Os-coated cathode for very high emission-density applications

Emission densities of 35 to 100 A/cm
²
, demanded by free-electron lasers (FEL's), have set far more stringent requirements on thermionic cathodes than what is considered state of the art with current technology. The purpose of the work described here is to evaluate Os-coated cathodes for these ultra-high-current-density applications. Organometallic CVD techniques were used to coat "B" type tungsten dispenser cathodes. The coatings were found to show good adhesion and rapid activation. The activation process involves not only the standard formation of a BaO layer on the surface but also the formation of an optimum surface alloy composition of near 50-percent Os and 50-percent W. These cathodes showed excellent emission capabilities, and at 1085°C
B
, a zero-field emission density near 50 A/cm
²
was obtained. Subsequent life studies at 1085° C
B
indicated that at an emission level of 40-50 A/cm
²
over 800 h of life is obtainable with this type of cathode. During life, the pulsed emission density and surface composition were monitored. Surface Ba/BaO coverage as well as surface Os content were observed to decrease with life, but an S impurity was found to increase with life. However, the surface alloy composition change was identified as being the major cause for the emission degradation.