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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.
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.
|
