BDSIntegratorSextupole.cc

/* 
Beam Delivery Simulation (BDSIM) Copyright (C) Royal Holloway, 
University of London 2001 - 2023.
 
This file is part of BDSIM.
 
BDSIM is free software: you can redistribute it and/or modify 
it under the terms of the GNU General Public License as published 
by the Free Software Foundation version 3 of the License.
 
BDSIM is distributed in the hope that it will be useful, but 
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.
 
You should have received a copy of the GNU General Public License
along with BDSIM.  If not, see <http://www.gnu.org/licenses/>.
*/
#include "BDSDebug.hh"
#include "BDSIntegratorSextupole.hh"
#include "BDSMagnetStrength.hh"
#include "BDSStep.hh"
#include "BDSUtilities.hh"
 
#include "globals.hh" // geant4 types / globals
#include "G4Mag_EqRhs.hh"
#include "G4ThreeVector.hh"
 
#include "CLHEP/Units/SystemOfUnits.h"
 
#include <cmath>
 
BDSIntegratorSextupole::BDSIntegratorSextupole(BDSMagnetStrength const* strength,
                                               G4double                 brho,
                                               G4Mag_EqRhs*             eqOfMIn):
  BDSIntegratorEulerOld(eqOfMIn)
{
  // B'' = d^2By/dx^2 = Brho * (1/Brho d^2By/dx^2) = Brho * k2
  bDoublePrime = brho * (*strength)["k2"] / CLHEP::m3;
  zeroStrength = !BDS::IsFinite(bDoublePrime);
#ifdef BDSDEBUG
  G4cout << __METHOD_NAME__ << "B'' = " << bDoublePrime << G4endl;
#endif
}
 
void BDSIntegratorSextupole::AdvanceHelix(const G4double  yIn[],
                                          G4double        h,
                                          G4double        yOut[],
                      G4double        yErr[])
{
  G4ThreeVector mom = G4ThreeVector(yIn[3], yIn[4], yIn[5]);
  G4double momUnit = mom.mag();
  G4double kappa = (-eqOfM->FCof()*bDoublePrime) / momUnit;
 
  if (std::abs(kappa) < 1e-12)
    {
      AdvanceDriftMag(yIn, h, yOut, yErr);
      SetDistChord(0);
      return;
    }
  
  G4ThreeVector pos          = G4ThreeVector( yIn[0], yIn[1], yIn[2]);
  BDSStep       localPosMom  = ConvertToLocal(pos, mom, h, false);
  G4ThreeVector localPos     = localPosMom.PreStepPoint();
  G4ThreeVector localMom     = localPosMom.PostStepPoint();
  G4ThreeVector localMomUnit = localMom.unit();
  
  G4double x0 = localPos.x();
  G4double y0 = localPos.y();
 
  G4double xp = localMomUnit.x();
  G4double yp = localMomUnit.y();
  G4double zp = localMomUnit.z();
  
  // Evaluate field at the approximate midpoint of the step.
  const G4double halfH = 0.5*h;
  x0 = x0 + localMomUnit.x()*halfH;
  y0 = y0 + localMomUnit.y()*halfH;
  
  G4double x02My02 = x0*x0 - y0*y0;
  
  // local r'' (for curvature)
  G4ThreeVector localA;
  localA.setX(zp*x02My02);
  localA.setY(-2*zp*x0*y0);
  localA.setZ(xp*x02My02-2*yp*x0*y0);
  
  localA *= kappa / 2; // 2 is actually a 2! factor.
  
  AdvanceChord(h,localPos,localMomUnit,localA);
  ConvertToGlobal(localPos, localMomUnit, yOut, yErr, momUnit);
}