Investigation On High-Power W-Band Confocal Gyrotron Travelling-Wave Amplifers

Millimeter-wave gyro-devices exhibit remarkable advantages over conventional linear-beam vacuum electron devices in terms of the output power and circuit gain.Among them,gyrotron traveling-wave tube?gyro-TWT?exhibits essentially applications in both military and civilian fields such as high-resolution ranging and imaging radar,electronic countermeasures,high-bit-rate communication systems and advanced medical imaging systems due to its high power,high frequency and broadband characteristics.Since the distributed-loss circuit was firstly proposed at the end of last century,various institutions over the world have adopted this circuit to develop high-performance gyro-TWTs.At present,gyro-TWTs operate at W-band and beyond are confronted with more obstacles compared to that operating at the lower millimeter-wave band such as Ka-band.Main reasons are as follows.The transverse dimension of circuit decreases with the increase in operating frequency since the gyro-TWA operates near cutoff frequency.As a result,the electron beam tunnel and power-handling capability of the circuits are singificantly reduced for gyro-amplifiers at low-terahertz band.Moreover,a small structural roughness and fabrication/assembly torlerence may result in dramatic decrase in the output performance.As far as be concerned,operating at higher-order modes?HOMs?are effective solution when considering the above factors.Higher-order mode operation can effectively increase the size of rf circuits,thereby increasing the average power capacity and electron beam tunnel,and reducing fabrication and assembly requirements.However,when the gyro-TWT operates at HOMs,the electron beam mode will intersect with the lower-order modes in the negative propagation constant region given by the dispersion diagram,thus backward-wave oscillations will occur.In order to alleviate the mode competition and parasitic oscillation for HOM operations,Massachusetts Institute of Technology?MIT?has proposed two novel rf circuits,namely the confocal?or quasi-optical?waveguide and photonic-band-gap?PBG?circuits.The two circuits can achieve overmoded operation whilst avoiding concomitant oscillation issues attributing to its inherent mode selectivity,which can significantly reduce the mode density of the circuits,making stable HOM operation possible.Between them,the PBG circuit seems unworkable at present,limited by the fabrication technology and thermal dissipation of rf circuits.Based on the development facilities of the Research Center of High-Power Millimeter-Wave Technology at the University of Electronic Science and Technology,this dissertation focuses on the investigation of high-power confocal gyro-TWTs for industrial use.In specific,for gyro-TWT operates at W-band,the diameter of cylindrical circuits is around 4 mm for fundamental TE₀₁-mode operation while it is 7 mm for HE₀₄-mode operation in confocal circuits.Although the HE_mn mode with m>0 in confocal circuits encounters heavey loss,the lower-order HE_0n modes are still potential oscillations due to a similar propagation property as the operating mode.According to the preliminary hot-test results at MIT and China Academy of Engineering Physics?CAEP?etc.,instability including BWO oscillation and near-cutoff self-oscillation,and limitations on the performance of the amplifier such as the narrow bandwidth and low efficiency of the input coupler remain unsolved.The dissertation mainly focuses on the investigation on the rf circuits of confocal gyro-TWTs.Specifically,design software of confocal gyro-TWT has been developed.In addition,very possible reason related to the instability of the circuits was found and solution on improving the performance of the input coupler are proposed after carefully examining the previous hot-test results at MIT and CAEP etc.More details are as follows:1).A generic method of calculting the diffraction loss of confocal waveguides is theoretically derived,which can be applied to calculate the diffraction loss of confocal waveguides with arbitrary mirror width for different modes.The developed method shows much better precision compared to the widely used quadratic polynomial fitting expression,especially for circuits composed of narrow mirrors.2).The state-of-the-art input coupler with wide bandwidth and high-efficiency for confocal circuits has been developed.This made much progress on developing confocal gyro-TWTs because one of the biggest challenges in MIT's experiments is the bandwidth and efficiency limitation of the input system.In addition,two novel broadband confocal mode exciters with flat transmission have been developed.Between them,one of the mode exciter exhibits the state-of-the-art performance in bandwidth,mode-converting efficiency and in-band flatness,which is a good candidate for circuit measurements.3).Feedback mechanism resulted from diffraction in confocal circuits is studied for the first time,which will result in significant decrease in total loss of rf circuits.As a result,the stability of the gyro-amplifier will be dramatically decreased.Simulation model is designed to explain and verify the feedback mechanism.Finally,the previously developed mode converters are applied to the principle-proof measurements of the mechanism and it has been successfully verified.4).Theory of beam-wave coupling impedance in confocal waveguide circuits is deduced in detail.On this basis,the kinetic theory and nonlinear theory of confocal gyro-TWT are derived.Numerical results show the two theories agree well in small-signal region.5).Based on the above theories,a versatile software for confocal gyro-TWTs design is developed,which mainly includes:dispersion diagram plotting module,generic diffraction-loss calculation,amplification analysis for generic circuits with arbitray magnetic field profile B?z?and starting-threshold analysis for both near-cutoff oscillation and back-wave oscillations.Finally,an example of W-band non-uniform distributed-loss confocal gyro-TWT based on the software is given,and then demonstrated by particle-in-cell?PIC?simulation.