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

The Transmission Electron Microscope 61
(used in cathode-ray tubes) are even more sensitive to ambient gas and are
not used in the TEM.
Compared to a tungsten filament, the LaB6 source is relatively expensive
(� $1000) but lasts longer, provided it is brought to and from its operating
temperature slowly to avoid thermal shock and the resulting mechanical
fracture. It provides comparable emission current from a smaller cathode
area, enabling the electron beam to be focused onto a smaller area of the
specimen. The resulting higher current density provides a brighter image at
the viewing screen (or camera) of the TEM, which is particularly important
at high image magnification.
Another important component of the electron gun (Fig. 3-1) is the
Wehnelt cylinder, a metal electrode that can be easily removed (to allow
changing the filament or LaB6 source) but which normally surrounds the
filament completely except for a small (< 1 mm diameter) hole through
which the electron beam emerges. The function of the Wehnelt electrode is
to control the emission current of the electron gun. For this purpose, its
potential is made more negative than that of the cathode. This negative
potential prevents electrons from leaving the cathode unless they are emitted
from a region near to its tip, which is located immediately above the hole in
the Wehnelt where the electrostatic potential is less negative. Increasing the
magnitude of the negative bias reduces both the emitting area and the
emission current Ie.
Although the Wehnelt bias could be provided by a voltage power supply,
it is usually achieved through the autobias (or self-bias) arrangement shown
in Figs. 3-1 and 3-6. A bias resistor Rb is inserted between one end of the
filament and the negative high-voltage supply (�V0) that is used to accelerate
the electrons. Because the electron current Ie emitted from the filament F
must pass through the bias resistor, a potential difference (IeRb) is developed
across it, making the filament less negative than the Wehnelt. Changing the
value of Rb provides a convenient way of intentionally varying the Wehnelt
bias and therefore the emission current Ie. A further advantage of this
autobias arrangement is that if the emission current Ie starts to increase
spontaneously (for example, due to upward drift in the filament temperature
T ) the Wehnelt bias becomes more negative, canceling out most of the
increase in electron emission. This is another example of the negative
feedback concept discussed previously (Section 1.7).
The dependence of electron-beam current on the filament heating current
is shown in Fig. 3-3. As the current is increased from zero, the filament
temperature eventually becomes high enough to give some emission current.
At this point, the Wehnelt bias (�IeRb) is not sufficient to control the emitting