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Magnetic Field Interface

Contacts

  • TOSCA computation: Vyacheslav Klyukhin
  • CMSSW interface: Nicola Amapane

Default configuration

The python configuration to get the default field map is, since CMSSW_7_4_3 and CMSSW_7_5_0_pre5 (see below for older releases):

process.load("Configuration.StandardSequences.MagneticField_cff")

This sets the the map so that the Run I/Run II map is chosen automatically based on the GT, and the nominal working point is set automatically depending on the magnet current stored in the conditions DB (RunInfo). Nominal working points are:

  • 0 T
  • 2 T (9500 A)
  • 3 T (14340 A)
  • 3.5 T (16730 A)
  • 3.8 T (18164 A)
  • 4 T (19140 A)

For the MC, the magnet current in conditions is fixed, and corresponds to the 3.8 T working point; so the above always results in the 3.8 T map for MC. In order to configure CMSSW for zero field (magnet off) in MC one needs to replace the config with:

process.load("Configuration.StandardSequences.MagneticField_0T_cff")

Similarly, to fix the map to other nominal working points, replace "0T" with one of the following: "20T", "30T", "35T". Note that this is not necessary for data.

Older releases

In releases prior to CMSSW_7_4_3 and CMSSW_7_5_0_pre5, default configurations used to be:

  • Configuration.StandardSequences.MagneticField_AutoFromDBCurrent_cff: Run I maps with automatic choice of nominal working point for data; fixed 3.8T for MC
  • Configuration.StandardSequences.MagneticField_38T_PostLS1_cff: Run II map for 3.8T only.

When moving to CMSSW_7_4_3 and CMSSW_7_5_0_pre5, please replace these with Configuration.StandardSequences.MagneticField_cff as mentioned above. Configurations for fixed nominal working points, eg. 0 T, did not change (cf above).

Access to the map

The magnetic field map is available in CMSSW from the EventSetup as an object of type CMS.MagneticField. An example of getting the magnetic field in CMSSW is available in

CMS.MagneticField/Engine/test/queryField.cc
CMS.MagneticField/Engine/test/queryField.py

This is everything needed for the general user.

Internals

The following sections include developer documentation and non-standard and advanced options that may be needed for specific studies.

How the field map works

Concrete implementations ("engines") of a MagneticField are available to deliver different maps in either the whole CMS or in specific parts of it. The default map is called VolumeBasedMagneticField, and is based on a geometrical description of the solenoid and yoke volumes, with a separate "field provider" for each volume. Normally a field provider is based on interpolation over a grid of points tabulated with a finite-element computation.
Providers can however also be based on e.g. parametrizations.
A special region is the inner tracker region. Here, VolumeBasedMagneticField allows to specify an ad-hoc parametrization to replace the normal volume/field provider mechanism, for either speed or accuracy purposes. Technically this is implemented by overlaying a "slave field engine" with a defined spatial validitity region. This behaviour is specified by the card:

VolumeBasedMagneticFieldESProducer.useParametrizedTrackerField = True

With this setting, a field map with label = 'parametrizedField' is searched for in the <nop>EventSetup and used for the inner tracker region.

History of map versions

Here is a brief description of the different development and production versions.

A collection of documents related to the field map computation, mapping, and comparison with measured data can be found here .

Version 85l_030919 ("old 4T map"; obsolete, no longer supported)

The 4T field map used in ORCA and in CMSSW up to series 2.0.X included is based on a TOSCA finite-element computation performed in 2003. It is based on a geometric model ("magnetic field geometry") of 271 volumes representing one 30 degree-sector in z<0 only (1/24 of CMS), thus the model is Z-symmetric and 12-fold phi-symmetric. For each volume, the field is computed on a regular (non-cartesian) grid defined appropriately. The CMSSW field interface returns the field at an arbitray point using 3D linear interpolation within the grid cell containing the point inside each volume. The nominal B value in this map (4T) cannot be changed. A parametrization of the TOSCA computation (by V. Karimaki) can be optionally used in the region r<1.2 m, |z|<3.0 m in place of the interpolation; the main advantage is speed (and it also provides smoother derivatives than the interpolation).

This has been the only full field map available after 2003 and before CMSSW_2_1_0.

