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Keywords: coordinate systems nonuniform electric field
Forum: Fields: Magnetic and Otherwise
Re: Question Local Electric Field: GetFieldValue() problem (Mario)
Date: 15 Feb, 2008
From: Gumplinger Peter <Gumplinger Peter>

Hi Mario,

My guess is that none of us will speculate what happens when you have overlapping volumes because it is a NO NO in Geant4, precisely because what the G4Navigator will do is unpredictable. You have to use special (G4) tools to test your geometry for overlaps. Please, see the documentation about that.

> The main concern is point[4] which comes in at the GetFieldValue()
> function.

point[3] is 'time' - ergo, you can have a time dependent field.

> I have displayed point[0], point[1], point[2] and calculated the
> magnitude of this vector and in some instances it is larger than my
> entire world volume

You can also turn on step-wise debugging (/tracking/verbose 1) to follow where your tracks go and since you seem to have a debugging print statement in your SetFieldValue method you can find out when it's called - should only be called when in your 'logicalVolume' - and at what global coordinate points. If you print Point[0,1,2] you should be able to verify whether Point is local or global. I think it is global - but I am not 100% sure. So, if you want to return a value easier calculated from the local system, you need to use a GlobalToLocal transform:

G4StepPoint* p = aStep->GetPreStepPoint();

G4ThreeVector globalPoint(Point[0],Point[1],Point[2]);

const G4AffineTransform global2local = p->GetTouchable()->GetHistory()->GetTopTransform();

G4ThreeVector localPoint= global2local.TransformPoint(globalPoint);

Except I am at a loss how you get a handle to G4Step aStep in your SetFieldValue method. One lengthy way might be:

G4Step aStep = G4EventManager::GetEventManager()->GetTrackingManager()->GetSteppingManager()->GetStep();

> I guess I am curious about the coordinate system that point[4] is
> using... does it use the logicalVolume in which the fieldManager of
> the field belongs to as the coordinate system, or is it with respect
> to the global volume??

I am afraid it's the global coordinate system.

> is there something special about a volume that is made of Vacuum and
> holds a field manager??

Not that I know off.

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1 None: Re: Local Electric Field: GetFieldValue() problem   (Mario - 15 Feb, 2008)
(_ Feedback: Re: Local Electric Field: GetFieldValue() problem   (Peter Gumplinger - 18 Feb, 2008)
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