Research Of The Terahertz Backward Wave Oscillator Based On The Hole-grating Slow Wave Structure

Terahertz(THz)wave(defined as electromagnetic radiation from 0.1 THz to 10 THz)has good penetrability,wide bandwidth and low photon energy.Hence it is attractive for many applications,such as medical imaging,high data-rate communication,gas sensing,and high-resolution radar.Radiation source plays a key role in the development of the THz technology.As one of the established vacuum electron device(VED),backward wave oscillator(BWO)has a simple configuration and could tune over a very wide frequency band by adjusting the beam voltage,so it is an important THz radiation source.However,there remain many challenges in the BWO development process,including improving the radiation intensity and efficiency,minimizing the weight and dimensions of the structures,and reducing the operating current density.Substantial research indicates that multiple electron beam can help to improve the radiation intensity and reduce the operating current density.The sheet electron beam has a flat shape and low space charge field,it is more suitable for use in small dimensional flat devices and can propagate closer to the surface of the SWS thereby enhancing the beam-wave interaction and reducing the requirement of the focusing system.Sub-wavelength hole array has the property of enhancing the electromagnetic wave transmission and could be used in the design of THz radiation sources.On these bases,a dual sheet beam THz BWO based on the hole-grating slow wave structure(SWS)has been proposed in this dissertation.Theoretical analyses,simulations and principle tests have been described in detail.The main achievements and innovations of this dissertation are as follows:1.A single-hole-grating SWS has been presented on the bases of the grating structure and sub-wavelength hole-array.The dispersion equation has been derived by the longitudinal field method and field matching theory,and has been validated by numerical calculations and simulations.Using HFSS software,the influence of the structure dimension on the high-frequency characteristic has been obtained,and the electric field distribution in the SWS has been discussed.The 3D particle-in-cell(PIC)simulations have been carried out by Chipic software.Results demonstrate that,because of the enhanced transmission effect of sub-wavelength hole array,the electric fields in the upper-and lower-beam tunnels are integrated.The single hole-grating BWO has obviously large beam-wave interaction region and is favorable to improving the beam-wave interaction power and shortening the time to the stable status.2.Based on the effect of the grating and hole on the surface electric field distribution characteristic,the number and position of the rectangular hole have been optimized.A double-hole-grating SWS has been proposed,which has improved the transverse distribution of the surface electric field in the beam tunnel and is beneficial to the stable transmission of the sheet beam.According to the analyses of high-frequency characteristics,a 0.14 THz double-hole-grating BWO has been designed and optimized.The 3D PIC simulations have been implemented by CST Particle Studio.The influence of the operating parameters on the output and its physical mechanism have been discussed in detail.Results indicate that the double-hole-grating BWO has many advantages,such as low voltage and low current density,and capability of producing W-level output power in the 7.6 GHz tuning region.Meanwhile,because of the introduction of the hole array,the current difference between two sheet beams couldn't affect the output of the double-hole-grating BWO,and the BWO could provide stable output as long as the current density of one electron beam is larger than the minimum starting current density.Thus,it could reduce the requirement of the dual sheet beam gun and ensure the feasibility of our dual-beam BWO.3.An E-plane power combiner which is suitable for the combining of two equal-amplitude and reversed-phase signals has been designed utilizing the phase modulation of the quarter-arc bent rectangular.This power combiner avoids the micro-geometry in the conventional T-junction combiner and reduces the difficulties of the designing and processing.Moreover,an approximate method based on the analytic geometrical analysis has been given to design this E-plane power combiner for different frequency bands.4.According to the structure symmetry and the enhanced transmission effect of sub-wavelength hole array,a multi-beam BWO composed of multilayer cascading of the hole-grating structure has been proposed.The electric field distribution,output characteristics and the physical process of the beam-wave interaction have been analyzed.Investigations show that,compared with the 1-layer hole-grating BWO,BWOs based on 2-and 3-layer cascading structures could produce higher output power,and their efficiency increases by 23% and 30%,respectively.Energy could be gathered from the external two beam tunnels,in which the electric fields have equal amplitudes and reversed phases even sheet beams have different current.Increasing current of the inner beams could improve the output power and efficiency of the device.5.The dual-hole-grating high-frequency system and the E-plane power combiner have been machined and tested.The transmission characteristics have been obtained.Combining the theoretical analyses,the test results have slight differences in the resonant frequencies because there is deviation of machining accuracy,but the trend is basically the same.Experiments verify the correctness of the design.