PHG4OuterHcalField.cc

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// $Id: $

#include "PHG4OuterHcalField.h"

#include <TSystem.h>

#include <Geant4/G4Field.hh>  // for G4Field
#include <Geant4/G4FieldManager.hh>
#include <Geant4/G4PhysicalConstants.hh>
#include <Geant4/G4SystemOfUnits.hh>
#include <Geant4/G4TransportationManager.hh>
#include <Geant4/G4Types.hh>  // for G4double, G4int
#include <Geant4/G4Vector3D.hh>

#include <cassert>  // for assert
#include <cmath>    // for atan2, cos, sin, sqrt
#include <cstdlib>  // for exit
#include <iostream>

PHG4OuterHcalField::PHG4OuterHcalField(bool isInIron, G4int steelPlates,
                                       G4double scintiGap, G4double tiltAngle)
  : is_in_iron(isInIron)
  , n_steel_plates(steelPlates)
  , scinti_gap(scintiGap)
  , tilt_angle(tiltAngle)
{
}

void PHG4OuterHcalField::GetFieldValue(const double Point[4], double* Bfield) const
{
  G4FieldManager* field_manager =
      G4TransportationManager::GetTransportationManager()->GetFieldManager();

  if (!field_manager)
  {
    std::cout << "PHG4OuterHcalField::GetFieldValue"
              << " - Error! can not find field manager in G4TransportationManager"
              << std::endl;
    gSystem->Exit(1);
    exit(1);
  }

  const G4Field* default_field = field_manager->GetDetectorField();

  if (default_field)
  {
    default_field->GetFieldValue(Point, Bfield);

    // scale_factor for field component along the plate surface
    double x = Point[0];
    double y = Point[1];
    //      double z = Point[2];

    assert(cos(tilt_angle) > 0);
    assert(n_steel_plates > 0);

    // input 2D magnetic field vector
    const G4Vector3D B0(Bfield[0], Bfield[1], Bfield[2]);
    const G4Vector3D B0XY(Bfield[0], Bfield[1], 0);
    const G4Vector3D B0Z(0, 0, Bfield[2]);

    const double R = sqrt(x * x + y * y);
    const double layer_RdPhi = R * twopi / n_steel_plates;
    const double layer_width = layer_RdPhi * cos(tilt_angle);
    const double gap_width = scinti_gap;
    if (gap_width >= layer_width)
    {
      std::cout << "PHG4OuterHcalField::GetFieldValue gap_width " << gap_width
                << " < layer_width: " << layer_width
                << " existing now, here is some debug info" << std::endl;
      std::cout << "coordinates: x: " << Point[0] / cm
                << ", y: " << Point[1] / cm
                << ", z: " << Point[2] / cm << std::endl;
      std::cout << "n_steel_plates: " << n_steel_plates << std::endl;
      std::cout << "Radius: " << R << std::endl;
      std::cout << "layer_RdPhi: " << layer_RdPhi << std::endl;
      std::cout << "layer_width: " << layer_width << std::endl;
      std::cout << "tilt_angle: " << tilt_angle << std::endl;
      gSystem->Exit(1);
      exit(1);
    }
    // sign definition of tilt_angle is rotation around the -z axis
    const G4Vector3D absorber_dir(cos(atan2(y, x) - tilt_angle),
                                  sin(atan2(y, x) - tilt_angle), 0);
    const G4Vector3D radial_dir(cos(atan2(y, x)), sin(atan2(y, x)), 0);

    const double radial_flux_per_layer = layer_RdPhi * (B0XY.dot(radial_dir));
    double B_XY_mag = radial_flux_per_layer / (relative_permeability_absorber * (layer_width - gap_width) + relative_permeability_gap * gap_width);
    B_XY_mag *=
        is_in_iron ? relative_permeability_absorber : relative_permeability_gap;

    const G4Vector3D B_New_XY = B_XY_mag * absorber_dir;

    const double z_flux_per_layer = layer_width * B0Z.z();
    double B_Z_mag = z_flux_per_layer / (relative_permeability_absorber * (layer_width - gap_width) + relative_permeability_gap * gap_width);
    B_Z_mag *=
        is_in_iron ? relative_permeability_absorber : relative_permeability_gap;
    const G4Vector3D B_New_Z(0, 0, B_Z_mag);

    // scale B_T component
    G4Vector3D B_New = B_New_Z + B_New_XY;
    Bfield[0] = B_New.x();
    Bfield[1] = B_New.y();
    Bfield[2] = B_New.z();
  }
  else
  {
    std::cout << "PHG4OuterHcalField::GetFieldValue"
              << " - Error! can not find detecor field in field manager!"
              << std::endl;
    gSystem->Exit(1);
    exit(1);
  }
}