In the directly heated oxide cathode, {HEAD ~ the core metal wire} comprises a tungsten core wire or {TAIL ~ a rhenium tungsten core wire} 
In the directly heated oxide cathode , { HEAD ~ the core metal wire } comprises a tungsten core wire or { TAIL ~ a rhenium tungsten core wire }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL