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Apart from a long lifetime, the main requirements of the electron source are high
brightness (electron current density per solid angle) and small spread of the kinetic
electron energy. This requires materials with a small work function.
Table 4963. 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) |
1700-2700 |
1800 |
1800 |
300 |
E (V/m) |
Low |
Low |
≈108 |
>109 |
Je (A/m2) |
≈104 |
≈106 |
≈107 |
≈109 |
Brightness
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 |
| Effective area |
Larger |
Smaller from a crystal vertex |
|
|
| Coherence |
Lower |
Higher |
|
|
** 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.
|
