Ba losses due to oxygen adsorption on Ba-dispenser carthodes

The emission stability against ion bombardment and oxygen adsorption was in vestigated on thermionic Be-dispenser cathodes with different top layers (W , Os/Ru, Re, Ir, and Re+Sc2O3) as a function of temperature and oxygen part ial pressure. From these data, Ba and BaO reactivation energies and oxygen desorption energies were derived. During ion bombardment, two degradation/r egeneration channels were observed due to the sequential carrying off of th e emitting dipole and the corresponding resupply of Ba and BaO. Also two de gradation/regeneration channels were detected during oxygen poisoning due t o adsorption of oxygen and oxygen induced desorption of the emitting dipole or pacts thereof and corresponding oxygen desorption and regeneration of t he BaO dipole. The different top layers showed Ba and BaO reactivation ener gies of 1.5-2.6 and 2.0-2.93 eV, respectively. The Values determined from t he poisoning measurements were 0.3-0.7 eV larger that those from ion bombar dment measurements. The oxygen desorption energies were between 1.5 (Sc/Re) and 2.91 eV (W). (C) 2000 American Vacuum Society. [S0734-211X(00)08302-5] .