ATLAS Inner Detector Magnetic Field

1
ATLAS Inner Detector Magnetic Field
Steve Snow
I am responsible for providing the magnetic field
map for the ATLAS Inner Detector.

• 6m long x 2m diameter cylindrical volume
• 2 Tesla (20000 Gauss) at Z0, dropping to 0.8 T
  at Z3m
• Defines momentum scale of all ID tracks.
• Require track sagitta error due to field
  uncertainty lt 0.05

End of coil at 2.65 m
2
2006the key year of the mapping project
Years of thinking and preparation I started back
in 1997 Months of building and installing the
mapping machine CERN team
Martin Aleksa, Felix Bergsma, Pierre-Ange
Giudici, Antoine Kehrli, Marcello Losasso, Xavier
Pons, Heidi Sandaker, started mid 2005 5 days of
data taking, 2nd to 7th August. Maps at 5000,
7000, 7730 and 8000 Amps. Months of data analysis
(John Hart, Paul Miyagawa, SS) now nearing
completion.
3
The Mapping Machine
Cards each hold 3 orthogonal sensors
Pneumatic motors with optical encoders. Move and
measure in Z and f.
4 fixed NMR probes at Z0
4 arms in windmill. Each arm equipped with 12
Hall cards
4
Typical data from 5kA map
Typical Bz, Br and Bf values from probes 3,6,9,12
(increasing radius) on arm AE. Bz peaks at 13000
G. Br peaks at 4500 G, Bf 0 except for return
conductor at 90 deg.
5
Surveys
A lot of them ! We need to know precisely where
the sensors are Typical accuracy 0.2 mm
Survey of mapping machine in Building 164 .
Radial positions of Hall cards Z separation
between arms Z thickness of arms Survey and
re-survey in situ before mapping.
Survey again in situ after mapping. Position
and rotation axis of each arm Position of
encoder zero Position of NMR probes All these
in the Inner Warm Vessel co-ordinate
system Survey of rail slope in IWV Survey the
offsets of 3 Hall sensors from a common point in
the card Final map will be in IWV system, as
will SCT TRT final survey
6
Calibration
Hall sensors
Low field calibration , up to 1.4 T, good to 2 G
High field calibration, up to 2.4 T, good to 10 G
NMR probes
Need no calibration (was done by whoever measured
Gp 42.57608 MHz/T ) Compare proton resonance
frequency with reference oscillator, accurate to
0.1 G.
7
Corrections derived from the data
Corrections to Hall probes normalisation and
angular alignment can be derived from the data
assuming Field obeys Maxwell Approximate
phi symmetry No high spatial frequencies
Normalisation corrections are small, as expected
from probe calibration Alignment corrections have
spread of 2 mrad, as expected from limited Hall
card mounting accuracy.
Corrections for the field due to magnetic
components of the machine.
Perturbations visible in the data, eg this phi
scan ? Correlated with identifiable
componenets. We subtract a dipole field, located
at component position, with dipole strength
chosen to make residuals look smooth.
Phi encoder
8
Fitting function
Conductor geometry model All based on surveys of
solenoid as built, except weld thickness chosen
to fit field map data
95 of the field is directly due to the solenoid
current. Use conductor geometry model and
integrate Biot-Savart law. 7 free parameters
scale factor and aspect ratio
(Length/Diameter) of conductor model Position
and orientation of field due to conductor 5 of
the field is due to magnetised iron (TileCal,
girders , shielding disks etc). For the moment
use 4 free parameters Fourier-Bessel series
9
Results
R.M.S. residuals of fit to 5000A data are Br 3.8,
Bz 3.9, Bf 3.2 Gauss R.M.S. residuals of
fit to 7730A data are Br 7.6, Bz 7.1, Bf 5.7
Gauss We expect to exceed the required accuracy
of 10 Gauss The fit tells us that solenoid axis
is 2 mm below IWV axis
the barrel tracker has been placed 2 mm low to
match.