Study Of Low-frequency 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 detectability in radar application and data rates in communication system.One-dimension and two- dimension particle-in-cell simulation are performed on ion noise in microwave tubes. In one-dimension simulation, the approach taken is a hybrid model where the ions are treated as distinct particles in one-dimension while the electron beam is treated using the beam envelope equation. Using an one-dimension particle simulation program, we analyze the characteristics of ion noise in a CPI TWT and a UHF klystron. The phenomenon of ion noise is caught, the mechanism of ion noise is explained, the influence factors are discussed and the effective control method of ion noise is presented. In two-dimension simulation, we use two-dimension OOPIC software to perform particle simulation on a klystron. Hydrogen ion lens are loaded and neutral hydric gas is filled in simulation in order to observe the interactional behavior of electrons and ions. We observe that the presence of positively charged ions lens will cause the electron beam to compress radially more than it would in vacuum and result in the phase shift of electron beam, and ions will oscillate radially in the electrostatic potential wells. These phenomena are very important for explaining the origin of ion noise. The results of one-dimension simulation and two-dimension simulation are identical in the catch of phenomenon and explaining of mechanism,and are somewhat complementary. The study of ion noise in microwave tubes is just underway overseas and there is no research as yet at home. The main innovation of this paper is that we enrich the study fruit of the past ,and discuss the influence of magnetic field, beam current and beam voltage on ion noise in one-dimension simulation, and the phenomena of electron beam phase shift and ion radial oscillation in the potential wells are caught in two-dimension simulation.