High current density operation of oxide cathodes

Oxide-coated cathodes have been used widely in electron tubes due to its high-emission efficiency at low operating temperatures. The maximum limit for dc emission of oxide cathodes has been considered at 0.5 A/cm
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due to limitations of sparking or overheating due to Joule effect. We have tested successfully oxide cathodes at operating temperatures of 1000°K at current levels from 1.0 to 4.0 A/cm
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with considerable life. Different types of oxide cathodes were tested in planar diodes and triodes. It was found that the operation limitations of an oxide cathode in a triode are far more severe than that in a diode. Diode testing at high current density dc was performed, while triode testing at dc, RF, and pulse conditions were carried out. Experimenting with high current density oxide cathodes in diodes indicated some limitations in the presently used cathode materials only above 4.0 A/cm
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. In triodes where the limitation was found at present at 2.0 A/cm
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, several interesting effects occurred. Grid overloading and beaming effects due to the closely spaced grids, produced intrusions on the anode face and poisoning on the cathode. Various grid materials have been tested and results are compared. Triodes in high power RF oscillators have been tested at about 1.0 A/cm
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average current density. In this mode of operation, the limiting factor is the grid design and its materials. Evaporation of the grid materials may cause in this mode sudden and catastrophic failure of the tube. Evaporated grid materials deposited on the insulators result in cracking the structure during RF operation. Experiments will be discussed which showed that this effect can be avoided by the proper selection of materials.