Challenge: external B-field #2
Description
Maybe starting with a pre-defined B-field in Geant4, target is to import an external B-field, for instance from COMSOL.
One method is shown in Advanced Examples, purging magnet.
Potential issues: Thread-local?
No repeated reading of file resource permitted -> must be a one-off effort.
Not a regular grid of values, realistically. Find good storage data structure and interpolation, different to 'purging magnet' example. Candidate methods could be as in SNDrift code with kd-tree.