1. 14 August 2008 1 Magnetic Field in the ATLAS Muon Spectrometer Masahiro Morii for the ATLAS Group Harvard University Laboratory for Particle Physics and Cosmology

2. 14 August 2008ATLAS Toroids2 ATLAS 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 m 3 BarrelEndcap Outer diameter20.1 m10.7 m Length25.3 m5.0 m # turns / coil120116 Current20.5 kA Field in the bore0.2–2.5 T0.2–3.5 T

3. 14 August 2008ATLAS Toroids3 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

4. 14 August 2008ATLAS Toroids4 Field Reconstruction 1730 3-dim. Hall probes are installed on the MDT chambers Sensor accuracy 0.2–1 mT 1. Measure the field 2. Subtract the perturbation 3. Fit for the coil position and shape 4. Generate the field map 0.2% field accuracy expected for the barrel toroid Harvard (Morii) collaborates with Saclay and Dubna on field fit and reconstruction

5. 14 August 2008ATLAS Toroids5 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

6. 14 August 2008ATLAS Toroids6 2007 Endcap Toroid Tests The 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

7. 14 August 2008ATLAS Toroids7 Summary Understanding 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