Magnetic field

The user magnetic field is in VMC defined via TVirtualMagField interface. The propagation of tracks inside the magnetic field in Geant4 can be performed to a user-defined accuracy. See more details at the Geant4 User Guide for Application Developers, section 4.3. Electromagnetic Field . Note that while Geant4 allows magnetic, electric and electromagnetic fields, the VMC is currently limited to magnetic fields only.

In Geant4 VMC, user can customize the default integration method and the default accuracy parameters with a set of dedicated command. It is a user responsibility to choose the type of equation of motion and the integration method compatible with the user field. (The defined commands are not limited to magnetic field only in order to make easier an eventual future extension of VMC to electric and electromagnetic fields.)

The available commands:

• /mcMagField/stepperType stepperType 

  Selects the integrator (stepper) of particle's equation of motion; the default stepper is G4ClassicalRK4.

• /mcMagField/equationType eqType 

  Selects the type of equation of motion of a particle in a field; the default is G4Mag_UsualEqRhs.

• /mcMagField/setDeltaIntersection value
  /mcMagField/setDeltaOneStep value 

  These commands set the accuracy parameters delta intersection and delta one step of the G4 Field Manager. Their default values in Geant4 are 0.001*mm and 0.01*mm respectively.

• /mcMagField/setStepMinimum value
  /mcMagField/setDeltaChord  value 

  These commands set the step minimum and delta chord parameters to G4 Chord Finder. Their default values in Geant4 are 0.01*mm and 0.25*mm respectively.

• /mcMagField/setMinimumEpsilonStep value
  /mcMagField/setMaximumEpsilonStep value 

  These commands set the MinimumEpsilonStep and the MaximumEpsilonStep parameters of the G4 Field Manager. Their default values in Geant4 are 5.0e-5 and 0.001 respectively.

How to apply Geant4 commands in a Root user session is explained at the section on User Interfaces.