| In the rapid development of terahertz(THz) science and technology, THz radiation sources are one of the key problems. Vacuum electron THz radiation sources can provide high power with high efficiency at room temperature. Among electron-beam(e-beam)-driven oscillator sources, with increased working frequency, some problems limit developments in the THz regime, such as the starting current density, interaction efficiency, and difficult manufacture. In the dissertation, THz radiation within 3-mirror quasi-optical cavity and hole-array structures have been studied, which could obtain high interaction efficiency and high radiantion intensity with relatively low current density and easy manufacture.The following results have been achieved in this dissertation:1. The interaction between square-shaped electron beam and guiding wave in the multiple stacked layers with sub-wavelength holes(MSLS) structure has been applied to generate coherent THz radiation. Different from the surface waves, the guiding wave mode has strong field intensity in the whole e-beam location, which could interact with the square-shaped e-beam sufficiently. The results show that such interaction is more efficient than electron beam-surface wave interaction in traditional grating and bi-grating structures. Moreover, the square-shaped e-beam emission gun and the hole-layers structure could be much easier to fabricate and the e-beam is easier to form into shape than sheet beam, which could reduce the difficulty of the experiment.2. Based on the study on the MSLS structure, coherent THz radiation from multiple electron beams excitation within a strucutre which is composed of multiple stacked layers with 3 × 3 subwavelength holes array(MSLS-array) has been proposed. It has been found that in the MSLS-array structure the electromagnetic fields in each hole can be coupled with each other to construct a composite mode which has very strong field intensity. The multiple electron beams injection can excite a very strong coherent terahertz radiation in such structure with high electron conversion efficiency. The results show that due to the coupling among the electron beams, the starting current density of this mechanism is much lower than that of traditional e-beam-driven terahertz sources. Therefore, this multi-beam radiation system may provide a favorable way generate middle, high power terahertz radiation with relatively low current density.3. Coherent THz radiation from high-harmonic component of a modulated free-electron beam in a tapered two-asymmetric grating structure and prebunched e-beam-driven THz coherent Smith-Purcell radiation(SPR) with a two-section cylindrical periodic structure have been presented, respectively. The results show that both the mechanisms can obtain relatively high frequency coherent THz radiation with relatively low current density. Because of the low required current density and relatively high efficiency, these two concepts show the application potential for e-beam-driven coherent THz sources.4. An e-beam-driven THz coherent SPR within a cylindrical quasi-optical cavity has been proposed. The studies show that in this system THz coherent SPR from the interaction between the annular e-beam and cylindrical surface wave is efficiently excited and a distinguished Hermite-Gaussian mode well established. The cylindrical quasi-optical cavity plays a very important role in the coherent SPR oscillation and energy feedback. Therefore, a pure 2nd harmonic coherent SPR spectrum and five-fold of radiation intensity improvement and almost one order of magnitude of radiated field energy enhancement have been achieved by this system. Moreover, such THz coherent SPR source could provide about 450-mW output power and work with 2nd harmonic coherent SPR at 0.607 THz with only about 30 A/cm2 which is a relatively low value. The concept of this paper presents an efficient way to develop e-beam-driven THz coherent SPR sources in cylindrical system, which could be an available way to develop THz sources. |
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