Magnetic Field Mapping of the CLAS12 Torus—A Comparative Study Between the Engineering Model and Measurements at JLab

This paper provides an overview of the magnetic field measurement and subsequent electromagnetic re-modeling of the CLAS12 torus during the commissioning of the magnet in the fall of 2016. The CLAS12 detector in Hall B is part of the 12 GeV Accelerator Upgrade project at Jefferson Lab. The torus magnet allows precise determination of particle momenta within a cone ~ (5°-40°) in the forward direction. The ability to do this requires that we know the ∫B·dl of the torus to within an accuracy of 0.5% or better. To achieve this, an accurate model of the field along the particle path is required. The TOSCA code is used to generate a full three-dimensional (3-D) simulation of the magnetic envelope of the magnet as designed. Experimentally, the actual magnetic field within the magnet was surveyed to confirm the model design and to measure the deviation from the ideal case. The magnetic field deviations are attributed to manufacturing variability and assembly tolerances. A final model was created with allowances for these deviations guided by the survey data to create a more precise field integral model, which greatly improves the momentum resolution capability, allowing it to deliver the required specifications.