Volume-based Representation of the Magnetic Field

The access to the magnetic field has a large impact on both CPU performance and accuracy of simulation, reconstruction and analysis software. An approach to the magnetic field access based on a volume geometry is described. The volumes are constructed in such a way that their boundaries correspond to field discontinuities, which are due to changes in magnetic permeability of the materials. The field in each volume is continuous. The value of the field at a given point of a volume is obtained by interpolation from a regular grid of values resulting from a TOSCA calculation or, when it is available, from a parameterization. To allow global access to the magnetic field, a volume finding algorithm that exploits explicitly the layout and the symmetries of the detector is used. The main clients of the magnetic field, which are the simulation (GEANT) and the propagation of track parameters and errors in the reconstruction, can be made aware of the magnetic field volumes by connecting the per-volume magnetic field providers to the corresponding volume in the respective geometries. In this way the global volume search is by-passed and the access to the field is sped up significantly.