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Question Re: Stepper for a Rapidly Changing Magnetic Field 

Forum: Fields: Magnetic and Otherwise
Re: Question Stepper for a Rapidly Changing Magnetic Field (Doris)
Re: None Re: Stepper for a Rapidly Changing Magnetic Field (John Apostolakis)
Date: 16 Nov, 2013
From: Doris <Doris>

Dear John,

    Thanks for the explanation. So the classical RK4 is a very robust one. 

    If you do not mind, could I ask you one more question? From the Geant4 document, I found an interesting magnetic field calculation option involving "cashing" of the old calculated value. I can keep the old value under certain conditions instead of calculation new one, which could reduce the CPU time consumption.

    I guess this "cashing" method may not be ideal for the iron/air interface region, but may work for the other detector area. Does the community have enough experience on this option?

    Best Regards,
    Doris

On Fri, 15 Nov 2013 17:26:27 GMT, John Apostolakis wrote:

> 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,
> John
> ===================================================
> 
> On Nov 15, 2013, at 6:19 AM, <dorisykim@ssu.ac.kr>
>  <dorisykim@ssu.ac.kr> 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 hypernews.slac.stanford.edu message (to reply or unsubscribe) at: 
> > http://hypernews.slac.stanford.edu/HyperNews/geant4/get/emfields/209.html 
> 

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