| The relativistic backward-wave oscillator (RBWO) is one of the most promising high-power microwave (HPM) sources with characteristics of high power, high efficiency and high repetitive rate. RBWO generating microwaves with multi-frequencies is becoming a novel interesting topic in recent years and has attracted more and more attention. Therefore, two kinds of RBWOs generating dual-band microwaves with a single beam and dual beams are discussed in this dissertation. Experiments of the dual-band RBWO with dual beams are carried out to verify the designing method of such a kind of device, and dual-band microwaves are generated. The detailed contents and innovative work consists of the following.1. An RBWO with a single beam is designed to be capable of generating C-band and X-band microwaves based on the model of RBWO with sectioned slow wave structures (SWSs), and investigated by use of a 2.5-dimensional particle-in-cell (PIC) code.With an electron beam of 650kV and 5.0kA guided by a magnetic field of 2.2T, dual-band microwaves with power of 380MW and dominant frequencies of 5.48GHz and 9.60GHz are produced. The design principle of the dual-band RBWO is obtained by the investigation on a simplified structure. The effect of the axial length of the tapered waveguide between two sections of SWSs on the dual-band microwaves is analyzed.2. An RBWO with dual beams generating the C-band and X-band microwaves is proposed and designed, and analyzed physically.The distribution of the electrostatic field on the surface of the coaxial dual-annular cathode at the voltage of 650 kV is simulated. The results reveal that the beam emission of the inner and outer cathode is independent except for the gap closure.The dispersion relation of the coaxial corrugated SWSs with infinitely length and guiding magnetic field is derived. And the longitudinal resonant modes of the C-band structure are investigated, among which the resonant reflector and the resonant cavity are investigaed by the microwave injection into the"cold"cavities and by the PIC simulation. The dispersion relation of the non-coaxial corrugated SWSs with infinitely length and guiding magnetic field is derived. And the longitudinal resonant modes of the X-band structure are investigated with the characteristic of each longitudinal resonant mode explained.3. The dual-band RBWO with dual beams is investigated in detail by use of a 2.5-dimensional PIC code, and the optimization of the structure is perfomed.With the diode voltage of 650kV, the inner and outer beam currents of 5.4kA and 6.4kA guided by a magnetic field of 2.2T, dual-band microwaves are generated with dominant frequencies of 4.625GHz and 8.450GHz. The powers of the dual-band microwaves are 0.92GW and 0.60GW with efficiencies of 21.8% and 17.1%, respectively. The transverse mode of the C-band microwave is quasi-TEM mode with a longitudinal mode ofπ-like, and that of the X-band microwave is TM01 mode with a longitudinal mode of 4π/7.The effects of structure parameters, guiding magnetic field strength, beam parameters, and microwave leakage on the dual-band microwaves are presented and discussed. The single-band operations of the dual-band RBWO are realized, and the powers and frequencies are the same as those from the complete structure. Such results demonstrate the independence of the generation process of the C-band microwave and that of the X-band microwave.4. The experiments of the dual-band RBWO with dual beams are performed.The dual-annular cathode, guiding magnetic field coils, the radiating antenna and polyethylene window are designed. The diagnostics of the beam parameters, powers, frequencies and radiation patterns are described in detail.When the diode voltage is 651kV, the total beam current of 14.9kA is obtained at a guiding magnetic field of about 1.7T, with the inner and outer beam current of 6.4kA and 8.5kA, respectively.With the diode voltage of 657kV and the total beam current of 14kA guided by a guiding magnetic field of about 1.7T, dual-band microwaves are generated with dominant frequencies of 4.58GHz and 8.30GHz both corresponding to a radiation pattern of the TM01 mode. The powers of them are 520MW and 113MW with efficiencies of 9.3% and 2.7%, and pulse widths of 20.2ns and 19.8ns. Besides, a dominant frequency of 8.46GHz is also generated in the X-band microwave with much smaller amplitude than that at the frequency of 8.30GHz.In addition, the effects of variations in the guiding magnetic field and diode voltage on the powers of the dual-band microwaves are presented and discussed. The single-band operations of the dual-band RBWO are realized, and the same frequencies and radiation patterns as the results of the complete structure are obtained, which indicates that the generation processes of the dual-band microwaves are independent.In order to suppress the multifrequency excitation in the generation process of the X-band microwave, the structure is modified and further experiments demonstrate that, when the diode voltage is 650kV and the guiding magnetic field is 1.7T, dual-band microwaves with dominant frequencies of 4.58GHz and 8.30GHz are generated, and the powers of them are 866MW and 206MW, respectively. Moreover, the frequency of the X-band microwave is approximately monochromatic. When the diode voltage is larger than the critical diode voltage which stands between 739kV and 789kV, the obvious multifrequency oscillation of the X-band microwave is observed. Furthermore, the critical diode voltage resulting in obvious multifrequency falls between 694kV and 728kV with a higher inner beam current by decreasing the inner A-K gap. 5. In order to simplify the structure, the structure of the X-band is redesigned, and both the PIC simulation and preliminary experiment are performed.With the diode voltage of 645kV, the inner and outer beam currents of 5.5kA and 6.4kA guided by a magnetic field of 2.7T, dual-band microwaves are generated with dominant frequencies of 4.625GHz and 8.750GHz by the PIC simulation. The powers of the dual-band microwaves are 0.93GW and 0.71GW with efficiencies of 22.5% and 20%, respectively.In the preliminary experiment, when the diode voltage is 650kV and the guiding magnetic field is 2.7T, dual-band microwaves with frequencies of 4.58GHz and 8.76GHz are obtained, and no multifrequency excitation is observed in the X-band microwave with the diode voltage ranging from 494kV to 650kV.
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