Study On Properties Of The Slow Wave In The NIM And Its Interaction With The Linear Beam

Based on the Cherenkov Effect,the Cherenkov devices including traveling-wave tubes and backward wave oscillators are the most important radiation sources,which play a remarkable role in the field of radar,communication and electronic countermeasure.In the Cherenkov devices,the linear electron beam interacts with slow surface waves to amplify or generate electromagnetic radiations.Researches focus on properties of the propagating slow surface modes and the beam-wave interaction during the development of such devices.The slow wave structures(SWSs)relating to the dielectric slab are generally used in the Cherenkov devices when the slab is made with the materials with positive permittivity and permeability.The studies on the double negative materials namely negative index materials(NIM)with simultaneously negative permittivity and permeability explode since 2000.Generally artificial,the NIM holds special characteristcis which do not exists in nature.The Cherenkov devices combing the NIM is an interdisciplinary area worth investigation.The key is whether the slow suface modes exist along the interface involved NIM in slow wave structures.In this thesis,the slow surface waves excited from the interfaces involved NIM based on the model of a NIM slab with and without perfect electric conductor(PEC)substrate and the slab with PEC in the bottom and both sides.The restrictions on the permeability and permittivity of the existence of surface waves,the slow wave modes characteristics and power flows,are investigated.Besides small signal beam wave interaction theory is carried out.The following is included:1.The physics conditons restricted by the existence of surface modes in the NIM slab with and without substrate is studied based on the corresponding two-dimensional models.Using the field match method,dispersion equations are obtained and the restrictions on the permittivity and permeability of different modes are given.2.With the linear beam loaded,the interaction between the beam and the slow surface modes with longitudinal electric field occus in the SWS of the NIM slab with and without PEC substrate.The properties of the propagating TM surface modes are carefuly studied since they are supported by these sturctures and can interact wth the beams.The dispersion characteristics,the cutoff frequency,field distribution and power flows are investigated in detail.Ddifferent values of the permittivity and permeability of the NIM gives quite different surface modes.3.Studies on the linear theory of beam wave interaction exsiting in the SWS are the basis of the theoretical study of the Cherenkov devices.The dispersion equations in the beam loaded model of the NIM slabs with and without PEC substrate are derived considering hydrodynamic method.The spatial growth of the waves is worked out,and the expression of the start current is given.As to the dielectric slab without substrate,the model with two electron beams is also investigated and the spatial growth rate increases significantly compared with the single beam model.4.PEC side walls are added along both sides of the dielectric slab with the PEC substrate,which makes the structure and the model three dimensional.Theoretical studies are carried out.Mode analysis shows that the LSM modes can progate along the NIM surface of the structure with longitudinal electric field.The dispersion equation is derived and numerically solved.The properties of the LSM surface modes are investigated.Studies shows that the three dimensional structures surport higher frequencies.The corresponding small signal theory is worked out.5.To prove the obtained theoretical results,a novel method named ‘electron bunch excitation method'(EBEM)is proposed to investigate the dispersion characteristics of the SWSs studied in the thesis which are open in transvers direction and non-periodic in longitudinal direction..This method is availabare needed.To test the method,dispersion characteristics of several different kinds of SWSs are obtained through EMEM.The results are accurate compared with the corresponding theoretical or simulation or experimental results.And the EBEM is used to obtain the dispersion characteristics of the structures studied in this thesis,and good agreement is achieved.