In the electron guns of conventional electron microscopes, resistance heating is produced in the gun filaments and electrons are boiled off the tip by thermionic emission.
Figure 1969a shows the structure of the electron probe-forming system in STEM mode in JEOL JEM-2010F TEMs.
Figure 1969a. Schematic illustration of the probe-forming electron optics in STEM mode in JEOL JEM-2010F TEMs.
In conventional EMs, the beam current increases with the increase of the filament current as shown in Figure 1969b. However, the filament current will finally be increased to a level which produces no greater beam current, called gun saturation.
Figure 1969b. (a) The change of beam shape with increase of filament current, and (b) The variation of beam current with increase of the filament current.