Study On Terahertz Harmonic Gyro-devices

THz radiating sources is one of the key topics in THz science and technology, the gyrotron is the most powerful source among various THz sources. Study on THz gyrotron plays an important role in the development of THz sources. However, the gyrotron operating higher frequency needs stronger magnetic field (1THz gyrotrons operating at fundamental cyclotron harmonic need 40 Tesla working magnetic field, so strong magnet is expensive and difficult to fabricate). In order to reduce the strength of magnetic field, the gyrotron needs to operate at higher cyclotron harmonics (when the gyrotron working at nth cyclotron harmonic, the corresponding working magnetic field can be reduced to 1/n). Study on high cyclotron harmonic gyrotron is important in THz region. In this dissertation, the beam-wave interactions of coaxial gyrotron with two electron beams are studied with the coupled-mode theory; meanwhile, with the linear and self-consistent nonlinear theories, the beam-wave interactions and electron bunching process of three-cavity frequency-quadrupling gyroklystron are studied and discussed in details; then, a new-type of gyrotron resonant cavity-photonic crystal cavity is studied and discussed with the plane wave expansion methods; finally, the THz gyrotron experiments the author participated in at FIR Center of Fukui University is detailed introduced.1. The beam-wave interactions of coaxial gyrotron with two electron beams are studied. With the coupled-mode theory, the beam-wave interactions of dual-frequency, single-frequency coaxial gyrotron with two electron beams are discussed in details, compared to the one-electron-beam coaxial gyrotron with the same geometric structure and parameters of electron beams, due to the coupling between two beams and two modes, two modes of the dual-frequency coaxial gyrotron with two electron beams are enhanced.2. three-cavity THz frequency-quadrupling gyroklystron with successive frequency doubling in each cavity is detailed studied. The cavities are designed and analyzed with the scattering matrix methods and simulated with the CST software, the geometric parameters' effects on the resonant frequency and quality factor of the resonant cavity are discussed; the mode selection, dispersion relation, starting current and possible competing modes are considered with the linear theory, the beam-wave interactions of three-cavity frequency-quadrupling gyroklystron are studied with the self-consistent nonlinear theory, the bunching process of electrons is studied, the effect of length of drift tubes and output cavity, voltage, current, velocity pitch and guiding center radius of electron beam on the output efficiency is discussed, meanwhile, the effect of the ohmic loss on the output efficiency is considered.3. The photonic crystal cavity is studied with the plane wave expansion methods. The first Brillouin zone and photonic band gap calculation of two-dimensional arbitrary parallelogram lattice dielectric rods and metallic photonic crystals are discussed in details. The photonic band gap structures of triangular lattice and square lattice two-dimensional photonic crystal calculated by the method described in this dissertation agree well with the conventional method, furthermore, the calculated photonic band gap in critical conditions of the first Brillouin zone are consistent with either hexagon or quadrangle Brillouin zone, which shows that the method described here is valid.4. The THz gyrotron experiment the author participated in at FIR Center of Fukui University including power spectrum, frequency, wavelength, the power measurement is introduced.