High Voltage Fluctuation in Microscopes
- Practical Electron Microscopy and Database -
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The resolution of electron microscopes (EMs) is partially limited by:
         i) The electrical stability of the EM systems, e.g. the stabilities of the high voltage and the lens currents;
         ii) External disturbances e.g. mechanical vibration, contamination, charging, fluctuation of stray magnetic fields, and the nonuniform magnetic properties of the pole-piece material used.

For instance, a million volt (MeV) TEM must maintain an accelerating voltage that is stable to ±0.5 volts. The instability of high voltage also limits the phase contrast transfer function (CTF) in TEMs. For instance, in EMs (especially in TEMs), the temporal coherency effects comes from the small instabilities in the accelerating voltage and electron gun emission over time, which will give the illumination a small energy spread, and from variations in the lens currents, which induces focus variation with time. Therefore, most of the CTF damping is caused by the fluctuations of the lens currents or the high voltage of the microscope, which affects the information limit (see page4772).

Moreover, various sources of instability in the microscopes also contribute to EELS spectrum drift: high voltage fluctuation, magnetic field creep (or stray magnetic field) etc. The high-voltage stability at 100 kV is typically 1 ppm (1 part in 106), resulting in an electron-energy spread of only 0.1 eV. This energy spread is much smaller than that in electron sources (see page3695).

However, shock absorbers and a shield box for the microscope high-voltage circuit can be installed to avoid degrading high-voltage stability through mechanical vibration or stray electromagnetic fields.

 

 

 

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