M type impregnated cathodes: life time data and model, application to microwave tubes

The M type impregnated cathode is the favourite electron source of microwave tubes, in particular travelling wave tubes (TWT´s) and Klystrons. A substantial life test program has been conducted at TED on TWT´s and gun vehicles, to determine, over the long term, the electron emission properties of the cathodes. The cathodes designed for this program were made with porous tungsten pellets impregnated with calcium and barium aluminates and coated with 5000 angstroms of osmium or osmium-ruthenium. This paper presents, in the first part, the main physical properties of these cathodes at the beginning of life: the value of the zero field current density J0 and its spread measured at the reference temperature of 1000 Cb, the variation of J0 with the temperature and the surface composition measured by Auger spectroscopy. The characteristics of the TWT´s for space applications selected for life test was presented as well as cathodes parameters. Some experimental data recorded on the TWT´s families was also presented. In the second part, we introduce a prediction model for cathode life that gives the decrease of the cathode current Jn (t) as a function of time. This model is based on the evolution over the time of the space charge current Jsc(t) and of the saturation current J0(t), which depends on the baryum coverage of the cathode surface. The model has been calibrated by fitting the predictions with the average experimental data of the 10 W Ku Band TWT´s presented above. The end of life of the M type cathodes is calculated as a function of the cathode temperature and of the current density. It was correlated in the presentation with the saturation current variation versus time J0(t) when Jn (t) ∼; J0 (t). Other duration´s was also presented for tubes and gun vehicles operating at a current density of 4 A/cm2 and cathode temperatures ranging from 965 °Cb up to 1045 °Cb. A comparison with the model approach was discussed. In the last part, we present the characteristics or several microwave tubes currently manufactured by Thales Electron Devices, which are using M type (4-1-1) impregnated cathodes and in particular we review the expected life time.