ALICE3FieldV3Magnet.C

// Copyright 2019-2020 CERN and copyright holders of ALICE O2.
// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
// All rights not expressly granted are reserved.
//
// This software is distributed under the terms of the GNU General Public
// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
//
// In applying this license CERN does not waive the privileges and immunities
// granted to it by virtue of its status as an Intergovernmental Organization
// or submit itself to any jurisdiction.
//
// Author: J. E. Munoz Mendez jesus.munoz@cern.ch

std::function<void(const double*, double*)> field()
{
  return [](const double* x, double* b) {
    const double Rc = 185.;                                  //[cm]
    const double R1 = 220.;                                  //[cm]
    const double R2 = 290.;                                  //[cm]
    const double B1 = 2.;                                    //[T]
    const double B2 = -Rc * Rc / ((R2 * R2 - R1 * R1) * B1); //[T]
    const double beamStart = 370.;                           //[cm]
    const double tokGauss = 1. / 0.1;                        // conversion from Tesla to kGauss

    const bool isMagAbs = true;

    const double r = sqrt(x[0] * x[0] + x[1] * x[1]);
    if ((abs(x[2]) <= beamStart) && (r < Rc)) { // We are inside of the central region
      b[0] = 0.;
      b[1] = 0.;
      b[2] = B1 * tokGauss;
    } else if ((abs(x[2]) <= beamStart) && (r >= Rc && r < R1)) { // We are in the transition region
      b[0] = 0.;
      b[1] = 0.;
      b[2] = 0.;
    } else if ((abs(x[2]) <= beamStart) && (r >= R1 && r < R2)) { // We are within the magnet
      b[0] = 0.;
      b[1] = 0.;
      if (isMagAbs) {
        b[2] = B2 * tokGauss;
      } else {
        b[2] = 0.;
      }
    } else { // We are outside of the magnet
      b[0] = 0.;
      b[1] = 0.;
      b[2] = 0.;
    }
  };
}

void ALICE3V3Magnet()
{
  auto fieldFunc = field();
  // RZ plane visualization
  TCanvas* cRZ = new TCanvas("cRZ", "Field in RZ plane", 800, 600);
  TH2F* hRZ = new TH2F("hRZ", "Magnetic Field B_z in RZ plane;Z [m];R [m]", 100, -10, 10, 100, -5, 5);
  hRZ->SetBit(TH1::kNoStats); // disable stats box
  for (int i = 1; i <= hRZ->GetNbinsX(); i++) {
    const double Z = hRZ->GetXaxis()->GetBinCenter(i);
    for (int j = 1; j <= hRZ->GetNbinsY(); j++) {
      const double R = hRZ->GetYaxis()->GetBinCenter(j);
      const double pos[3] = {R * 100, 0, Z * 100}; // convert to cm
      double b[3] = {0, 0, 0};
      fieldFunc(pos, b);
      hRZ->SetBinContent(i, j, b[2]);
    }
  }

  hRZ->Draw("COLZ");
  cRZ->Update();

  // XY plane visualization
  TCanvas* cXY = new TCanvas("cXY", "Field in XY plane", 800, 600);
  TH2F* hXY = new TH2F("hXY", "Magnetic Field B_z in XY plane;X [m];Y [m]", 100, -5, 5, 100, -5, 5);
  hXY->SetBit(TH1::kNoStats); // disable stats box

  for (int i = 1; i <= hXY->GetNbinsX(); i++) {
    const double X = hXY->GetXaxis()->GetBinCenter(i);
    for (int j = 1; j <= hXY->GetNbinsY(); j++) {
      const double Y = hXY->GetYaxis()->GetBinCenter(j);
      const double pos[3] = {X * 100, Y * 100, 0}; // convert to cm
      double b[3] = {0, 0, 0};
      fieldFunc(pos, b);
      hXY->SetBinContent(i, j, b[2]);
    }
  }

  hXY->Draw("COLZ");
  cXY->Update();
}