Handling fields in Key4hep

Juraj Smieško

CERN

FCC Detector Full Sim Working Meeting

CERN, 26 Jun 2023

Key4hep

  • Set of common software packages, tools, and standards for different Detector concepts
  • Common for FCC, CLIC/ILC, CEPC, EIC, …
  • Individual participants can mix and match their stack
  • Main ingredients:
    • Data processing framework: Gaudi
    • Event data model: EDM4hep
    • Detector description: DD4hep
    • Software distribution: Spack

EDM4hep I.

Describes event data with the set of standard objects.

  • Specification in a single YAML file
  • Generated with the help of Podio

EDM4hep II.

Example object: 

#-------------  CalorimeterHit
edm4hep::CalorimeterHit:
  Description: "Calorimeter hit"
  Author : "F.Gaede, DESY"
  Members:
    - uint64_t cellID            //detector specific (geometrical) cell id.
    - float energy               //energy of the hit in [GeV].
    - float energyError          //error of the hit energy in [GeV].
    - float time                 //time of the hit in [ns].
    - edm4hep::Vector3f position //position of the hit in world coordinates in [mm].
    - int32_t type               //type of hit. Mapping of integer types to names via collection parameters "CalorimeterHitTypeNames" and "CalorimeterHitTypeValues".
  • Current version: v0.8.0
  • Objects can be extended / new created
  • Bi-weekly discussion: Indico

FullSim in Key4hep

DDsim

  • Part of the DD4hep
  • Used to simulate CLD
  • Steering with Python script

k4SimGeant4

  • Set of Gaudi algorithms/tools
  • Used to simulate FCC LAr
  • Can be part of the larger steering
Both simulations output EDM4hep format, but there are some minor differences

Fields in Geant4

  • Geant4 can propagate particle through magnetic, electric, electromagnetic and gravitational fields
  • The tracking can be done to arbitrary accuracy
  • Equation of motion of the particle in the field is integrated usually by Runge-Kutta method
  • There are several method implementations, suitable for different conditions
  • Inside one step, the path is broken up into small segments: chords
  • The magnetic field is managed by G4FieldManager
  • User needs to implement G4Field method:
    GetFieldValue(const double Point[4], double *fieldArr)

Field in DD4hep Compact File

  • Electric or magnetic field(s) described in field(s) tag
  • DD4hep creates combined field: OverlayedField
  • Constant Electric or Magnetic Fields are defined as follows:
  • Magnetic Dipoles are defined as follows: 
    
  1.0*tesla
  0.1*tesla/pow(cm,1)
  0.01*tesla/pow(cm,2)
  • Other notable field types: solenoid, FieldXYZ
More details in DD4hep Manual

DDSim

  • Simulation runs field(s) provided in the field tag
    • Constant, solenoid, 3D Fieldmap, ...
  • Integration parameters can be provided in the Python steering: 
    ################################################################################
## Configuration for the magnetic field (stepper) 
################################################################################
SIM.field.delta_chord = 0.25*mm
SIM.field.delta_intersection = 0.001*mm
SIM.field.delta_one_step = 0.01*mm
SIM.field.eps_max = 0.001*mm
SIM.field.eps_min = 5e-05*mm
SIM.field.equation = "Mag_UsualEqRhs"
SIM.field.largest_step = 10.0*m
SIM.field.min_chord_step = 0.01*mm
SIM.field.stepper = "ClassicalRK4"
    Example taken from CLICPerformance

k4SimGeant4

  • Simulation service SimG4Svc needs mag. field tool
    • Interface: ISimG4MagneticFieldTool
  • Three tools implemented:
    • SimG4ConstantMagneticFieldTool: Constant field in barrel
    • SimG4MagneticFieldFromMapTool: 2D Comsol map, 3D map from ROOT file
    • SimG4MagneticFieldTool(PR #37): Propagates field defined in compact file
  • Example of constant mag field: 
    from Configurables import SimG4ConstantMagneticFieldTool
field = SimG4ConstantMagneticFieldTool("SimG4ConstantMagneticFieldTool")
field.FieldComponentZ = -2 * units.tesla
field.FieldOn = True
field.IntegratorStepper="ClassicalRK4"
    Example taken from k4RecCalorimeter

Conclusions

  • With PR #37 both simulation methods can run with various fields
  • DD4hep offers ability to specify mag. field alongside the other detector "parts" in the compact file
  • Tool approach of k4SimGeant4 allows to change the field in the steering
  • Remark: Field maps interface is not well defined

Backup

MagFieldScanner

  • With PR #37 there is now possibility to probe your magnetic field with MagFieldScanner
  • There are three probe types:
    • XYPlane: shows mag. field on the XY plane at any z
    • ZPlane: shows mag. field on the plane lying on z-axis at any phi (angle from x-axis)
    • Tube: shows mag. field on a tube with radius r
  • Example result: Mag. field: constant B_z = -2T, rMax = 150cm, abs(zMax) = 20m