Lifetime of Electron Gun
- Practical Electron Microscopy and Database -
- An Online Book -

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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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The designed lifetime of the principal electron sources are 100 hr for tungsten (W) gun, 1000 hr for LaB6 gun, > 5000 hr for Schottky FEG Gun, and > 5000 hr for cold FEG, respectively. Table 4962 shows the operating parameters and characteristics of electron sources, including gun lifetime in general.

Table 4962. Operating parameters and characteristics of electron sources

Type of source
Tungsten
thermionic
LaB6
thermionic
Schottky
emission
Cold field
emission
Material
W
LaB6
ZrO/W
W
ds (µm)
≈40
≈10
≈0.02
≈0.01
ΔE (eV)
1.5
1.0
0.5
0.3
φ (eV)
4.5
2.7
2.8
4.5
T (K)
2700
1800
1800
300
E (V/m)
Low
Low
≈108
>109
Je (A/m2)
≈104
≈106
≈107
≈109
β (Am-2/sr-1)
≈109
≈1010
≈1011
≈1012
Vacuum (Pa)
<10-2
<10-4
<10-7
≈10-8
Lifetime (hours)
100
1000
104
104

** ds -- Effective (or virtual) source diameter; ΔE -- Energy spread of the electron beam; φ -- Work function; T -- Operating temperature; E -- Electric field; Je -- current density of electron beam; β -- Electron-optical brightness at the cathode.

However, the actual lifetime of the electron gun depends on the operation conditions such as vacuum, operating temperature, current, and high voltage. For instance, the following figure shows the dependence of lifetime on operating temperature.

FILAMENT LIFETIME vs OPERATING TEMPERATURE

Figure 4962. Filament lifetime vs operating temperature.

With increase of heating temperature in thermionic emission guns, the emission current also increases, the energy spread of emitted electrons becomes wider, and the lifetime of the filament becomes shorter.

 


 

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