Imapct of Non-Uniform Magnetic Field on the Operation of Magnetrons

Abstract

Summary form only given. The magnetron is a well-known power source because of its high efficiency, low cost, compactness and small volume. In the magnetron design, a uniform magnetic field is always pursued by shaping the pole pieces connected to ordinary magnets. However, in practice, the perfect uniform magnetic field distribution is hard to achieve and there is always some degree of nonlinearity for the magnetic field distribution. Therefore, this paper investigates the effect of the non-uniform magnetic field on the magnetron performance, using the 3D version of the MAGIC code. The computer model of an X band rising-sun magnetron with 18 cavities is set up based on e2v MG5241 marine magnetron. The simulations of the magnetron performance are carried out using both the uniform and the non-uniform magnetic field distribution obtained from the practice measurement. The simulations show that with the non-uniform magnetic field, the distribution of the emitted current density along the cathode length is Gaussian, while with a uniform magnetic field, the cathode emitted current density is smaller and there are extra two spikes in the distribution curve near the end hats. It is this higher cathode emitted current and the non-uniform magnetic field (mainly the azimuthal magnetic field) that enable the more electrons being emitted and accumulating to substantially reduce the start up time of pi mode oscillation and improve the frequency spectrum. The results suggest that non-uniformed magnetic field is a better choice for the magnetron operation, which challenges the conventional approach of pursuing a uniform magnetic field.