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Forum: Fields: Magnetic and Otherwise
Re: Question Stepper for a Rapidly Changing Magnetic Field (Doris)
Date: 15 Nov, 2013
From: John Apostolakis <John Apostolakis>

Dear Doris,

ClassicalRK4 is the most robust field integration stepper, and for this reason it is the default.  It should cope well with the case that you mention - where the field changes at the interface between iron and air.  

You will have a challenge to describe this field accurately, of course, in this region because of its complexity - but it will not cause a significant problem to ClassicalRK4.

It is possible to choose a different stepper for other reasons - for example if tracking in field is a significant CPU host spot for your application, or if you want to check whether your results or CPU speed are influenced by it.

Two suggestions for steppers which could be interesting:
  - G4NystromRK4 was developed in ATLAS (Gavrilenko and colleagues - there is a paper written about it) and has fewer field evaluations per step - a plus if your field is costly to calculate. Note: this can be used only for pure magnetic fields - it does not work in any other case;
  - G4SimpleHeum is general, and *may* cope a bit better with a very-discontinuous magnetic field - if its value is not smooth when you approximate it and if it has large steps (discontinuities).

Since replacing the default stepper requires a few operations, I refer you to the Users Guide for Applications Developers to see how to do it.

Best regards,

On Nov 15, 2013, at 6:19 AM, <>
 <> wrote:

> *** Discussion title: Fields: Magnetic and Otherwise
> Hello,
>     In my understanding, RK4 is the default integration stepper for the magnetic field. I also heard that this method would not work for a rapidly changing magnetic field.
>     For example, a track would go through a detector consisting of big chunks of iron and air gaps in between. I guess the magnetic field would change rapidly at every boundary between iron and air. In such cases, which stepper would you recommend to use? And what kind of commands should I use to implement the change?
>     Regards,
>     Doris
> -------------------------------------------------------------
> Visit this GEANT4 at message (to reply or unsubscribe) at: 

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1 Question: Re: Stepper for a Rapidly Changing Magnetic Field   (CuriousDoris - 16 Nov, 2013)
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