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Forum: Fields: Magnetic and Otherwise
Re: Sad electron stuck on volume boundary with (local) uniform magnetic field (Brian Fisher)
Re: Feedback Re: electron stuck on volume boundary with (local) uniform magnetic field (Gumplinger Peter)
Date: 21 Jul, 2008
From: Brian Fisher <Brian Fisher>

Thanks for your help. Sadly, neither solution worked.

Try 1: I adjusted the minimum steplength in

 IntgrDriver = new G4MagInt_Driver(1.0*micrometer, fStepper, fStepper->GetNumberOfVariables(), 2);

from 1 m to 1 cm to 1 mm to 1 micrometer, and the electron seems to get stuck at the same point each time.

I then tried explicitly changing the stepper used. I tried G4ExactHelixStepper (as before), G4ClassicalRK4, and G4HelixImplicitEuler. The electron gets stuck at the same place.

Try 2: I then commented out the stuff that manually sets the stepper and driver and was left with just:

	fMagField = new G4UniformMagField(G4ThreeVector(fFieldValue, 0.0, 0.0));
	fEquation = new G4Mag_UsualEqRhs(fMagField);
	fFieldMgr = new G4FieldManager(fMagField);
	fFieldMgr->CreateChordFinder(fMagField); 
	logicBFieldZone->SetFieldManager(fFieldMgr, true);

and got the same results.

I can't help but notice that the momentum vector of this particular electron is nearly entirely in the +z direction:

WARNING - G4Navigator::ComputeStep()
          Track stuck, not moving for 10 steps
          in volume -expHall- at point (-8.257530138,-9.52505,18.49000716)
          direction: (-0.01057780492,7.290631171e-05,0.9999440508).
          Potential geometry or navigation problem !
          Trying pushing it of 9e-10 mm ...

The Lorentz force on it wants to push it back into the BFieldZone volume, though the electron's momentum is carrying it across the boundary into a zero-field region. Could this be why this is happening?

Thanks for any more help you can give.

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