SHiP Field Maps

The SHiP magnetic field maps in the SHiP field-map format, plus a manifest that says where each map sits in the SHiP geometry. aegir and Trout read the maps through field_service. The conda package is ship-field-maps.

Contents

The package installs into $PREFIX/share/field/:

File	Map	Magnet	Placement (mm)	Symmetry
muon_shield_TRY_2026.root	muon_shield_TRY_2026	/SHiP/muon_shield	(0, 0, 2040)	quadrant dipole
spectrometer_V21_2455.root	spectrometer_V21_2455	/SHiP/magnet	(0, 0, 89570)	none
placements.json
field_maps.libsonnet

The muon shield map starts at the shield entrance. The spectrometer map is centred on the magnet.

Both maps are converted with fairship_to_fieldmap from FairShip's files/ directory at the commit given in placements.json:

• muon_shield_TRY_2026 comes from TRY_2026.root.
• spectrometer_V21_2455 comes from 2026_07_02_MainSpectrometerField_V21_2455.root.

The values are unchanged. Every other map in FairShip is outdated or was never adopted, so none of them are included.

Placements

The field-map format leaves placement to the consumer, so the translations live in placements.json. They match the volume positions of the shipgeometry version recorded in its geometry entry. Bump that version together with the translations when the geometry moves.

FairShip uses its own geometry and places the maps differently: the shield at about z = 2140 mm and the spectrometer at z = 89720 mm.

Use

Bare filenames resolve under $SHIPFIELD_ROOT/share/field/. In a pixi or conda environment, SHIPFIELD_ROOT is the environment prefix.

In jsonnet, add the directory to the import path (-J $CONDA_PREFIX/share/field) and take the magnet list from the library:

local field_maps = import 'field_maps.libsonnet';

{
  field: { cpp: 'field_map_provider', magnets: field_maps.magnets },
}

field_maps.magnets has one {name, volume_pattern, file, map, translation} entry per magnet, which is the shape SHiP::FieldMapSource takes. Other code can read placements.json directly.

Build

pixi run build   # downloads the FairShip maps and converts them into build/
pixi run test    # compares the converted maps with the FairShip originals
pixi run install # installs into the pixi environment

scripts/build_maps.py downloads each map from FairShip at the pinned commit and checks it against the sha256 in placements.json. Downloads are cached in $TMPDIR/ship-field-maps. To use a local FairShip checkout instead, pass --input-dir path/to/FairShip/files.

The test evaluates the converted maps at 2000 random grid nodes and compares the result with FairShip's Data tree. For the muon shield it also checks the three mirrored quadrants against the sign rules of FairShip's ShipBFieldMap. It then checks that field_maps.libsonnet expands to the expected magnet entries.

Updating a map

1. Point fairship.commit in share/field/placements.json at the FairShip commit with the new file.
2. Update the map entry's source and sha256. The sha256 is the oid in the file's git-lfs pointer: git show <commit>:files/<name>.root | grep oid.
3. Run pixi run test.