Thermionic cathode life-test studies

A NASA-Lewis Research Center program for life testing commercial, high-current-density thermionic cathodes has been in progress since 1971. The purpose of the program is to develop long-life power microwave tubes for space communications. Four commercial-type cathodes are being evaluated in this investigation. They are the "Tungstate," "S" type, "B" type, and "M" type cathodes, all of which are capable of delivering 1 A/cm²or more of emission current at an operating temperature in the range of 1000-1100°C. The life test vehicles used in these studies are similar in construction to that of a high-power microwave tube and employ a high-convergence electron-gun structure; in contrast to earlier studies that used close-space diodes. These guns were designed for operation at 2 A/cm²of cathode loading. Life tests of two of the cathode types, "Tungstate" and "S," have been completed, but the other two cathodes are still being studied. The "Tungstate" cathodes failed at 7000 h or less and the "S" cathode exhibited a lifetime of about 20 000 h. One "B\´" cathode has failed after 27 000 h, the remaining units continuing to operate after up to 30 000 h. Only limited data are now available for the "M" cathode, because only one has been operated for as long as 19 000 h. However, the preliminary results indicate the emission current from the "M" cathode is more stable than the "B" cathode and that it can be operated at a true temperature approximately 100°C lower than for the "B" cathode.