| Millimeter-wave technologies have great potential applications in many military and civilian fields,including 5G communications,imaging,radar,electronic countermeasures and material detection.Millimeter-wave amplifiers/oscillators are one of the very important components in millimeter wave technology,which have great research values.As one of the solutions for millimeter-wave radiation sources,millimeter wave vacuum electronic devices(VEDs)have received great attentions in recent years.Researchers around the world have conducted extensive research of VEDs in the low frequency band of the millimeter-wave.Some progresses have been made and some preliminary solutions have been achieved.To further improve VEDs' performances working in in the low frequency band of the millimeter-wave(30-100GHz),it is necessary to further study some new types of VEDs,such as the sheet beam devices.Radiation sources in the high frequency band of the millimeter-wave(100-300GHz)are still in the exploratory stage.So far,there is a lack of compact high power millimeter-wave sources,thus requiring every effort around the world to make a further progress.Based on the research status regarding VEDs both in the low and high frequency band of the millimeter wave,this paper has made some progress in the following two aspects:(1)In the low-frequency band of the millimeter-wave,the RF system of a Q band(30-50GHz)sheet beam travelling wave tube(SB-TWT)has been in-depth investigated,with an aim of upgrading the tube's performance and thus achieving high power compact millimeter wave amplifiers.(2)In the high frequency band of the millimeter-wave,a 0.2 THz extended interaction oscillator(EIO)driven by a pseudospark-sourced sheet electron beam was studied,which is committed to providing some possible solutions for high power compact millimeter-wave and terahertz oscillation sources.Compared with traditional round beam TWTs,SB-TWTs have the characteristic of high output power.Compared with gyro-TWTs,SB-TWTs have the advantage of miniaturization.Therefore,a SB-TWT is one of the most promising millimeter-wave amplifiers.However,the design and fabrication of SB-TWTs is so far not very mature,thus having great research significances.For the research of SB-TWTs,this thesis has mainly concentrated on the high frequency system,which is composed of the 2-4th chapter.In Chapter 2,the input/output structures for SB-TWTs were studied.Four kinds of novel input and output couplers were put forward:(1)A L-type branch waveguide coupler and its extended structures;(2)A Y-type branch waveguide coupler and its extended structures;(3)A multiple branch waveguide coupler;(4)A single branch waveguide coupler.The above four kinds of structures were achieved by two main innovative ideas:(a)Couplers widely used in other fields were firstly introduced to SBTWTs;(b)Based on the traditional structures,the coupler performances can be improved by introducing some other structures,such as a reflection cavity,a symmetric cavity and wave-absorbing materials.The above several kinds of structures were investigated through theoretical analysis,simulation and millimeter wave cold test.Compared with the couplers in the published literatures,the above several couplers not only have wonderful electrical properties,such as ultra-wide frequency band,but also have achieved great structural performances,such as a large electron beam channel and a compact configuration.The proposed input and output couplers can be well used to separate or converge the beam and the wave.Chapter 3 has conducted in-depth research in slow-wave structures(SWSs)for SBTWTs to achieve wide bandwidth,high efficiency,high gain,and stability operation.The operation characteristics of the staggered-vane SWS have been theoretically analyzed.Dual-mode operation for the staggered-vane SWS was proposed to broaden the working bandwidth.PIC simulation results show that the bandwidth of the SWS has been improved from 9 GHz to 15 GHz after adopting dual-mode operation.Cold test experiment was carried out for the high frequency system,and the simulation and experimental results were in good agreements on the curve trend.Hot test experiment was conducted,and output power was detected in the frequency band corresponding to the 2th mode,verifying the feasibility of dual-mode operation.Based on the traditional staggered-vane SWS,an improved SWS with a curved beam tunnel was put forward.PIC simulation results showed that the output power,gain and efficiency have increased by 1kW,3dB,and 2%,respectively.In addition,other methods to improve the efficiency and gain were also studied,including the narrow band high efficiency method and the velocity phase tapering method.Studies show that these methods can also effectively improve the efficiency and gain of the whole tube.Both the reflection oscillation and the backwardwave oscillation were studied,and the corresponding oscillation suppression scheme was proposed.A new kind of dielectric attenuator was put forward,which can effectively suppress the oscillation and meanwhile reduce the difficulty of practical implementations.Dielectric materials are widely used in the high frequency system of SB-TWTs.Accurate measurement of the complex permittivity for low-loss materials is of great significance to the design of SB-TWTs.In the measurement of dielectric materials,it is very difficult to achieve the complex permittivity of low-loss dielectric materials,which is of great research value.This paper developed a measurement system for testing the complex permittivity for low-loss dielectric materials based on the quasi-optical cavity.The complex permittivity was achieved through measuring the S21 resonant curve in the traditional quasi-optical cavity measurement system,and the quasi-optical cavity usually has two coupling apertures.In this paper,the complex permittivity was measured through S11 resonant curve.And the quasi-optical cavity usually has only one single coupling hole,which has greatly simplified the coupling structure of the quasi-optical cavity,thus benefiting for the fabrication and assembly.In order to verify the feasibility of this scheme,a W-band quasi-optical cavity was designed through theoretical analysis and simulation.Based on the fabricated quasi-optical cavity,a complex permittivity measurement system was built.A sapphire window was tested using the system.Test results were in good agreement with the simulated results.As for the research of the 0.2 THz sheet beam extended interaction oscillator(SBEIO),a new idea was put forwarded and preliminary exploration research has been conducted.When the operating frequency increases up to the high frequency band of the millimeter-wave or the terahertz band,a series of problems and challenges will become more serious,including small electron beam current,large copper loss,the difficulty of processing and assembly,the difficulty of electron beam focusing.In order to reduce the disadvantageous effects caused by these problems,this dissertation put forward to combine with the advantages of plasma cathode electron gun(high current density,ion channel focusing),sheet electron beam(large beam area)and EIOs(high gain per unit length,compact configuration,and short interaction length)to obtain a high power millimeter-wave or terahertz-wave pulse source with a compact configuration.To verify this idea,the following work has been done in this thesis:(1)Experimental studies of the plasma cathode electron gun and preliminary investigations of the pseudospark-sourced sheet electron beam have been conducted;(2)Optimal design,fabrication and cold test of a high frequency system has been finished,and test results were in good agreement with simulations on the trend;(3)Assembling the whole tube as well as conducting the preliminary hot test experiment,millimeter-wave signals were detected in the experiment. |
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