Comparison of heat electron sources: CeB6 cathode, LaB6 cathode, and tungsten cathode
CeB6 cathode is not the only option for hot electron source, and LaB6 and tungsten cathodes are also frequently used.
Tungsten filament is a curved thin filament used to reduce the size of the emitting surface. They are usually heated to the temperature of 2500-3000 K to achieve high current density, with a work function of 4.5 eV. At 2800 K, the actual value of current density is 3A/cm2.
The service life of tungsten cathode ranges from 40 to 200 hours, which is limited by the evaporation of the cathode material. When it becomes too thin, the filament is easy to break. To prevent excessive oxidation, tungsten cathodes need to be stored in a vacuum environment of 10-3 Pa.
The cerium hexaborate and Lanthanum hexaboride (CeB6 and LaB6) cathodes are rods with sharp tips, and their operating temperatures are 1400-2000 K, because their work functions are lower than those of tungsten cathodes (LaB6 is 2.7 eV, CeB6 is 2.5 eV). Lower work function and lower temperature generate higher current density than tungsten cathodes, ranging from 20-50A/cm2.
Typically, the hexaboride cathode is 10 times brighter than the tungsten cathode, which means that they provide a higher electron beam current with a smaller spot size. Moreover, the service life of the hexaboride cathode is longer, usually 10 times longer than that of the tungsten cathode.
However, the hexaboride cathode requires a vacuum environment higher than 10-4Pa to prevent oxidation. The performance of lanthanum hexaboride cathode depends largely on vacuum and temperature. Research has shown that the CeB6 cathode is less affected by carbon pollution than the LaB6 cathode. Moreover, compared with LaB6, CeB6 cathode has a lower evaporation rate at an operating temperature of 1800K. Therefore, the service life of CeB6 cathode is longer.