TABLE OF CONTENTS After emission from the cathode, electrons are accelerated to their final kinetic energy E 0 by means of an electric field parallel to the optic axis. This field is generated by applying a potential difference V 0 between the cathode and an anode, a round metal plate containing a central hole (vertically below the cathode) through which the beam of accelerated electrons emerges. Many of the accelerated electrons are absorbed in the anode plate and only around 1 % pass through the hole, so the beam current in a TEM is typically 1% of the emission current from the cathode.
To produce electron acceleration, it is only necessary that the anode be positive relative to the cathode. This situation is most conveniently arranged by having the anode (and the rest of the microscope column) at ground potential and the electron source at a high negative potential (- V 0). Therefore the cathode and its control electrode are mounted below a high-voltage insulator (see Fig. 3-1) made of a ceramic (metal-oxide) material, with a smooth surface (to deter electrical breakdown) and long enough to withstand the applied high voltage, which is usually at least 100 kV.
Because the thermal energy kT is small (<< 1 eV), we can take the kinetic energy (KE) of an electron to be zero before acceleration and its potential energy (PE) to be the product of its electrostatic charge (- e) and the...
