Accelerating Voltage/Tube in EMs
- Practical Electron Microscopy and Database -
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Manufacturers have attempted to use van de Graaff generators to supply high voltages to TEM systems, but the favorite voltage source is still Cockcroft–Walton voltage generator.

The high-voltage generated with a high-voltage generator (e.g. Cockcroft–Walton voltage generator) is supplied to acceleration tube through the high voltage cable of electron microscopes, and thus the electrons are accelerated in the acceleration tube. In TEM systems, a multi-stage acceleration electrode is used as shown in Figures 1967a and 1967b. The accelerator tube is composed of a stack of porcelain insulators sealed together by metal flanges and O rings. The accelerating electrodes are usually made of highly polished metal.

Schematic illustration of the probe-forming electron optics in STEM mode in JEOL JEM-2010F TEMs

Figure 1967a. Schematic illustration of the probe-forming electron optics in STEM mode in JEOL JEM-2010F TEMs.

For convenience of microscope operation, we need to consider the coupling between the accelerator and the condenser system. The input electron beam to the condenser system should not vary significantly as the working voltage is changed. The position of the crossover of the beam leaving the accelerator especially should not move greatly along the axis.

The schematic illustration in Figure 1967b presents the position of acceleration tube in typical TEM systems.

Schematic illustration of structure of TEM systems

1. Electron gun
2. Wehnelt unit
3. Anode
4. Electron gun second beam delector coil
5. Anode chamber isolation valve
6. 1st condenser lens coil
7. Condenser polepiece
8. 3rd condenser lens coil
9. Condenser aperture assembly
10. Specimen chamber
11. Goniometer
12. specimen holder
13. Stigmator screening cylinder
14. Objective lens coil
15. Objective lens liner tube
16. Field limiting aperture
17. Intermediate lens stigmator
18. Intermediate polepiece
19. Intermediate lens linear tube
20. Projector lens beam deflector coil
21. Projector upper polepiece
22. Projector lower polepiece
23. Binoculars
24. Viewing chamber
25. Viewing window
26. Dispensing magazine
27. Receiving magazine
28. Camera chamber
29. Lift arm
30. HT cable to high voltage tank
31. Anode chamber, or called acceleration tube
32. Gas inlet
33. Electron gun 1st beam deflector coil
34. Condenser lens stigmator coil
35. Spot alignment coil
36. Condenser lens 1st beam deflector coil
37. Condenser lens 2nd beam deflector coil
38. Condenser minilens (CM) lens coil
39. Stage heater
40. Objective polepiece
41. Objective lens stigmator coil
42. 1st image shift coil
43. Objective minilens (OM) lens coil
44. 2nd image shift coil
45. 1st intermediate lens coil
46. 2nd intermediate lens coil
47. 3rd intermediate lens coil
48. Projector lens coil
49. Viewing chamber isolation valve
50. High resolution diffraction chamber
51. Small screen
52. Large screen

Figure 1967b. Schematic illustration of the structure of typical TEM systems (e.g. JEM-2010F here).

The output of Cockcroft–Walton voltage generator is then connected to the electrode stages in the accelerator tube as shown in Figure 1967c. Each electrode stage is powered by a divided voltage supplied through the bleeder resistors that are connected to the output of the voltage generator. In the case that the high voltage is higher than several hundred kV, more than 15 stages of acceleration electrodes are often employed.

Electronic connection between Cockcroft–Walton voltage generator and accelerator tub in TEMs

Figure 1967c. Electronic connection between Cockcroft–Walton voltage generator and accelerator tub in TEMs.

Such voltage generators may be either air-insulated or confined in a tank filled with an insulating gas (e.g. freon, nitrogen, SF6) under pressure. For such systems, a large room or even a separate building is required to provide enough clearance against spark-over. Note that some systems have both generator and accelerator in a common tank; others use two separate tanks to minimize electrical interaction and make the service more convenient.

 

 

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