Electron guns can be classified into two types
- Thermionic gun
- Field emission gun
|Size (nm)||1 x 105||2 x 104||0.2|
|Brightness (A/cm2.steradian)||104 - 105||105 - 106||107 - 109|
|Energy Spread (eV)||1 - 5||0.5 – 3.0||0.2 – 0.3|
|Operating Lifetime (hrs)||>20||>100||>300|
|Vacuum (torr)||10-4 – 10-5||10-6 – 10-7||10-9 – 10-10|
The typical thermionic electron gun consists of three parts, the filament, Wehnelt cap and the anode. Thermoelectrons produced by the filament are accelerated through the application of a voltage between the filament and the anode, thus creating an electron beam that streams down the microscope column.
The filament load current of the thermionic gun has to be set correctly. Too low a current will yield an image of inadequate brightness, whereas excessive current will reduce the working life of the filament.
Some samples are especially sensitive to the electron beam and may even melt. If this happens, the electron beam should be turned down and the specimen removed from the microscope.
Filament types, their specifications and principles of operation are discussed in more detail in Principles of operation - The electron gun.