G4SextupoleMagField.cc

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// G4SextupoleMagField implementation
//
// Author: Helmut Burkhardt (CERN), 23.10.2019
// -------------------------------------------------------------------
 
#include "G4SextupoleMagField.hh"
#include "G4RotationMatrix.hh"
 
// -------------------------------------------------------------------
 
namespace
{
   G4RotationMatrix IdentityMatrix;
}
 
G4SextupoleMagField::G4SextupoleMagField(G4double pGradient)
{
   fGradient = pGradient;
   fpMatrix  = &IdentityMatrix;
}
 
G4SextupoleMagField::G4SextupoleMagField(G4double pGradient,
                                         const G4ThreeVector& pOrigin,
                                         G4RotationMatrix* pMatrix)
{
  fGradient = pGradient ;
  fOrigin   = pOrigin ;
  fpMatrix  = pMatrix ;
}
 
G4Field* G4SextupoleMagField::Clone() const
{
  return new G4SextupoleMagField(fGradient, fOrigin, fpMatrix);
}
 
// -------------------------------------------------------------------
 
void G4SextupoleMagField::GetFieldValue( const G4double y[],         // [4]
                                               G4double B[]  ) const // [3]
//  with displaced origin and rotation
{
  G4ThreeVector r_global = G4ThreeVector(
                                         y[0] - fOrigin.x(),
                                         y[1] - fOrigin.y(),
                                         y[2] - fOrigin.z());
 
  const G4ThreeVector r_local = (*fpMatrix) * r_global;
  const G4ThreeVector B_local( fGradient * r_local.x() * r_local.y(),fGradient * ( std::pow(r_local.x(),2) - std::pow(r_local.y(),2) )/2 ,0);
  const G4ThreeVector B_global = fpMatrix->inverse() * B_local;
 
  B[0] = B_global.x() ;
  B[1] = B_global.y() ;
  B[2] = B_global.z() ;
}