Research On Terahertz Miniaturized Vacuum Electronic Radiation Source

The unique properties of terahertz waves have attracted numerous researchers to delve into the field of terahertz technology.However,the advancement of terahertz technology heavily relies on high-quality terahertz radiation sources.High-power and miniaturized radiation sources play a pivotal role across various application scenarios.Among the plethora of radiation sources,vacuum electronic devices stand out as the optimal solution for achieving miniaturization and high-power radiation sources in the low terahertz frequency bands by modulating electron beams to convert electrical energy into electromagnetic wave energy.Within the high-frequency structures of vacuum electronic devices,both modulated direct current electron beams and equidistant continuous electron bunches carry specific frequency information after appropriate modulation by high-frequency fields.According to Fourier series expansion,an electron bunch at a certain frequency can be viewed as a direct current electron beam,overlaid with numerous harmonically related electron beams.Hence,by extracting the harmonics modulating the electron beam,higher-frequency electromagnetic wave outputs can be obtained,enabling the attainment of higher operating frequencies within the same-sized device.The development of vacuum electronic devices is primarily constrained by material performance and manufacturing capabilities.For instance,limitations exist in the emission capability of cathode materials,control over machining part tolerances,and precision in component assembly.Presently,these challenges prevent the complete fulfillment of the requirements for the advancement of vacuum electronic devices in the terahertz frequency range.Overcoming these complex issues within a short timeframe is currently unfeasible.Therefore,devices operating in the harmonic can enhance the output frequency level of vacuum electronic devices under current conditions.This approach offers advantages such as mitigating the size-commensurate effects and reducing the device’s threshold current for oscillation.Enhancing device output capacity through the development of novel harmonic operating mechanisms is an important research avenue for terahertz vacuum electronic devices within current constraints.In the terahertz frequency range,traditional structures remain effective,highlighting the significance of exploring traditional high-frequency structures that align with current manufacturing capabilities.Innovating upon these structures to enhance output capability represents an important developmental path.This paper primarily focuses on the computational and experimental exploration of new structures,each achieving design objectives of high frequency,high power,and high bandwidth.The main works and innovations are as follows:1.Research on terahertz super-radiation.First,based on the parameters of the electron beam used in the experiment,we designed a high-frequency structure for a planar grating working at the π point with a second harmonic super-radiation angle of 90°.Furthermore,theoretical and simulation-based analyses have been conducted to investigate their dispersion characteristics.Calculations of the interaction between the electron beam and the planar grating revealed certain issues,such as a long starting oscillation time and low radiation power.We conducted an analysis of the causes behind these problems and proposed optimization solutions.The optimized structure not only reduced the starting oscillation time but also increased the one-fold super-radiation power.Subsequently,the optimized grating structure was fabricated,and an experimental platform for terahertz super-radiation research was designed and assembled.Preliminary experiments have unveiled certain shortcomings within the design process,prompting the proposal of improvement strategies based on these identified issues.2.Research of terahertz harmonic enhancement mechanism.In order to address the issue of weak harmonic components in the electron beam resulting in low superradiant power,research focused on harmonic enhancement strategies.This involved studying the interaction between cascaded grating high-frequency structures and electron beam selfexcited oscillations to amplify the harmonics within the electron beam.It was observed that this approach did not provide stable enhancement of electron beam harmonics.A detailed simulation analysis was carried out to identify factors causing the instability in harmonic enhancement.By summarizing the unstable factors,adjustments were made to the type of harmonic grating and the coupling position to address the issues encountered in the study of electron beam second harmonic enhancement.This resulted in stable oscillation and enhancement of electron beam second harmonics.A G-band device was designed to operate on the harmonic enhancement mechanism.Interaction with a 31.5ke V electron beam produced a single-frequency signal at 202 GHz with an output power of 4.59 W.As the electron beam voltage was adjusted within the range of 23 k V to 38.5k V,the output frequency varied from 195.71 GHz to 205.37 GHz,with a relative bandwidth of approximately 5%.During the operation of this structure,the highfrequency field associated with the harmonics exhibited an extended interaction mode distribution,and the oscillation threshold current was lower by an order of magnitude compared to same-frequency range extended interaction oscillators.3.Experimental research of the terahertz harmonic enhancement