Modern thermionic cathodes

This paper reviews the current state-of-the-art in modern thermionic dispenser cathodes. Over the past seven years there has been a resurgence of interest in extending the performance limits of thermionic cathodes, driven by the needs of advanced microwave tubes, in particular high-power millimeter-wave tubes and higher frequency longer life space communications tubes. More recently the development of high efficiency free electron lasers with the potential to transfer prodigious peak and mean powers to the radiation field has further increased the demands upon cathode performance in terms of current density and intrinsic brightness. The long-term benefits of the increase in R&D funding which has been directed toward the study of thermionic cathodes since the early 1980s are now becoming apparent. The development of advanced dispenser cathodes is no longer based upon empirical experimentation but is guided by engineering and understanding based upon our increased knowledge of cathode physics. Advanced mixed-metal-matrix cathodes have shown lives of over 30,000 hours at 8 A/cm². Fully space charge limited emission capability of 100 A/cm²has been demonstrated and design lifetimes of 100,000 hours at 6-10 A/cm²are now a practical possibility.