||Experimental Studies on Terahertz Backward Wave Oscillator Driven by Weakly Relativistic Electron Beam
66 , 2016-03-23 , 新潟大学
学位の種類: 博士（工学）. 報告番号: 甲第4188号. 学位記番号: 新大院博（工）甲第445号. 学位授与年月日: 平成28年3月23日
In this dissertation, the results of theoretical and experimental studies on a vacuum backward wave oscillator (BWO) and cylindrical surface waves (CSWs) resonator are presented. Slow-wave devices such as backward wave oscillator (BWO) have been studied extensively as a candidate for high or moderate power microwave sources. In BWOs, slow-wave structure (SWS) is used to form slow wave by reducing the phase velocity of electromagnetic wave to the beam velocity. To increase the operation frequency and the power handling capability, the diameter D of SWS is increased larger than the free-space wavelength λ of output electromagnetic wave, i.e. D/λ > 1. In oversized cases, the slow wave becomes a surface wave concentrated on SWS wall. The designed BWOs and CSW-resonators are with upper cut off frequencies in the modern IEEE G-band (110-300GHz). The G-band SWS used in this work is an oversized hollow waveguide and a metal cylinder with periodical corrugations as G-band BWO and CSW-resonator. The corrugation parameters are the average radius R_0, corrugation width d, periodic length z_0, and corrugation amplitude h. The radius R_0 is the center point between the top and bottom of the corrugation. The corrugation wavenumber is given by k_0=2π/z_0. The dispersion characteristics of CSW are determined by changing R_0, h, d, and z_0. Dispersion curves of the CSW are obtained by a numerical method based on a modal expansion of the electromagnetic field for corrugated feed-horns. We have been studying oversized BWOs driven by a weakly relativistic beam of less than 100kV. The beam is generated by a disk cold cathode and the propagation of the electron beam has been investigated in detail. The intense radiations in terahertz region based on the BWO and CSW-resonator are reported. We examine the starting conditions for intense G-band BWO and CSW-resonator operation by varying the beam parameters and SWS lengths. There are two threshold conditions for such intense radiations. One is the well-known starting current, which is common in many microwave sources. The other is a starting energy, whose existence has been pointed out for the non-oversized X-band BWO and the oversized K-band BWO. It is shown that a starting energy exists for the G-band BWO and CSW-resonator which is more critical than the starting current in the oversized BWOs. We estimated the starting energy for G-band BWOs oscillation by numerical analysis and the experimental obtained starting energy values are in excellent agreement with the calculated ones. Above the starting energy, very intense output powers on the order of kW are obtained. The figure of merit Pf^2 is on the order of 10^5kW･GHz^2 and nearly the same as that for the K-and Q-band BWOs. The oscillation-starting condition for such intense operation may play an important role in high-intensity terahertz wave sources.