interaction klystron carried out.The reason for enhancing klystron beam-wave interaction efficiency of multiple in- teraction gaps cavity is analyzed. Based on equivalent circuit model, and in ideal condition, klystron efficiency can enhanced by increasing gap number, thereby, increasing frequency bandwidth and enhancing power capability, so long as velocity of beam is matching with phase velocity of electromagnetic wave.There are several key points in extended interaction klystron high frequency structure, such as choice of cavity structure, choice of work mode, match between waveguide with input cavity and stability of each cavity. Field in every gap is not expected of extended interaction cavity, therefore, π mode and 2π mode is available. To realize maximal efficiency, π mode is used as work mode. Theoretical analysis represents interaction efficiency is associated with uniformity of field in each gap. To improve uniformity of π mode in each gap, bi-periodic structure is applied.Since matching between waveguide with input cavity and stability of extended interaction klystron lie on beam loading, so it is separately derived based on kinetic theory and space charge wave theory in small signal condition. Beam loading based on kinetic theory is right only when current is small, while beam loading based on kinetic theory is right all the time. The characteristic of beam loading is analyzed both extended interaction cavity and traditional single gap cavity. The results show that beam loading of extended interaction cavity can change in a bigger extension than that of traditional single ggap cavity. Consequently, the loaded Q of extended interaction cavity can adjust in a bi- gger extension to realize a desired Q which request by klystron.Based on above theory analysis, a three dimension whole tube model of an W band extended interaction klystron amplifier is designed. With the integrated model, the high frequency characteristic analysis and optimization of the extended interaction klystron amplifier are accomplished. With an electron beam of 17 k V and 0.34 A, an average power of 591 W with frequency of 94.8 GHz, gain of 43 d B, power conversion efficiency of 10.2%, instantaneous – 3 d B bandwidth of about 150 MHz is obtained. The PIC simulation result show that the input cavity matches well with the input waveguide, that is, most of the input microwave energy is absorbed by electron beam and input cavity.In same work condition, PIC simulation result of same structure in CST software is average power of 600 W, corresponding frequency of 94.82 GHz, gain of 43 d B, power conversion efficiency of 10.4%. The simulation result by CST software and MAGIC software is consistent. |
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