This version is no longer supported after CMSSW_2_2_10.

Version 1103l_071212 (Pre-CRAFT default; deprecated since 3_1_X; still used for some nominal operating points other than 3.8T)

This map is in use since CMSSW 2.2. and includes improvements based on the experience of the Magnet Test, which included a mapping campaign of the field within the solenoid, as well as measurements in the yoke (with flux loops during fast discharges, and with Hall probes). It is based on a TOSCA finite-element computation with a model that includes the Z-asymmetry of the coil spires and more accurate description of a full half (x>0) of CMS including chimneys. However field data is still provided only for sector 1 (30 degrees, representing 1/12 of CMS with a total of 312 volumes), so the CMSSW map is 12-fold phi-symmetric (but not Z-symmetric).

The map with this model is available for several nominal field values: 2, 3, 3.5, 3.8 and 4T. The latter is expected to be different (more realistic) w.r.t. the previous 4T computation (85l_030919). Physics analyses should use the 3.8T map.
A parametrization of the TOSCA computation within r<1.15 m, |z|<2.8 m is available for all field values (V. Karimaki). This parametrization matches very well with the computed data (difference of ~5mT at the edges of the validity region) and is used as a slave field in the default configuration (cf section above), mainly to save CPU time in production and HLT.


A different parametrization of the real mapping data (V. Maroussov) is available as an option. It is defined in the region r<1.9 m, |z|<3.5m, excluding the "corners" |z|+2.5*r>6.7 m. This parametrization reproduces the mapping data more precisely, but is slower; it is the suggested option for application where ultimate precision is necessary and CPU speed can be sacrificed. The agreement of this map with the mapping data is better than 0.4 mT (agreement to ~10^-4) in the tracker region and up to 0.8 mT at the edges of the validity region. To activate it specify the following:

process.ParametrizedMagneticFieldProducer.version = 'PolyFit2D'
process.ParametrizedMagneticFieldProducer.parameters = cms.PSet(
         BValue = cms.double(3.81143)
)
process.VolumeBasedMagneticFieldESProducer.useParametrizedTrackerField = True;

(the latter card is actually set to True by default in the standard config).

In 3.1.X releases and later, where this map version is superseeded by 1103l_090322_2pi_scaled, it can be activated using:

process.load("MagneticField.Engine.volumeBasedMagneticField_1103l_cfi").

Version 1103l_090216 (Intermediate development; no longer supported)

Improved computation w.r.t. 1103l_071212, with enlarged radius (R=30 m) of the total region where the field is computed, plus some additional details outside CMS (e.g. iron plate in the hall's floor). The same 312-volume 12-fold phi-symmetric geometry is used in the CMSSW interface. This map shows much better agreement with measurements with tracks in the iron yoke. This version was used for development only, and is no longer supported.

Version 1103l_090322 (deprecated)

Based on 1103l_090216, this computation is done in volume extended also in |Z| (up to 35 m). The same 312-volume 12-fold phi-symmetric geometry is used in the CMSSW interface. Changes w.r.t. 1103l_090216 are visible basically only in the forward region.
This version was used for development only, and is deprecated in favor of later versions.

This version will is available since recent 2_2_X versions with the configuration:

process.load("MagneticField.Engine.volumeBasedMagneticField_1103l_090322_cfi")

Version 1103l_090322_2pi (deprecated)

Based on 1103l_090322, uses different tables for the barrel yoke iron layers in the chimney sectors (S3, S4) and for S9, S10, S11 in the third barrel layer and in the endcaps. This version was used for development only, and is deprecated in favor of later versions.

Version 1103l_090322_2pi_scaled (Run I default , adopted starting from 3_1_0)

Same as 1103l_090322_2pi; in addition the field is scaled with factors measured from CRAFT data.

This version is used by default in CMSSW_3_1_0 and later releases.
In CMSSW_2_2_12 and CMSSW_2_2_13 it can be optionally used replacing whatever other \CMS.MagneticField config fragment with:

process.load("MagneticField.Engine.volumeBasedMagneticField_090322_2pi_scaled_cfi")

Version 1103l_090601 (Intermediate development; no longer supported)

Same as 1103l_090322_2pi; with small correction of the TOSCA geometry in L3 of barrel sectors 9,10,11. Since in S9,S1 the thicker L3 platesextend into air volumes 297-307, a finer grid was used for these volumes/sectors. Tables with this configuration were specified in MagneticFieldParameters _07_2pi_air.xml.

