| More and more attention has been paid to millimeter wave because of its intrinsic advantages in many aspects. It has become the keypoint to further increase the output power. Slow wave devices have been used widely to generate high power microwave for their high efficiency, simple mechanism and stability. The power capacities of millimeter wave devices are limited due to their small dimension. To increase the radial dimension reasonably, i.e., to adopt overmoded slow wave structures, can effectively increase the power capacity. However, when overmoded slow wave structures are used, high order modes may be excited within the device. According to the modes analysis, single-mode operation can be achieved. In millimeter wave devices, beams with less spacial fluctuation are required. So, stronger magnetic field is usually needed. Based on theoretic analysis, low magnetic field operation can be achieved. In this dissertation, an overmoded slow wave type high power millimeter wave generator is designed. And it can be operated at a relatively low magnetic field to generate microwaves of quasi-single-mode. The main content and innovative work are listed as follows.Firstly, the overmoded slow wave structures (SWS) are investigated systematically. Fourier-series method is used to investigate the dispersion characteristics. The dispersion curves of rectangular SWS and edge-rounded rectangular SWS are calculated. The effects of different structural parameters on dispersion characteristics are investigated. Taking the rectangular SWS for example, the coupling impedances of TM01 mode are investigated. And this will help to design the device.Secondly, mode selection in overmoded devices is analysed. The field distribution characteristics of overmoded slow wave structures have been analysed. It is found that the distribution ofπmode of TM01 mode is surface wave while the fields of TM02 and TM03 are volume waves. So it is possible for us to choose proper parameters of the SWSs as well as the beam to only excite TM01 mode. Strong magnetic field is usually needed to confine electron beam in millimeter wave devices, but in practical use, lower magnetic field will be more convenient. So the feasibility of low magnetic field operation is investigated. And a relatively low magnetic field of about 0.80T is selected according to Particle-in-cell (PIC) simulation.Thirdly, PIC simulation is used to investigate the mechanism of the proposed device of generating high power millimeter waves. The detailed beam-wave interaction process is analyzed with the help of the 2.5-dimensional PIC code. The influence of structure parameters and operation parameters on the performances of the device is investigated, such as SWS ripple depth, the number of the periods of the two SWSs, the length and radius of the output cavity, the beam voltage and the strength of the external guiding magnetic field. In the optimized configuration with rectangular SWSs, a microwave of TM01 mode at 33.20GHz is generated in simulation with diode voltage of 600kV, beam current of 5.05kA and guiding magnetic field of 0.85T. The output microwave power is 1.05GW. The microwave modes have been studied in the beam-wave interaction region and at the output port. In the beam-wave interaction region, it is typical surface wave of the TM01 mode. While at the output port, it is the TM01 mode in smooth waveguide. With edge-rounded rectangular SWSs, a microwave of TM01 mode at 33.10GHz with the power of 1.02GW is generated, with diode voltage of 600kV, beam current of 5.05kA and guiding magnetic field of 0.85T. A permanent magnets system of 197.6kg is designed for the device to achieve gigawatt output power.Finally, experiments are carried out to investigate the proposed high power millimeter wave generator. Experiments are carried out on the TORCH-01 accelerator. For the device with rectangular SWSs, with the diode voltage of 600kV, the current of 5.2kA and the guiding-magnetic field of 0.8T, the radiated microwave with frequency of 33.56GHz, power of 113MW and pulse duration of 7-8ns is generated. The edges of rectangular SWSs are rounded to improve output microwave power and efficiency. For the improved device, with the diode voltage of 590kV, the current of 5.2kA and the guiding-magnetic field of0.8T, the radiated microwave with frequency of 33.56GHz, power of 320MW and pulse duration of 13ns is generated, and the power efficiency is about 10%.
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