Study Of A New Complex Cavity For Gyrotron Applications

Resonant cavity is an important part of microwave devices and system, which has been applied in klystron, magnetron and so on. In range of millimeter wavelength (such as 8mm), resonant cavity operates in high order mode in gyrotron based on relativistic effect, where mode competition is a very important problem affecting its electrical feature and stability. Open cavity has some action for improving mode competition. But the open cavity is usually long along axial direction because of long tapering section, which limits its use through the combination of the several cavities to enhance gain, bandwidth, output power, and efficiency of Gyrotron.In this thesis, a new mode-elective complex open-cavity is discussed, which has the good feature for mode selection and suppression with a short length and can be used to form clustered cavity. The mode electivity of this cavity is studied in detail in order to understand its operating mechanism. The resonant characteristics of the TE₀₂₁/TE₀₃₁ as operating mode and the other competing modes in the cavity are numerically simulated and analyzed with Ansoft HFSS code. The results have shown that the relative stored energy of two main competing modes the TE₅₁₁ and TE₂₂₁ in the cavity can respectively be reduced to 58% and 12% of that in cylindrical cavity.In order to apply this new complex cavity in millimeter wave Gyrotron, the effects of thickening the cavity wall and constructing a pair of drift tube in the cavity on the operating mode stability for engineering design are analyzed. Some compensating methods for alleviating the effects are discussed, which shows that rationally adjusting the radius ratio between the inner and outer cavity in the complex cavity may effectively realize high purity and stability of the operating mode.In addition, a cold test model is designed based on a set of designed clustered cavities, the amplitude frequency response of each cavity is measured with network analyzer. Via the comparison of the simulation results of the cold test model with the cold test data of the model, a method for determining the resonant characteristics of the operating mode TE₀₂₁/TE₀₃₁ and the main competing modes TE₅₁₁ and TE₂₂₁ is tried. The difference between the cold test way and real operation status is discussed, which is useful for the application of this new complex cavity.