mechanism.Initially,a device was designed to operate in the W-band with a mechanism for second harmonic enhancement.Simulational results indicated that as the electron beam energy was adjusted from 12.5 ke V to 15 ke V,the output signal frequency varied from 94.69 GHz to 94.91 GHz,with a maximum output power of 7.9 W.Subsequently,based on the device’s output characteristics,the output window was designed and subjected to joint simulation with an ideal model,yielding consistent results with the ideal model.Theoretical analysis was carried out to assess the negative impact of processing techniques on the high-frequency characteristics of the device,and correction measures were proposed.Following this,various components were processed,and the finished components were cleaned,assembled,welded,encapsulated,and tested.The test results for the output window were consistent with the design,meeting usage requirements.Moreover,the results indicated that the structural design based on processing techniques did not introduce significant differences in the transmission characteristics of the two ports of the output window.During the device’s thermal testing,it was noted that there was no electron beam emission.Subsequently,the emission capacity of the bare cathode was tested,and the analysis of the experimental results attributed the suboptimal electron beam emission to a quality issue with the batch of cathodes,presumably related to salt impregnation.4.Research on pre-bunching electron beams for terahertz harmonic enhancement.The proposal involves cascading two fundamental grating structures to prebunch and control the modulation intensity of the electron beam’s fundamental component.This is aimed at addressing the uncontrollable fundamental modulation intensity of the electron beam observed in research on enhancing electron beam harmonics via cascaded grating self-excited oscillations.The proposal involved using two sections of fundamental grating cascades for pre-bunching to control the modulation intensity of the electron beam fundamental frequency.An analysis was conducted to understand why the second harmonic enhancement structure was not suitable for enhancing third and higher-order harmonics.Finally,based on the characteristics of electron beam modulation by planar gratings,a method involving the cascading of harmonic and fundamental grating structures was adopted to achieve enhancement in the third harmonic of the electron beam.Computational analysis was performed to assess the impact on the enhancement of the electron beam’s third harmonic and output power under various fundamental modulation intensities.The designed device for third harmonic enhancement,with a center frequency of 360 GHz,could output up to 584 m W of third harmonic signal and several watts of fundamental amplification signal when provided with an 80 m W fundamental input power.5.Research on terahertz ultra-wideband backward wave oscillator.A design proposal was introduced for a ultra-wideband traveling-wave tube based on a doublegrating structure with waveguide loading.By adjusting two parameters of the planar grating,the individual tunable bandwidths of each were maximized.Furthermore,the tunable ranges of the working regions for these two-parameter planar gratings were interlinked,significantly expanding the tunable range of a single device.In addition to optimizing the structural parameters of the gratings,an analysis of energy coupling between the gratings was conducted,and the extent of energy coupling under different parameters was compared.The minimization of structural dimensions and the optimization of interaction structural parameters were utilized to prevent excessive energy coupling.Having a high number of periodic structures can introduce negative effects on the purity of the output signal spectrum.An analysis of the reasons behind this phenomenon was undertaken,and a solution was found through the use of lossy materials in the loading process.Ultimately,calculations demonstrated that this traveling-wave tube could provide tunable output within the range of 185.63 GHz to 291.93 GHz,achieving a relative bandwidth of 36.4%.The research initially aimed to generate high-power terahertz harmonic output using a planar grating structure that is more friendly to processing requirements,driven by a high-power electron beam.However,calculations and preliminary experimental studies revealed that the characteristics of the planar grating’s modulation of the electron beam resulted in relatively low super-radiation power.In response,the research shifted its focus to generating high-power and high-frequency terahertz radiation by harnessing electron beam harmonic enhancement.Two mechanisms were proposed and explored: harmonic enhancement and pre-modulation of electron beam harmonic enhancement.Both mechanisms successfully achieved the research objectives,and experimental investigations were conducted to understand the underlying mechanisms of harmonic enhancement.Finally,an interaction structure incorporating waveguide loading and double-grating arrangements was designed to create an ultra-wideband backward wave oscillator,achieving a relative bandwidth of 36.4%.This realization fulfilled the design objective of a high-bandwidth terahertz radiation source.The research outlined in this paper has opened up new avenues for the development of high-power,high-frequency,and high-bandwidth terahertz radiation sources,which can significantly impact the study of miniaturized vacuum electronic devices in the terahertz band.