The electron gun chamber normally consists of an emission source (cathode) for emitting the electron beam and an electron gun lens (C0) for intersecting the electron trajectories with the optical axis. The gun lens is normally a small electrostatic lens with a variable potential in the range of 0.5 - 2 kV. Figure 3587 shows that a strongly excited gun lens brings the gun crossover close to the gun, whereas a weakly excited gun lens has the gun crossover lower (closer to the first condenser lens, C1).
Figure 3587. Schematic illustration of the effects of the strengths of gun lens:
(a) Strongly excited gun lens; (b) Weakly excited gun lens.
In most modern, practical TEMs/STEMs, the gun lens is used to position the first crossover in relation to the beam-defining aperture (normally the C2 aperture). The crossover locates high above the aperture with a strong gun lens, while the crossover is close to the aperture with a weak gun lens. Both the beam current and aberrations on the beam are higher for the latter case. Therefore, when small, intense and low-aberration electron beams are needed, e.g. for diffraction in TEM and analytical STEM, a strong gun lens is selected; while a weak gun lens is selected when high probe currents are needed (e.g. TEM imaging). In the TEM mode, the beam is spreaded, therefore, the aberrations do not significantly affect the small imaging area.