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Forum: Fields: Magnetic and Otherwise
Re: None Rotating a field & obtaining the EM field in a certain point (Giovanni Marchiori)
Date: 27 Sep, 2006
From: John Apostolakis <John Apostolakis>

Dear Giovanni,

Thank for your good questions.

> 1) ... when I [] rotate the logical volume and place it inside the world, the field
> is not rotated at all: is this behaviour expected? 

It is indeed the expected (and I think documented) behaviour that the field is expressed in global coordinates, despite being hooked into the logical volume.

I emphasize that both position and field are in global coordinates:
  - the position you give to the field must be in global coordinates, and
  - the value of the field returned will be in global coordinates.

I will try to inspect your provided code to provide feedback soon.

> 2) What is the easiest way to know, once I have setup the
> geometry of the fields, what is the total electromagnetic field - in 
> global coordinates - in a generic point in space? Does Geant4 
> provide such a functionality?

This is a very good question - and likely should be in our FAQ.

First I note that only a single field exists for any point of space in Geant4 currently. It can be either a global field (the one chosen for all the setup) or a local field (that is set for a logical volume and can be propagated to ones below, and which overrides any global field.) [ Extensions which allow different field descriptions have been created, including BeamTools from D. Elvira at Fermilab. ]

A single method to get the global field value is not currently available. You will need to call several objects to do this currently and tailor it to your current field needs (eg B field only, EM field, custom field.) I will communicate privately a trial version of this, and once it is ironed out post it.

Best regards, John Apostolakis.

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1 None: Re: Rotating a field & obtaining the EM field in a certain point   (Giovanni Marchiori - 27 Sep, 2006)
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