14 August 2008 1

Magnetic Field in the ATLAS Muon Spectrometer

Masahiro Moriifor

the ATLAS Group

Harvard UniversityLaboratory for Particle Physics and

Cosmology

14 August 2008 ATLAS Toroids 2

ATLAS ToroidsATLAS Muon Spectrometer uses a barrel and two endcap air-core toroids, each with 8 superconducting coils

Large (bending power x lever arm) p/p = 10% @ 1 TeV

Complex and inhomogeneous magnetic field Need precise (< 0.5%) field map over 20 x 20 x 25 m3

Barrel EndcapOuter

diameter20.1 m 10.7 m

Length 25.3 m 5.0 m# turns /

coil120 116

Current 20.5 kA 20.5 kAField in the

bore0.2–2.5

T0.2–3.5

T

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Field Calculation

We calculate the field from the current in the coils Perturbation due to iron structures is evaluated numerically

Positions and shapes of the coils must be known to a few mm

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14 August 2008 ATLAS Toroids 4

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Field Reconstruction1730 3-dim. Hall probes are installed on the MDT chambers Sensor accuracy 0.2–1 mT 1. Measure the field2. Subtract the perturbation 3. Fit for the coil position and shape4. Generate the field map 0.2% field accuracy expectedfor the barrel toroid

Harvard (Morii) collaborateswith Saclay and Dubna on field fit and reconstruction

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Endcap Field Sensors

Endcap toroids are encased inside one cryostat per side Not enough nearby measurements to constrain coil positions

64 sensors were added on the cryostat surfaces Based on a Harvard study

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2007 Endcap Toroid TestsThe endcap toroids were energized to 10 kA in Nov. 2007 Field measurements were used to test the reconstruction Sensors on the cryostats proved very useful

Mounted directly on large solid bodies Positions well known

Analysis (by Harvard) uncovered a surprise:Coils were closer to the beam line than the design by 12 cm CERN engineer discovered inconsistency in drawings Shifts are (likely) real Larger gap between barrel and endcaps

Nominal field map has been updated to study the impact on muon reconstruction, trigger

Also: Toroid axes are ~1 cm below the beam line Corroborated by mutual inductance data

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SummaryUnderstanding the magnetic field is prerequisite for physics Harvard is contributing to this effort through field reconstruction

Magnetic field measurement and reconstruction progressed rapidly in the past year Basic idea has been demonstrated to work 2007 test data yielded useful information on coil geometry

All toroids were energized to 20.5 kA in June–July 2008 Data are being analyzed to validate the calculation of the field perturbation due to shielding structures

Combined (barrel + endcaps) test data will be used to constrain all 24 coils simultaneously to produce field map for the first run