The　rapidly　development　of　high　power　Vacuum　Electron　Devices(VEDs)　at　millimeter-wave　and　THz　range　puts　strong　requirement　on　high　current　density,　long　lifetime　cathodes　[1].　Thermionic　cathode　is　at　least　one　of　the　candidates　to　take　this　task　as　a　work　horse.　Determined　by　the　surface　construction　and　the　resultant　work　function,　the　Ba-W　cathodes　can　normally　provide　moderate　current　density　though　they　exhibit　excellent　performance　while　working　at　20　-　30　A/cm2.　High　current　density　scandate　cathode　is,　thus,　again　attracted　great　attentions　recently.　Scandia　Doped　Dispenser　(SDD)　cathode　has　been　continuously　investigated　since　2000　[2][3][4][5].　Repeatable　high　emission　data　have　been　widely　demonstrated.　A　better　understanding　of　the　operation　process　of　the　cathode　would　be　helpful　for　the　further　development.　©　2010　IEEE.