Study On Terahertz Novel Slow-wave Structure

Terahertz(THz) wave is considered as the electromagnetic spectrum whose frequency ranges from 0.1 THz to 10 THz. Recently, there has been great effort in the development of THz wave radiation source which have many potential applications, such as biology, imaging, material measurement, and space applications. Many approaches to generate THz wave radiation have been investigated including solid state electron devices, optical devices, and vacuum electron devices(VEDs). Based on vacuum electronic principle, traveling-wave tube(TWT) and backward-wave oscillator(BWO) are two kinds of the important radiation sources. As the core part of the VEDs, the slow-wave structures(SWS) determine the performances of the VEDs. Since the conventional SWSs have been encountered some restrictions in the THz band applications, such as the difficulties of fabricating, high transmission loss, high signal reflection, low interaction impedance, low output power and electronic efficiency. It is meaningful to quest and research novel type of SWSs which can overcome those problems. This thesis does a detailed analysis for the novel THz SWS in two aspects of high-frequency characteristics and beam-wave interaction characteristics. And the results illustrate that the proposed novel SWS can be applied in the research of TWT and BWO to improve the performance of these devices. The innovations and main works of this paper are concluded as follows:1. To increase the interaction impedance, enhance the output power of the TWT, and to reduce the difficulties of fabricating the beam tunnel, two kinds of novel folded waveguide(FWG) with large dimensional beam tunnel are proposed, which are suitable for circular-beam and sheet-beam, respectively. The investigation of the high-frequency characteristic illustrate that the electric field distribution of the novel FWG is more concentrated in the electron beam tunnel area than that of the conventional FWG, thus the interaction impedance of the novel FWG is increased. Besides, the transmission loss of the novel FWG is lower than that of the conventional FWG. On the basis of the high-frequency characteristic research, the 140 GHz and 220 GHz TWTs with novel FWG are investigated, including input/output coupler designing, and the three-dimensional(3D) beam-wave interaction model with ideal attenuator. By analyzing the energy exchange process of the beam-wave interaction, the performances of each operation frequency of the TWT are calculated. The particle-in-cell(PIC) results show that the output power of the novel FWG TWT is about 50% and 30% higher than that of the conventional FWG TWT at 140 GHz and 220 GHz, respectively, corresponding to the gain of 7% and 3.3%.2. A novel SWS called folded rectangular groove waveguide(FRGWG) is put forward in order to reduce the influences caused by electron beam bunnel, such as signal reflection and oscillation risk. The dispersion characteristic and interaction impedance of the FRGWG are investigated by field theory and equivalent circuit theory. The calculation results of the theory and simulation are compared to verify the correctness of the calculations. Finally, the small signal equation of the FRGWG is obtained. The FRGWG with large dimension circular electron beam tunnel is proposed. The high-frequency characteristic analysis shows that the circular electron beam tunnel with large dimension has almost no influences on the transmission characteristic of the SWS. Compared with conventional FWG TWT, the FRGWG TWT has higher output power and signal gain, and the oscillation risk of the FRGWG TWT is reduced.3. The restrictions of the conventional FWG for BWO application is analysised. A novel SWS called U-shaped quasi-parallel-plate(QPP) is proposed. A design scheme of the 140 GHz U-shaped QPP BWO is proposed and the influences caused by every size parameters are studied. Based on the high-frequency characteristic investigation, the operating parameters of the U-shaped QPP BWO are determined, including operating voltage, beam current, focusing magnetic field, and length of the whole slow-wave circuit. Finally, the performance of the U-shaped QPP BWO is calculated. The BWO can produce over 10 W peak power output in the frequency range of 120 GHz to 148 GHz by tuning the operating voltage from 6 k V to 12 k V, the electronic efficiency can reach 14%.4. For 1 THz frequency band, the W-class N-shaped QPP BWO is designed. The SWS of the BWO is simplified, which is easier for frabrication. The high-frequency characteristic of the N-shaped QPP is compared with the conventional FWG, which indicate that the QPP has wider “cold” bandwidth and lower operating voltage. Since the n=0 space harmonic regime selection, the interaction impedance of the QPP is higher than that of the conventional FWG which is working in higher space harmonic regime. At last, the PIC simulation is carried out, and the PIC results shows that the N-shaped QPP has certain advantages compared with that of the three kinds of novel SWS BWO including sine waveguide BWO, corrugated rectangular BWO and double-grating BWO. The comparison results show that the QPP BWO has wider operation band width(0.82THz~1.0THz), lower operation voltage(5k V~10k V), lower starting current(3m A), higher electronic efficiency(2.8%~6.6%), and more simple fabrication.5. The fabricating method of the QPP SWS is studied. The 140 GHz QPP BWO with the input/output coupler is obtained by WIRE-EDM technique and micro mlectro mechanical system(MEMS). The machining error of the fabricated structure is mesured. Finally, the transmission characteristic of the QPP SWS is tested by the vector network analyzer(VNA).