Investigation Of A P-band Coaxial Relativistic Backward Wave Oscillator

The backward wave oscillator (BWO) is one of the promising devices that can produce high-power microwaves and is intensively investigated because of high power, high efficiency and good stability. Now there are many reports on BWO's that operate in the frequency regime of X-band (8GHz-12GHz) or C-band (4GHz-6GHz), but reports on P-band (less than 1 GHz) are rare. However,it may be mentioned that the HPM source with frequency less than 1GHz still has very important applications in both military and industrial fields, therefore research on the compact P-band BWO device is certainly interesting. This thesis presents preliminary investigation on the P-band BWO using linear theory, particle simulation, and experimental design. A series of valuable results are obtained from this work.The present thesis gives theoretical investigations on the coaxial arbitrary-shaped outer periodic SWS using the linear fluid model. The TM mode"cold"dispersion equation of the coaxial BWO in the infinite guide magnetic field is derived. Then the"hot"dispersion equation of the coaxial arbitrary-shaped periodic SWS considering the electron beam is also derived and solved numerically. The result gives us the approximate dimensions of the main SWS for a BWO device that can work in frequency range of P-band. We can use these approximate dimensions of the SWS in the particle simulation code to start the optimum design process of the P-band BWO device. Studies also show that the radius of the coaxial P-band BWO can be smaller than that of the non-coaxial P-band BWO. The coaxial P-band BWO can be very compact and convenient for real application. So investigation on the compact coaxial P-band BWO is of great interest to the high-power microwave field.With the use of the 2.5D fully electromagnetic particle-in-cell (PIC) code, the coaxial P-band BWO is investigated based on the uniform dual-ripple SWS model, showing the dependence of microwave radiation on the structure parameters, the beam parameters, and the guide magnetic field parameters. The electromagnetic structure of the uniform coaxial P-band BWO can be improved, and the simulation result shows that a 767MHz, 2.51GW high power microwave after saturation can be obtained when using an elelctron beam of 715keV and 12kA in the case in which the guide magnetic field is 0.83T. The efficiency is about 30.8%. The total length of the uniform coaxial P-band BWO is about 126 cm and its maximum radius is 5.9cm.Simulation results show that the efficiency of the nonuniform BWO is considerably higher than that of the multi-stage BWO while their maximum radius of the ripples are the same. An approach of designing the coaxial BWO is proposed. The efficiency of the RBWO could keep a high value while the amplitude (2r1) and period (z0) of the ripple of the slow-wave structure (SWS) and the wavelength (λ) of the generated microwave satisfy the relations of z0≈2λ/5 and 2r1≈λ/20. Using the advantages of coaxial SWS and nonuniform SWS, a compact P-band high-power tapering nonuniform dual-ripple coaxial BWO is proposed. Using a beam of 717keV and 11.3kA, a high power microwave is abtained from the model with frequency of 767MHz and output power of 2.76GW when the guide magnetic field is about 0.83T.The power conversion efficiency is anout 33.9%. However, when the guide magnetic field reduces to about 0.69T, a 767MHz, 2.37GW high power microwave is still obtained in the simulation if the beam energy of 710keV and the beam current is about 12.5kA.In addition, we design the device considering three aspects including the matching with accelerator, the radiation system, and the engineering design. The radiation system is designed using a high-frequency-field numerical simulation software, which provides many functions such as mode converting from TEM to TM01, keeping the output waveguide and the space impedance in matching, making the inner and the outer conductor grounded, and mechanically supporting the inner conductor. In the final part of this thesis, the engineering design of P-band tapering nonuniform dual-ripple coaxial BWO is presented, considering the simulation result and the design of radiation system.These results in this thesis are of interest to potential application of the P-band high power microwave generated with BWO in the future.