@misc{oai:niigata-u.repo.nii.ac.jp:00006042,
 author = {MIN, THU SAN},
 month = {Mar},
 note = {In this dissertation, intense and broadband terahertz radiations by backward wave oscillator (BWO) are experimentally and numerically studied. Compact as well as tunable devices are preferable for practical use in terahertz applications. To predict the operation of the oscillator, numerical study for the dispersion relation of the slow-wave structure (SWS) is presented in the study. By the dispersion relation, the operating frequency and operation region of the oscillator can be estimated. In the experiment, cylindrical waveguides with rectangular corrugations in the wall are used as SWSs. Various lengths of SWS are employed in the study. Electron beam with energy less than 100 kV from pule power system drives disk-type cold cathode to generate annular electron beam. External magnetic field of 0.82 T is used to confine the flow of electron beam in the axial direction. Radiations are detected by horn antennas and radiated frequency is confirmed by high pass filters. Three types of SWSs with upper cutoff frequency of 0.1, 0.35, and 0.5 THz are employed in the BWO for terahertz radiations. Major difference among them is length of the corrugation amplitude; corrugation amplitude of 0.1 THz SWS is two (four) times larger than that of 0.35 THz (0.5 THz) SWS. The former SWS has relatively flat dispersion relation and operates as a surface-wave oscillator (SWO). Radiated output power is of ~ 20 kW with radiation frequency of 0.1 THz. Due to short length of the corrugation amplitude, 0.35 THz (0.5 THz) SWS has steep dispersion relation and operates as a surface-wave resonator (SWR). Both SWRs have tunable radiation frequency bandwidth of ~ 0.1 THz, and output power is of some kW. Peak radiation frequency of 0.4 THz for the SWR (0.5 THz) is detected in the study. To study the operation regions of the SWO (0.1 THz), SWS lengths are varied as 20-, 40-, 80-, and 120-period. By the dispersion relation, operation regions of the SWO can be predicted that the SWO operates at π-point around the beam energy of 32.5 keV, and in BWO and travelling wave tube (TWT) regions when the beam energy is less and greater than 32.5 keV. When the length of the SWS is relatively long: 80- and 120-period, radiations are detected at the beam voltage of less and greater than 32.5 kV, and no meaningful radiation is occurred by that voltage. When the length of the SWS is short enough: 20- and 40-period, meaningful radiations are detected even around 32.5 kV. The experimental results show the SWO with short length of the SWS operates in all regions, and with long length of the SWS operates in BWO and TWT regions. To recognize the π-point operation, two numerical methods: real wave number analysis and saddle-point analysis, are studied. The numerical results show that Cherenkov instability is of an absolute instability when the SWO operates around the π-point region. To analysis the oscillation starting conditions of the SWRs, SWSs of 40-, 60-, 80-, and 120-period for the SWR (0.35 THz), and 120-, 160-, 320-, and 400-period for the SWR (0.5 THz) are studied. Numerical studies for oscillation starting conditions: starting energy and starting current, are also presented. For both types, the experimental results of the starting current are totally different from the numerical anticipation, and the oscillation starting condition could not be explained by the starting current condition. For the SWR (0.35 THz), the experimental results of the starting energy are decreased increasing the length of the SWS and cathode diameter, and in a good agreement with the numerical results. For the SWR (0.5 THz), the experimental results of the starting energy agree with the numerical results until when the length of the SWS is relatively short of up to 160-preiod. In the experiment, the starting energy of the SWR (0.5 THz) could not be reduced anymore when the length of the SWS is relatively long of 320-period and above., 学位の種類: 博士（工学）. 報告番号: 甲第4470号. 学位記番号: 新大院博（工）甲第477号. 学位授与年月日: 平成30年3月23日, 新大院博（工）甲第477号},
 title = {Experimental Study of Intense and Broadband Terahertz Wave Radiation by Backward Wave Oscillator},
 year = {2018}
}