Reduction of field emission dark current for high-field gradient electron gun by using a molybdenum cathode and titanium anode

Dark current measurements are performed for Mo and Ti electrodes and the results are analyzed to separate the primary field emission current from the total observed dark current. The analysis shows that Mo exhibits very low primary field emission current as a cathode, reaching 1 nA of primary field emission current at a DC field gradient of 170 MV/m. The equivalent measure for Ti is 124 MV/m, and Ti also exhibits a lower enhancement effect of dark current due to electron and ion bombardments of cathode and anode. An electrode configuration with Mo cathode and Ti anode is thus examined, and it is confirmed by experiments that a field gradient of as high as 130 MV/m for 1 nA total dark current is possible for an electrode gap of 0.5 mm and an effective cathode area of 7 mm2.