The Study Of The Ion Noise In Microwave Tubes

Since there is always some background gas in microwave tubes, the collision between electron beam and ambient gas is inevitable, and ions are created in the system. For long pulse times and especially for CW mode, the coupling between ions and electrostatic potential wells in the devices will lead to ion noise, which generally manifests itself as a slow phase fluctuation on the output signal. The low frequency ion noise has negative influence on delectability in radar application and data rates in communication system. In this dissertation, we have made detailed theoretical study and computer simulation on this issue. The major achievements are listed as the following:1. The producing, motion, accumulation and balance of the ion are studied in the microwave tube, this is the fundament to study the ion noise phenomenon2. Characteristics of ion noise in the PPM TWT and the Klystron have been discussed by means of a hybrid model. One-dimensioned Particle-in-Cell method (PIC) simulation code is developed and the relation between ion noise and phase distortion was obtain. The numerical simulation result shows that increasing beam current; the ion noise amplitude will decrease and become stable. There exist optimal value of magnetic focusing field and beam voltage. That can minimize the ion noise amplitude.3. The time series of ion relaxation oscillation are obtained by the presented method. The ion relaxation oscillation is treated as the response of a complex nonlinear dynamical system, and the time series is analyzed by power spectrum; restructure phase diagram and Lyapunov exponent. From the analysis results we find that the ion relaxation oscillation has chaotic character at the first time.4. The ion noise of TWTs is analyzed using small signal by motion theory. The relation between noise to signal ration and signal frequency, voltage motipn amplitude is obtained. From the analysis results we find when the amplitude of voltage motion and signal frequency increase, the noise to signal ration would increase.5. The ion noise in microwave tube is studied using two-dimensional particle-in-cell method. The fluctuation of the ion amount, which is the source of ion noise of the tube, is observed in the simulation process. It is found that the sufficient ions amount in drift tube and beam envelope scalloping appear to be responsible for ion noise after changing the gas pressure, magnetic focusing field, and drift tube length. A great number ions escape radically and their amount is about half of the beam...