Low Magnetic Field Of The X-band High Efficiency Backward Wave Oscillator

This dissertation focus mainly on a X band high efficiency low guide magnetic field Backward Wave Oscillator, including the following the studies: (1) Beginning with the non-uniform slow wave structure, we discussed its electrical properties from two aspects of the amplitude and the period of the corrugated wall which greatly affect the beam-wave interaction efficiency and output power. Furthermore, we established a linear theory of the non-uniform slow wave structures and formulated the expression of the starting current. (2) Secondly, to face to the problem of pulse shortening that commonly exists in the research of high power devices, we discussed the reasons of causing it as well as some methods to eliminate it. One of these methods is to use slow wave structures with large radius, but it also brings out another problem, that is mode competition. Then we introduced some kinds of effective mode selection devices, especially the resonant cavity. We gave a systematical process of designing a resonant cavity and some PIC simulations by using a self-designed one. (3) After that, we analyzed with special emphasis of the slow wave structures with a coaxial structure inside it. We focused our study on the TM01 mode electromagnetic wave. We described in detail the effects of the amplitude, period and the radius of the coaxial structure on the backward wave oscillators. Then we established a linear theory of the slow wave structures with a coaxial conductor inside it. Finally we studied the effects of the electron beam parameters such as voltage, current, the radius of the beam, the guide magnetic field and the beam current density on the output power, interaction efficiency and microwave radiation frequency. The results of the study show that RBWO with a coaxial structure inside it has unique advantages: the inner coaxial structure can largely increase the cutoff frequency of the system, thus it allows a SWS with a more larger radius which will increase the system power capacity; Additionally, it also allows a larger radius electron beam to pass through the SWS, this will heavily decrease the space charge effects compared to a smaller radius beam with the same current, hence the BWOs can work well at a lower axial guide magnetic field. Besides that, the electromagnetic field distribution is of great advantage of effective electron beam and the electromagnetic wave interaction. An efficiency of 60% at frequency of 11.09GHz was obtained in the PIC simulation with a 0.6Tesla axial guide magnetic field. RBWO with a coaxial structure is a kind of promising vacuum high power device.