| Great progress has been made with the high-power microwave (HPM) technologyat high power, high efficiency, long pulse and high repetition operation, and the researchtrends on some other kind of HPM source emerge, such as dual-frequency andmulti-frequency HPM generators. HPM sources with dual-frequency andmulti-frequency are worthy of great academic value and potential project application,which could be applied to electronic warfare, as well as various communication systems.The relativistic backward-wave oscillator (RBWO) plays an important role in HPMfield and is one of the most promising HPM sources due to its essential characteristics,such as high power, high efficiency and high repetitive rate.In this context, a thorough and comprehensive research on the dual-frequencycoaxial relativistic backward-wave oscillator (CRBWO) has been conducted in thisdissertation, including theoretical research, numerical simulation and calculation,particle-in-cell (PIC) simulation, as well as experimental research, and experimentallyan X-band dual-frequency output is got with a single electron beam in a CRBWOdevice, which not only successful explored a new technological path for generatingdual-frequency microwave with a single electron beam, but also lays a theoreticalfoundation and provides an experimental basis for the pioneering research on theCRBWO applications in dual-frequency and multi-frequency HPM fields. Thefollowing contents have been studied in this dissertation:1． The linear and self-consistent dispersion equation for coaxial periodicslow-wave structure (SWS) is derived on the electronic field matching theorem andFourier series theory, in which the factors of inner and outer corrugations, the phaseshift between inner and outer conductors and the radial thickness of the electron beamare included, and can be applied to arbitrary coaxial periodic SWS, and compared toother models, this theory gets closer to the real HPM experiments. In comparison withhallow SWS, coaxial SWS permits higher space-charge limiting current, is competent toachieve more efficient microwave production and wider electronic frequency tuningbandwidth. The characteristics of dispersion curves of the experimental model with the electron beam (“cold” cavities) and without the electron beam (“hot” cavities) andmicrowave time growth rate and the coupling impedance are numerically analyzedbased on the derived dispersion equation, which indicate that: when the ripples remainthe same, the upper cut-off frequencies of the dispersion curves of outer-ripple-onlycoaxial SWS are higher than the curves of inner-ripple-only and double-ripple (bothinner and outer) coaxial SWSs, which is beneficial for frequency tuning; the couplingimpedance increases with the increase of the ripple, whereas decreases with the increaseof the SWS periodic length; the coupling impedance of the1st harmonic of TEMmode andTM01of double-ripple coaxial SWS within the cross section of the electronbeam are enhanced, which permits more efficient synchronous beam-wave interactionand a higher RF power conversion efficiency; the microwave time growth rate increaseswith the increase of the ripple and decreases with the increase of the SWS periodiclength; the strength of the surface wave and the coupling impedance, as well as themicrowave time growth rate, increase as the electron beam gets close to the surface ofthe outer SWS; the microwave time growth rate increases as the electron currentbecomes larger, which is conducive to the microwave oscillation of CRBWO.2．A dual-frequency CRBWO with a single electron beam is proposed, and athorough and comprehensive research on the dual-frequency operational mechanismand performance is conducted by use of a2.5-dimentional PIC code. The simulationresults show that with an electron beam of500kV and8.7kA guided by a magnetic fieldof0.82T andL2=0.5, an X-band dual-frequency microwave output is acquired, the dualfrequencies are10.06GHz and10.49GHz, respectively, both corresponding to aradiation pattern ofTM01mode. Clear and obvious beat frequency electric field signalsis observed with peak microwave power of2.3GW and average power of700MW, inwhich the power of the two frequency components are383MW and317MW,respectively, and the average microwave efficiency of the generator is16.1%.Time-frequency analysis of the microwave output demonstrates that the times of startingoscillation and saturation of the two frequency components are different and thesaturation time of the dual-frequency output is about8ns. The independence of theoperating frequency of the two SWS sections (SWS1and SWS2) are studied andverified by use of PIC simulation.3．A dual-frequency CRBWO with coaxial resonator is firstly proposed and investigated in detail by use of a2.5-D PIC code. When the cathode potential is520kV,an electron beam of-8.5kA is emitted and microwave with average power of1.07GW isproduced, the power conversion efficiency is24.2%and the dual frequencies are9.43GHz and10.30GHz. Furthermore, quasi-periodic occurrence of the dual-frequencyspectrum is obtained by changingLd with the cycle length about a half of thewavelength off2, simultaneously, a frequency agility bandwidth of about400MHz off1is acquired.4．Extended research on the dual-frequency output is performed and a CRBWOwith an asymmetric reflector cavity generating the C-band and X-band microwaves isproposed and designed. Improved sectioned coaxial SWS by introducing stepwisevariation of coupling impedance and phase velocity is designed and introduced tooptimize the axial electric field in the SWS sections and decrease the energy scatter ofthe electron beam. The model is investigated by use of a PIC code which reveals thatwith an electron beam of510kV and9.03kA and an axial guiding magnetic field of0.73T, dual-band microwaves are generated with dominant frequencies of8.09GHz and9.91GHz, corresponding to C-band and X-band, respectively. The power of thedual-band microwave is1.0GW with efficiency of21.9%, which is higher than otherdual-band HPM devices. Meanwhile, the dual-band frequencies demonstrateperiodic-like dependence on the length of the tapered waveguide between SWS1andSWS2, and this is the first time to obtain dual-band frequency agility in thedual-frequency HPM field.5．The experiments of the X-band dual-frequency CRBWO are performed with apulsed accelerator. Firstly, the measurement methods of the electron beam parametersare described briefly, then a detailed description of the microwave diagnostics(including radiation pattern, microwave frequencies and power) is provided, after whichthe calibration methods of the experimental measurement systems and the design of theguiding magnetic field coils and the double-row coaxial support post, as well as theradiation system, are given. When the magnetic field is0.82T, the diode voltage is512kV and the beam current is11.5kA, microwave radiation with power of509MW isobtained, the power efficiency is8.6%and the pulse width is about50ns.Dual-frequency output with frequencies of9.97GHz and10.52GHz and frequencydifference of550MHz are achieved, which are close to the PIC simulation results. Clear and stable beating phenomenon of the electric field in frequency of about500MHz isobserved, which is in stark contrast to the single frequency radiations. The microwaveradiation pattern is measured both with the far-field method and the fluorescencemethod, which jointly prove that the radiation pattern isTM01mode. In a word, thetheoretical and numerical analysis and the PIC simulation results and experimentalresults coincide with each other, which confirm the idea of the design of adual-frequency CRBWO with a single electron beam proposed in this dissertation.Finally, the primary work and innovation of this thesis are summarized andprospected the next research work. |
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