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Let us assume we have a very strong electron microscope. One that would need multiple electro-static and electro-magnetic lenses.

The electrostatic lens is used to accelerate the electrons to a desired energy.

The electro-magnetic lens is used to change the path of the beam.

Now I'd assume that especially the electro-magnetic lenses, created by running high currents through a coil, can create significant magnetic fields.

On one hand I'd expect that:

  1. These magnetic fields interact with each other if we have more than 1 electro-magnetic lens.
  2. That the magnetic fields generated could cause problems outside of the E-microscope machine, like pulling metal objects towards it. Just like the MRIs in hospitals.

But neither seems to be the case. Why? When I was there, I asked about that but got only a very short answer:

'We use Helmholtz coils to stabilize the magnetic field'

What exactly does that mean? Can anyone help?

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Your question suggests that there is some external source of magnetic fields (this can be any high-power instrument in the lab, a tram on the street etc.), and you want to reduce its harmful influence on the electron beam in your microscope. This is often the case indeed, an accelerating tram pulls rouhly 1000 A from the overhead lines and the magnetic field can deflect the e-beam by fractions of a micrometer, making a high-resolution SEM image wobbly. In such situations, big Helmholtz coils are built around the whole SEM, with a sensitive magnetometer and feedback electronics that compensates the external field.

You are however asking about the interference of multiple magnetic coils within the SEM. First of all, the magnetic field of the coils should be linear and additive with respect to the individual fields of each coil. Second, the coils are enclosed in their own magnetic "cores" (or, shieldings) that guide the magnetic field into a well-defined volume within the e-beam column. So there is negligible overlap of individual coils' field anyway. You can see these "cores" in any cross-section diagram of a SEM online.

As a closing note, there are no "electric lenses" in a common SEM nowadays (excepting the Wehnelt cathode assembly on the top). There is "acceleration voltage" around the cathode, but this is not called a "lens". The rest of beam shaping is done with magnetic lenses.

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  • $\begingroup$ Ah, now things make some sense! That being said how does the magnetometer + helmholz coils work? I imagine that the helmholz coils 'cover' the whole machine in a uniform magnetic field like some sort of protection. But are they constantly generating a magnetic field or only when it is necessary (for example when sth. like a tram causes a magnetic field). In other words I am asking whether the whole helmholz apparatus always only generates exactly a magnetic field when it is necessary or whether there is always a magnetic field generated by the Helmholz coils? $\endgroup$ – Caito Oct 2 '19 at 12:54
  • $\begingroup$ And thanks for the rest information! Things start to make sense! $\endgroup$ – Caito Oct 2 '19 at 13:00
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    $\begingroup$ You will usually want to compensate only the variable component of the magnetic field. The static magnetic field (e.g. 30-50 μT field of Earth) does not cause any image deterioration, it only slightly shifts the image which nobody notices. $\endgroup$ – dominecf Oct 3 '19 at 14:40

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