The operation of coated tungsten-based dispenser cathodes in nonideal vacuum

The behavior of iridium- and osmium-coated dispenser cathodes in background pressures of oxygen and water vapor is discussed. Poisoning is predominantly chemical in nature and occurs at significantly lower pressures than would be expected for physical sputtering damage to the cathode surface. Measurements to determine the maximum background pressure in which the cathode can maintain their full emission capability are presented. The osmium-coated cathode has greater resistance to poisoning than the iridium-coated both at room and operating temperatures. The coated cathodes can operate in about three times as great a pressure of water vapor as oxygen. The results suggest ways that these cathodes can be optimized in nonideal vacuum. They also suggest that it should be possible to design an osmium-coated that will be able to emit fully 1310 K to about 5×10^-8 torr of water vapor