For further higher brightness, another electron source called a field emission gun is used. A field emission gun typically uses a single crystal tungsten filament that has a very fine tip (Figure 17.1B). The electrons are not ejected by heating the filament but by applying a very strong electric field called an extraction voltage. The field at the pointed tip is very large (>109 V/m) and results in electron emission through tunneling. As more number of electrons can be emitted compared to field thermionic emission, field emission guns have very high brightness (>1013 A·m-2 ·sr-1 ).
Lenses for electrons
The lenses that focus the electron beam constitute the heart of an electron microscope. While studying mass spectrometry (Lecture 11), we learnt how electric and magnetic field can bend the moving charged particles. The lenses and condensers that are used in electron microscopes are electromagnets. Let us see how a magnetic field acts as a lens in focusing the electrons. A typical electromagnetic lens is shown in figure 17.2. The deflection experienced by a charged particle in a magnetic field is given by the Lorentz force law (discussed in lecture 11):
F = q (v × B) ......................................................... (11.4)
The magnetic field is largely, but not completely, parallel to the direction of the electron motion. The magnetic field in an electromagnetic lens can be resolved into radial and axial components as shown in figure 17.2B. An electron entering the lens does not experience the axial component but gets deflected by the radial component of the magnetic field. This deflection imparts a radial velocity component to the electron that takes a spiral path while going down the lens. The radial component of the electron causes the electron to respond to the axial component of the magnetic field; the force thus experienced decreases the radius of the spiral as shown in figure 17.2C and thereby resulting in a focused electron beam.
Figure 17.2 An electromagnetic lens and the magnetic field direction (A), the axial and radial components of the magnetic field in the lens (B), and the trajectory an electron takes while passing through the lens (C).