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Forum: Fields: Magnetic and Otherwise
Re: Question Generated electron is not affected by static magnetic field? (Kayo Matsuura)
Re: Idea Re: Generated electron is not affected by static magnetic field? (Michael H. Kelsey)
Re: Question Re: Generated electron is not affected by static magnetic field? (Kayo Matsuura)
Date: 29 Oct, 2014
From: Tom Roberts <Tom Roberts>

This has to do with the difficulty of getting good results when attempting to simulate an unphysical world.

A magnetic field as you describe, 6 mm wide with hard edges, cannot be constructed in the real world, and Geant4 has difficulty simulating it. There are two basic problems:

 1) small features in the world require that steps be limited to comparable
    size, or the features may be missed completely. Solids usually do not
    have this problem, fields always do.
 2) hard-edge fields always cause problems.

You have mentioned (1), but not (2).

To deal with (1) you need to limit steps to something less than 6 mm, perhaps a factor of 10 less.

(2) can be seen this way: if a track happens to start a step right on the edge and remains in the field for the entire step, it will see nonzero field throughout the step and will get the result you expect. If the step spans the hard edge, the result can vary considerably, depending on how the integrator splits the step into integration points, and how those points just happen to fall relative to the hard edge of the field.

The best way to avoid (2) is to not use hard-edge fields, use physical ones (i.e. satisfy Maxwell's equations, which rule out hard edges). A workaround that can work in some/many cases is to use a solid with a geometrical boundary right at the hard edge(s) -- this prevents any step from spanning the geometrical boundary, and thus spanning the hard edge of the field..

In practice, without a boundary at the hard edge, (2) makes the result depend on stepsize. In a quick test I had to limit steps to 0.02 mm before the positions of tracks after the field had an accuracy of 0.1 mm (i.e. reducing stepsize further did not move them more than 0.1 mm).

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1 Ok: Re: Generated electron is not affected by static magnetic field?   (Kayo Matsuura - 04 Nov, 2014)
2 None: Re: Generated electron is not affected by static magnetic field?   (John Apostolakis - 04 Nov, 2014)
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