This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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Figure 3608 shows schematic diagram of a thermionic electron gun. A high voltage is placed between the cathode and the anode, modified by a potential on the Wehnelt which acts as the grid in a triode system. The electric field from the Wehnelt focuses the electrons into a crossover. In other words, the Wehnelt cylinder has a small negative bias to confine the beam and enable it to pass through the anode into the column.

Figure 3608. Schematic diagram of tungsten (W) electron source.