Two sets of scaling factors were prepared, based on CRAFT08 (ScalingFactors _090601_091022_cfi.py) and CRAFT08+09 (ScalingFactors _090601_091116_cfi.py). These were however never released.

Configuration (no longer supported)

process.load("MagneticField.Engine.volumeBasedMagneticField_090601_cfi")

or, to also apply scaling factors from CRAFT data:

process.load("MagneticField.Engine.volumeBasedMagneticField_090601_scaled_cfi")

Version 090812 (Intermediate development; no longer supported)

Full TOSCA model made of two halves ( _1 for X>0 and _2 for X<0). Has not been released into CMSSW.

Version 120812 (Default for Run II since CMSSW_6_2_0; replaced for data since 743/750pre5)

Full TOSCA model made of two halves ( _1 for X>0 and _2 for X<0). Volume numbers are 1001-1360 and 2001-2360.Two version exist, with the additional YE4 plate (post-LS1) and without (2010-2012 geometry.)

To use it in CMSSW_6_2_0, you need to use:

process.load("MagneticField.Engine.volumeBasedMagneticField_120812_largeYE4_cfi")

and for the 2010/2012 geometry:

process.load("MagneticField.Engine.volumeBasedMagneticField_120812_cfi")
Version 130503 (Default for data only, starting from 743/750pre5)

Full TOSCA model of the entire CMS; reduced memory footprint cf. this presentation. 804 volumes per sector; volumes outside any detector are replicated starting from sector 1 (volumes extending from R~7.6 m to to R=9 m, ie 1011, 1028-1029,1034-1035,1042-1043,1050-1051,1058-1059,1064-1065,1072-1073,1078-1079; volumes beyond |Z|>10.86 ie 1084-1129,1136-1137; and the corresponding +1000). Volumes 1134-1135 (for the volume outside CASTOR) are replicated from sector 4 instead than 1, to avoid aliasing effect due to the plates in the cylinder gap.

Map sets and configuration in db

A map set consists of six maps for the five nominal operation currents, corresponding to 2, 3, 3.5, 3.8 and 4T, plus a zero-field map. The configuration for each map is stored in the condition DB in a MagneticFieldConfig object i, which includes:

  • a MF geometry version (int);
  • a data table version (string);
  • a map of table names to be associated to volumes;
  • optional scaling factors for specific tables;
  • name and parameters of a slave parametrized field for the tracker region.

In each GT, configuration for all six maps of the set must be present. They are stored as separate MagneticFieldConfig objects with the pre-defined labels "0T", "2T", "3T", "3.5T", "3.8T", "4T". The appropriate one is retrieved by label based on the coil current stored in the RunInfoRecord,

The geometry version of the required configuration is used to retrieve the correct geometry from the db by label. Each GT thus contains all required geometries, attached to the record MFGeometryFileRcd, with labels like "120812", "130505", etc.

To find what maps are included in a given GT, search fror the GT in conddb, eg for data and Run1/Run2 MC:

Search for "MagFieldConfigRcd" in the Tags List table. You should get six entries with the labels "0T","2T", "3T", "3.5T", "3.8T", "4T" for the six maps of the set.

Note that for data, the map should have IOVs for Run1/Run2 as needed. For MC, there should be only 1 valid IOV, depending on whether the GT is for Run1 or Run2.

Available geometries can instead be found searching for "MagFieldConfigRcd".

The following table summarizes the currently used map sets.

