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Forum: Fields: Magnetic and Otherwise
Re: None Questions about SetLargestAcceptableStep()
Re: Feedback Re: Questions about SetLargestAcceptableStep() (Gumplinger Peter)
Date: 22 Jul, 2007
From: John Apostolakis <John Apostolakis>

The original use of  SetLargestAcceptableStep() was to make sure that a
low energy track that is spiraling in a field does not try step size
that would be impossible for the integration methods (eg in lab vacuum a
particle would likely be given a very very large step before the next
interaction, and this is a protection to ensure that work is not wasted
trying to integrate really far - whereas the setup/aparatus/detector is
of much smaller distances.)

It used to be that such a 'lever' could be needed to create smooth
tracks (for visualisation purposes only -- the tracks really are already
simulated as being curved!) - but now there are better ways to ensure
that the visualisation of tracks shows their curvature.   If you need
that, I suggest that you check the documentation for smooth trajectories
-- and ask in the forum for visualisation. 

In summary this parameter is not relevant for the purposes of accuracy.
It is meant to make sure that the field module is more robust -- and in
cases where the setup is very much larger than typical lab setups (eg
planet size) or much smaller, it is necessary to change its value.  Only
then.

Best regards,
John Apostolakis
------------------------------------------------------------------------
John Apostolakis                   Email:  John.Apostolakis@cern.ch
  

> -----Original Message-----
> From: Peter Gumplinger [mailto:gum@triumf.ca] 
> Sent: Saturday, July 21, 2007 7:10 PM
> To: PublicHyperNews@slac.stanford.edu
> Subject: Re: Questions about SetLargestAcceptableStep() 
> 
> *** Discussion title: Fields: Magnetic and Otherwise Email 
> replies to PublicHyperNews@slac.stanford.edu must include:
>   In-Reply-To: <"/emfields/136/1"@geant4-hn.slac.stanford.edu>
>   Subject: ...change this to be about your reply.
> 
> > When do i need to set the maximum acceptable step using the
> 
> > SetLargestAcceptableStep() method ?
> 
> for the visualization of smooth trajectories.
> 
> > whether the maximum acceptable step setting will affect the final
> 
> > result or not?
> 
> should not affect your results.
> 
> What can affect your results and why is illustrated in this 
> presentation by John Apostolakis:
> 
> http://geant4.in2p3.fr/2007/prog/JohnApostolakis/EMField.pdf
> 

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