Physical Principles of Electron Microscopy: An Introduction to TEM ...

70 Chapter 3
where G is a large amplification factor (or gain), dependent on the design of
the oscillator and of the step-up transformer. Changing Vi (by altering the
reference voltage V+ in Fig. 3-6) allows V0 to be intentionally changed (for
example, from 100 kV to 200 kV). However, V0 could drift from its original
value as a result of a slow change in G caused by drift in the oscillator or
diode circuitry, or a change in the emission current Ie for example. Such HV
instability would lead to chromatic changes in focusing and is generally
unwanted. To stabilize the high voltage, a feedback resistor R f is connected
between the HV output and the oscillator input, as in Fig. 3-6. If G were to
increase slightly, V0 would change in proportion but the increase in feedback
current I f would drive the input of the oscillator more negative, opposing the
change in G. In this way, the high voltage is stabilized by negative feedback,
in
a similar way to stabilization of the emission current by the bias resistor.
To provide adequate insulation of the high-voltage components in the HV
generator, they are immersed in transformer oil (used in HV power
transformers) or in a gas such as sulfur hexafluoride (SF6) at a few
atmospheres pressure. The high-voltage “tank” also contains the bias resistor
Rb and a transformer that supplies the heating current for a thermionic or
Schottky source. Because the source is operating at high voltage, this second
transformer must also have good insulation between its primary and
secondary windings. Its primary is driven by a second voltage-controlled
oscillator, whose input is controlled by a potentiometer that the TEM
operator turns to adjust the filament temperature; see Fig. 3-6.
Although the electric field accelerating the electrons is primarily along
the optic axis, the electric-field lines curve in the vicinity of the hole in the
Wehnelt control electrode; see Fig. 3-7. This curvature arises because the
electron source (just above the hole) is less negative than the electrode, by an
amount equal to the Wehnelt bias. The curvature results in an electrostatic
lens action that is equivalent to a weak convex lens, bringing the electrons to
a focus (crossover) just below the Wehnelt. Similarly, electric field lines
curve above the hole in the anode plate, giving the equivalent of a concave
lens and a diverging effect on the electron beam. As a result, electrons
entering the lens column appear to come from a virtual source, whose
diameter is typically 40 �m and divergence semi-angle �1 (relative to the
optic axis) about 1 mrad (0.06 degree) in the case of a thermionic source.
3.3 Condenser-Lens System
The TEM may be required to produce a highly magnified (e.g, M = 10 5 )
image of a specimen on a fluorescent screen, of diameter typically 15 cm. To
ensure that the screen image is not too dim, most of the electrons that pass