Run 1 default ("090322_2pi_scaled") - data and MC
Current (A) B(T) geometryVersion Table set
(version)
Parametrization type
(paramLabel)
paramData scaling factors
0-4779 0 90322 grid_1103l_071212_4t Uniform 0 -
4779-11987 2 90322 grid_1103l_071212_2t OAE_1103l_071212 2 -
11987-15617 3 90322 grid_1103l_071212_3t OAE_1103l_071212 3 -
15617-17543 3.5 90322 grid_1103l_071212_3_5t OAE_1103l_071212 3.5 -
17543-18765 3.8 90322 grid_1103l_090322_3_8t OAE_1103l_071212 3.8 ScalingFactors_090322_2pi_090520_cfi
>18765 4 90322 grid_1103l_071212_4t OAE_1103l_071212 4 -

Run 2, MC
Current (A) B(T) geometryVersion Table set
(version)
Parametrization type
(paramLabel)
paramData scaling factors
0-4779 0 90322 grid_1103l_071212_4t Uniform 0 -
4779-11987 2 90322 grid_1103l_071212_2t OAE_1103l_071212 2 -
11987-15617 3 90322 grid_1103l_071212_3t OAE_1103l_071212 3 -
15617-17543 3.5 130503 grid_130503_3_8t_v9_large OAE_1103l_071212 3.5 -
17543-18765 3.8 120812 grid_120812_3_8t_v7_large OAE_1103l_071212 3.8 -
>18765 4 90322 grid_1103l_071212_4t OAE_1103l_071212 4 -

Run 2, data
Current (A) B(T) geometryVersion Table set
(version)
Parametrization type
(paramLabel)
paramData scaling factors
0-4779 0 90322 grid_1103l_071212_4t Uniform 0 -
4779-11987 2 90322 grid_1103l_071212_2t OAE_1103l_071212 2 -
11987-15617 3 90322 grid_1103l_071212_3t OAE_1103l_071212 3 -
15617-17543 3.5 130503 grid_130503_3_5t_v9_large OAE_1103l_071212 3.5 -
17543-18765 3.8 130503 grid_130503_3_8t_v9_large OAE_1103l_071212 3.8 -
>18765 4 90322 grid_1103l_071212_4t OAE_1103l_071212 4 -

Future map set, based on 160812, configuration labels per IOV:

Current (A) B(T) Run I (from:1)
Run II (from:220641)
Note
0-4779 0 MFConfig_71212_0T requires grid_1103l_071212_4t
4779-11987 2 MFConfig_71212_2T Run I version, but we don't really care about 2T
11987-15617 3 MFConfig_160812_Run2_3T A specific Run I version is not needed
15617-17543 3.5 MFConfig_160812_Run2_3_5T ditto
17543-18765 3.8

MC tag:
MFConfig_90322_2pi_scaled_3_8T

OR
MFConfig_160812_Run1_3_8T

MC tag:
MFConfig_160812_3_8T

Must decide whether to use the new model for Run I as well, or not.

The data tag with 2 IOVs will be either MFConfig_RI90322_RII160812_3_8T
or MFConfig_RI_RII_160812_3_8T depending on what is chosen fro Run I.

>18765 4 MFConfig_71212_4T Not really needed?
With this map set, the following grids are no longer used:

grid_1103l_071212_3t
grid_1103l_071212_3_5t
grid_1103l_071212_3_8t
grid_120812_3_8t_v7_large
grid_120812_3_8t_v7_small
grid_130503_3_8t_v9_large
grid_130503_3_8t_v9_small
grid_1103l_090322_3_8t (ONLY IF NOT KEPT FOR RUN I)

Setting these new tables as default in the glibalTag: cf TagsForMagneticField160812

Magnetic field map geometries

XML files for the different geometry versions are located under MagneticField /GeomBuilder/data:

  • version 90322: MagneticFieldVolumes_1103l.xml. This geometry is suitable for all older maps in different releases.
  • version 120812: MagneticFieldVolumes_1_v7_large.xml, MagneticFieldVolumes_2_v7_large.xml
  • version 130503: MagneticFieldVolumes_1_v9_large.xml, MagneticFieldVolumes_2_v9_large.xml
  • version 160812: MagneticFieldVolumes_160812_1.xml, MagneticFieldVolumes_160812_2.xml

Other options

How to load two field maps in the same job

Any number of different maps can be loaded in the same job and accessed in parallel, for e.g. comparison. One of them will be the "standard" one which is visible by all modules; it will be the one with a null label., the others are available in the job using their specific non-null label. In the following example we specify a second map with label '090216_3_8t' in addition to the standard one (default 3.8T map).

