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The reliability of a TWT is strongly determined by the quality of its cathode. Therefore, to a n ever increasing extent MMcathodes (mixed metal matrix cathodes) are used as they are capable of meeting the growing requirements concerning life and current density. In two separate life test programs, 57 MM-cathodes with a W/Os matrix were investigated in test vehicles (tetrodes), where the cathode environmental was similar to that of a tube. In parallel, a further 100 MM-cathodes in other types of test vehicles were operated for supplementary investigations and cathode design optimization.The operational temperatures were between 880"CB and 12OO0CB (brightness). One group of cathodes was operated at constant anode voltage with a current density of 0.75 A/cm2 at begin of life, the other group with a loading of 2 A/cmZ for as long as the anode voltage could be adjusted. The cathodes at lower temperatures ( 5 llOO°CB) have accumulated more than 40 000 h of operation.The gathered results are used to establish cathode life prediction models as a function of temperature and current density. The excellent capability of the cathode type for long life space applications has been demonstrated.