Research On Beam-Wave Interaction Technologies In Terahertz Extended Interaction Oscillators

Terahertz waves are electromagnetic waves in the frequency range 0.1-10 THz,as with radio,microwaves,infrared light and visible light.Extended interaction devices(EIDs)provide the advantage of high gain per unit length through multi-gap slow wave structure,which makes these devices essential for their compactness and durability.As important terahertz source devices,EIDs have been used in space communications,active denial systems,dynamic nuclear polarisation applications and many other high frequency technology fields.Nevertheless,extensive realisations in the lower terahertz region(0.1-0.3 THz)are still difficult due to the fact that many high frequency technology solutions for EIDs(e.g.higher order mode solutions,sheet-beam solutions,etc.)are not yet fully mature.Furthermore,there are no reported experimental results of thermal measurements in the frequency band greater than 0.3 THz.This is mainly due to the many contradictions and key challenges in device operation when the device frequency rises to the terahertz frequency band:Firstly,the skin depth decreases,but the processing roughness remains the same,a contradiction that leads to a sharp increase in surface loss.Secondly,as the size of the device becomes smaller,the problems of magnetic focusing,interaction modulation and transmission due to space charge effects become more severe.In view of the important research value of EIDs,this dissertation addresses the following three main aspects of extended interaction oscillators(EIOs):(1)Firstly,terahertz EIOs operate at high loss and low current states.To solve the technical problem that enables operation at high frequencies,this paper applies to the one-dimensional linear theory equivalent circuit method to study the basic EID oscillation-starting current determinants(equations).Two different procedures of terahertz EIO low-current optimisation techniques are carried out:the eigenmode analysis-based and the particle simulation analysis-based starting current optimisation techniques.(2)To address the requirements of small device size and high current density,and to obtain a larger cross-sectional area of the beam tunnel for high ohmic losses in the terahertz EIO,this dissertation investigates the mechanisms of high-order mode operation based on the double beamTM₂₁ mode model and the sheet beamTM₃₁mode model respectively.(3)In this research,it is found that the high ohmic losses associated with high frequencies require circuits with longer longitudinal lengths or higher currents(such as multi beams and sheet beam)to compensate for the shortcomings of low coupling impedance R/Q in higher order modes.For further enhancement of the beam-wave interaction at finite lengths in EIO,the mechanism of the extended interaction field distribution is investigated to optimise the longitudinal field distribution profile and the effective coupling impedanceM² R/Q value at a given circuit length.Based on a number of theoretical studies of fundamental mode extended interaction structures,for this dissertation,the first key aspect of the main investigations is low starting current design techniques,in order to solve the problem of the rapidly rising surface losses in terahertz EIOs.Accordingly,two technical methods(processes)for the design of lower starting currents have been developed based on the starting current determination analysis.The first method is an eigenmode analysis-based optimization technique,which is designed to facilitate rapid design optimisation of low-start circuits by carefully selecting the number of gaps and the period length.The advantage is the speed of optimisation in the design phase,but the disadvantage is that losses are not fully taken into account.The optimization results are achieved for a 0.38THz fundamental mode EIO with a starting current density of 250 A/cm²(the beam current is 0.088 A),which corresponds to an output power of 1.27 W.The second technique for optimising the starting current is based on particle simulation analysis,which introduces a secondary optimisation of starting current applicable in the beam-wave interaction implementation phase.This can be used as an extension for the eigenmode-optimisation technical solution for the design of low current EIO circuits.The optimised starting current density for a 0.1 THz circular beam EIO is 60 A/cm²(0.17 A),with an output power of 254 W at 100 A/cm²(0.28 A)and 16.5 k V electron beam injected.For the dissertation,the second key aspect is to address the problem for the small size in terahertz EIOs by investigating two high-order mode technical approaches:the TM₂₁ mode double-beam model and theTM₃₁ mode sheet-beam model.The theoretical study of the high-order mode model,based on an equivalent parallel circuit,shows that the high-order mode circuit is theoretically at least twice as low in R/Q and twice as high in power capacity compared to the fundamental mode.TheTM₂₁ mode double beam EIO contains two 2?modes with similar frequencies and high values of R/Q,enabling intrinsic analysis of multi-mode operation for broadband and other potential applications.For another approach,the high cy model design technique and single mode-operation stabilisation(suppression of parasitic modes)techniques have been investigated.The established 16-periodTM₃₁ mode structure has an output power of620 W at 353.71 GHz with an electron throughput of 82%and an efficiency of more than 2%(the electron beam is of 41 k V and 0.6 A).The third key aspect is the optimisation of the longitudinal standing wave field,which is carried out in two main directions:one is the optimisation of the flatness of the longitudinal field and the other is the optimisation of the longitudinal field amplitude.The flatness optimisation applies the transmission principle in the gap to establish the critical cut-off condition;the equivalent electrical boundary cut-off characteristics are established using frequency and intrinsic characteristic analysis.After optimisations,the electron efficiency of the flat distributed field is optimised to be at least 0.6%higher compared to other types;the middle-concentrated field reduces ohmic losses by at least26%.In the other aspect,the longitudinal field amplitude studies are also applied to the transmission-principle,and a 0.35 THz strongly coupled 2?-mode EIO model is established using low-current design techniques,high-order mode structure design techniques,and staggered slot bi-periodic structure.In combination with several studies in this dissertation,a high-efficiency structure achieves an output power of 189 W at 0.3A(652 A/cm2)and 25 k V beam injection(2.52%electronic efficiency);a low-current structure achieves aM² R/Q value of 79.31?,an operating current density of only434 A/cm2 corresponding to 24 k V,an efficiency of 1.84%and a starting current density of only 325.5 A/cm²(corresponding to a current of 0.15 A).In addition,two 0.1 THz EIOs,a staggered-slotted bi-periodic structure and a conventional single-period structure,were processed and cold-tested.The cold-test frequencies were in good agreement with the simulated frequencies.Furthermore,two0.1 THz EIOs with different flatness of longitudinal field distribution were cold tested using perturbation method,and the two field distribution curves with different flatness were measured correctly.This verifies the correctness of the design and provides a basis for subsequent thermal experiments.