# The standard one
process.load("Configuration.StandardSequences.MagneticField_38T_cff")



# The new one with label "090216_3_8t"
# Note I use a different process name than what us used in the default.
process.VolumeBasedMagneticFieldESProducerNew = 
cms.ESProducer("VolumeBasedMagneticFieldESProducer",
   timerOn = cms.untracked.bool(False),
   useParametrizedTrackerField = cms.bool(False),
   label = cms.untracked.string('090216_3_8t'),
   version = cms.string('grid_1103l_090216_3_8t'),
   debugBuilder = cms.untracked.bool(False),
   cacheLastVolume = cms.untracked.bool(True),
   scalingVolumes = cms.vint32(),
   scalingFactors = cms.vdouble()
)

In the code, you will then be able to get both fields maps:

ESHandle<CMS.MagneticField> magfield;
setup.get<IdealMagneticFieldRecord>().get(magfield);

ESHandle<CMS.MagneticField> newMagfield;
setup.get<IdealMagneticFieldRecord>().get("090216_3_8t",newMagfield);

How to apply per-volume rescaling factors

A scaling factor for |B| can be specified independently for each of the 312 volumes in each sector.

The scaling is provided with two matching vectors. The first vector lists the volumes, encoded as (100*volume number + sector). Entries with sector=0 will match all sectors for which a specific sector entry is not set.
The second vector specifies the corresponding scaling factors.

The volume numbers for the iron yoke plates can be obtained from this schema.

In version, 1103l_090322_2pi_scaled, the default scaling factors are implemented in MagneticField/Engine/python/ScalingFactors_090322_2pi_090520_cfi.py. If you need to test modified scaling factor, please use this file as a starting point.

Annotated lists of volumes (actual factors are NOT significant):

Development of new maps

The steps described below must be followed, in their order, to prepare, validate and deploy a new set of maps, assuming the starting point is a new xml geometry and a corresponding set of TOSCA grid tables in txt format. For new tables for an already existing geometry, the first part can be skipped.

Preparing a new geometry

Add the new xml file(s) in Magfield/GeomBuilder/data. Test geometry building by preparing a config file like MagneticField/Engine/python/volumeBasedMagneticField_130503_cfi.py, specifying:

  • the new xml files in magfield.geomXMLFiles
  • an empty VolumeBasedMagneticFieldESProducer.gridFiles list

Set this cfi in MagneticField/Engine/test/testMagneticField_cfg.py instead of the default one, and set process.VolumeBasedMagneticFieldESProducer.debugBuilder = True . Run this and search for errors (“ERROR”). Some WARNINGs are normal if mismatches are small (normally <0.04 cm; can get up to 0.2 cm) .

The debugBuilder flag activates the test MagGeoBuilderFromDDD::testInside, which tests that the center of a volume is inside it and outside all other volumes. There must be no error.

Also run with process.testVolumeGeometry = cms.EDAnalyzer("testMagGeometryAnalyzer"). This calls MagGeometryExerciser::testFindVolume, which checks random points: the corresponding volume should be found, and the points must be inside() one and only one volume. There must be no error. Note: internally this uses uses MagGeometry::findVolume, that has a retry mechanism with increased tolerance to avoid tiny gaps.

Table conversion

To convert txt tables (extracted from TOSCA), use MagneticField/Interpolation/test/BinaryTablesGeneration/convertTables.py. Check for ERROR s in the output. Note that files are provided both with cartesian (xyz) and cylindrical coordinates (rpz). They are equivalent, but it is better to use rpz for all volume types except than for Box and Trapezoid volumes.

Once completion is completed with no error, copy the new set of maps into MagneticField/Interpolation/data following the existing directory structure. Edit the cfi file (eg. MagneticField/Engine/python/volumeBasedMagneticField_130503_cfi.py) and set VolumeBasedMagneticFieldESProducer.version to the name of the newly created directory in MagneticField/Interpolation/data, and specify the matching between grid files and volumes, eg:

    gridFiles = cms.VPSet(
           cms.PSet(
               volumes   = cms.string('1001-1464,2001-2464'),
               sectors   = cms.string('0') ,
               master    = cms.int32(=),
               path      = cms.string('s[s]/grid.[v].bin'),
           ),

With this syntax, all volumes numbered with range 1001-1464 and 2001-2464 for all sectors (0=all) are associated to the file s[sector]/grid.[volumeNumber].bin. The master specification can be used to use the table for a given sector for other sector as well by rotation, in order to save memory; 0 means that each sector will use its own specific table.

