Gyrotron devices are known as high power sources of coherent electromagnetic radiation. In the millimeter and sub-millimeter region, the power of gyrotron exceeds the power of classical microwave by many orders. In recent years, THz issues become popular due to its numerous applications showing the great importance of THz gyrotron.
For the terahertz operation, there are two important issues. The first one is mode competition. THz wave damps very fast for the same distance compared with millimeter wave; as a consequence, high-order modes are preferable. On the other hand, some THz applications require a broad tuning range; because of this, high order mode is unavoidable with changing magnetic field, indicating that mode competition becomes an important issue. The other issue is that generating terahertz wave require high magnetic field which is too difficult to achieve. Hence, cyclotron harmonic interactions are a key physics issue of critical importance to the generation of THz radiation via the electron cyclotron maser instability in a manageable magnetic field. I present an inherent mechanism, as well as a deciding factor, which governs the competition between low and high harmonic interactions. Multi-mode simulations reveal the physical process in which a significant advantage develops for the lower-harmonic interaction, which eventually dominates in the fully nonlinear stage. Results also suggest a start-up scenario for persistent higher-harmonic operation and the stabilization methods of high harmonic operation.
For some applications with wide magnetic field tuning range, there is likelihood that mode competition between same harmonic modes. The results, which conclude that the mode with smaller propagating constant has the advantage, shown through multi-mode simulations, would be explained by considering the intrinsic bunching mechanism.
The purpose of this dissertation is discussing the physics principle of mode competitions, as well as in providing strategies for high power THz operations.

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