Research On Dielectric-metal Terahertz Slow Wave Structure

With the development of terahertz technology,high-power output terahertz wave sources have become an urgent need to realize the popularization of various terahertz technologies.Traveling wave tube is a kind of vacuum electronic device.It has the advantages of high power capacity and wide bandwidth,so it is widely used in communication,military,aerospace and other fields.The slow wave structure is the core component of the traveling wave tube(TWT),which not only has the function of reducing the phase velocity of the electromagnetic wave,but also is the place of the beam-wave interaction.The existing full metal staggered double-gate slow wave structure and foldedwaveguide slow-wave structure have many excellent simulation and experimental results in the millimeter waveband and sub-terahertz frequency band.However,as the frequency increases,the surface roughness of the metal structure due to the machining accuracy is similar to its skin depth in the terahertz frequency band,so that the high frequency loss of the terahertz wave in the full metal structure increases sharply,which will greatly reduces the output power of the TWT.Therefore,it is very urgent to develop low-loss slow wave structures.In this thesis,two schemes for reducing the loss of slow wave structures are proposed,two existing terahertz slow wave structures are improved,and the effectiveness of these two schemes is verified by simulation.The first scheme is to increase the reflectivity of the overall structure by covering the metal surface with high-density polyethylene,and covering the dielectric with graphene to prevent electron aggregation.Therefore,an 850 GHz staggered double-gate slow wave structure based on dielectric and graphene is designed.Compared with the full metal staggered double-gate slow wave structure in the same frequency band,the transmission loss of this structure is reduced by 0.4dB/mm-0.8dB/mm.Then a matching input-output coupling structure is designed,and the beamwave interaction of the structure is simulated.The results show that the saturated output power at 850 GHz is 1.475 W and the gain is 21.7dB,and the operating bandwidth is about20 GHz,the maximum output power in the band is 2.4W.The second solution is to reduce the right-angle structure of the metal in the slow wave structure,improve the existing rectangular column photonic crystal slow wave structure into an elliptical cylinder photonic crystal slow-wave structure,and replace the rectangular metal column in the former structure with an elliptical metal cylinder,and then redesigned the tapered coupling structure to match the new structure.The loss of the slow wave structure of the elliptical cylinder photonic crystal was reduced by0.02 dB/mm-0.04 dB/mm.By simulating the beam-wave interaction process of this structure,it is obtained that the saturated output power of the traveling wave tube at215 GHz is 16.94 W and the gain is 17.1dB,the operating bandwidth is 40 GHz,and the maximum in-band output power can reach 25.9W.