Run testMagneticField_cfg.py with debugBuilder = True as explained above. It should now try to build interppolators for all volumes, reading the data tables. In addition a test is done, that each grid point should be inside() the corresponding volume. Search for ERRORs. They can be either due to errors in the tables (e.g. irregular grid spacing), or in the geometry (volume boundaries not matching the actual grid). To debug possible problems with the grid tables, MagneticField/Interpolation/test/BinaryTablesGeneration/GridFileReader.cpp can be used to dump table grid and values. To dump the memory representation of the yable, opem MagneticField/Engine/test/testMagneticField.cc and uncomment/edit the block containing vol->provider() according to specific needs.

The final check is to run a regression of the field map vs the data tables, using Interpolation/test/BinaryTablesGeneration/validateField_TOSCATables.py. Set the cfi.py for the new geometry; the list of tables is taken from a file tableList.txt, that is written during conversion by convertTables.py. Numerical discrepancies can be present in a handful of cases up to ~0.00065 T, especially in rotated tables. This could be be due to the appearance of spurious ~0 step parameters due to rounding in rotations. This level of accuracy is acceptable. Further study/development is needed to understand how to avoid these roundings.

Publishing new tables for release

Once data tables are ready, they should be published submitting a PR to https://github.com/cms-data/MagneticField-Interpolation.

Conversion of the geometry to db

Edit and run MagneticField/GeomBuilder/test/mfxmlwriter,py and mfwriter.py. The first writes the geometry in a single, self-contained xml ; the second converts it into a .db file and creates the corresponding metadata faile for upload to the condition db.

To load the geometry in the database:

uploadConditions.py <mfGeometryXXXXX.db>

Preparing a db configuration

See above for an explanation of how map configuration is stored in the condition DB.

A map configuration can be written into a .db file with /CondFormats/MFObjects/test/writeMagFieldConfigDB.py. The the corresponding metadata faile for upload to the condition db is also created.

In general, before uploading to the condition db this .db file needs to be combined with others to get a single file with all IOVs for Run I and II maps.

Debugging tools

Regression Testing

Regression testing is available to validate changes that are not expected to change the field numerically in any point. It is done comparing the field returned by the map with a reference file created for each map. To run the regression testing, use the head version of:

CMS.MagneticField/Engine/test/regression.py

This file loads the default map version and points to the corresponding reference file (on AFS).

Failures are reported in the standard output.

Visualization of the field map in CMSSW

The magnetic field and MF geometry can be visualized in Eve. The field visualization is implemented in Fireworks/Geometry/plugins/DisplayGeom.cc. An example .py configuration is shown at: Fireworks/Geometry/plugins/cmsRun_displayProdMFGeom_cfg.py.

Some notes:

  • In the Eve window, click "Camera Home" in the bottom left panel (to return to this panel later, double-click on Viewers and select Viewer 1).
  • To select/deselect individual volumes, double-click on cms:World and again on each new level of the hierarchy.
  • To choose a pre-defined perspective or orthographic view, hover with the mouse on the bar below the "Viewer 1" tab. A new menu bar appears, with predefined views under Camera. Toggle auto-hide un-ticking "File->Hide Menus" in this bar.
  • Help on visualization commands (zooming, panning, rotation etc can be found under Help in the menu bar mentioned above.
  • Exit with .q from the root shell (closing the graphical window causes a crash).

Review status

Reviewer/Editor and Date (copy from screen) Comments
KatiLassilaPerini - 23 Jan 2007 created template page
NicolaAmapane - 01 Jul 2008 first edit

Responsible: Nicola Amapane
Last reviewed by: Never reviewed

CMS.MagneticField/GeomBuilder
Topic attachments
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Texttxt EndcapScalingFactors.py.txt r1 manage 0.4 K 2009-05-14 - 16:10 NicolaAmapane Scaling factors for endcap yoke volumes (example syntax)
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