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Simulation of the Thermionic Electron Gun
    C: 3 of 3

Try changing the three parameters which control the electron gun:

1. Filament current, A

The filament current controls the temperature of the filament and hence the number of electrons emitted or 'beam current'. Generally we want a large number of electrons emitted from a small region of the filament. This is done by saturating the filament - increasing the filament current until the beam current no longer rises.

2. Bias

The bias potential controls the size of the region of filament which emits electrons and hence it affects both the source size and the beam current. If the bias is too high no region of the filament will emit and the beam is said to be pinched off.

The main reason to alter bias is to change the brightness of the beam.

Use the controls on the right to start the simulation and try to set the gun at saturation.

3. High Tension (HT), kV

In the TEM, higher energy electrons permit the examination of thicker specimens, but may cause specimen damage. Higher voltage microscopes are also more expensive. Most TEMs have a maximum HT of ~ 200kV. In the SEM, the maximum HT is usually ~ 25kV and its choice is determined by a compromise between penetration (which increases with HT ) and beam diameter (which decreases with kV).

What are the units of brightness? How is the brightness of an electron beam changed as it passes through a condenser lens with a magnification of 0.05?

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  Electron-atom interactions | Electron gun | Lenses

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