Developing the technique of measurements of magnetic field in the CMS steel yoke elements with flux-loops and Hall probes

CMS (Compact Muon Solenoid) is a general-purpose detector designed to run at the highest luminosity at the CERN Large Hadron Collider (LHC) . Its distinctive features include a 4 T superconducting solenoid with 6 m diameter by 12.5 m long free bore, enclosed inside a 10000- ton return yoke. Accurate characterization of the magnetic field everywhere in the CMS detector is required. To measure the field in and around the steel a set of flux-loops and Hall probe sensors will be installed on several of the steel pieces. Fast discharges of the solenoid during system commissioning tests will be used to induce voltages in the flux-loops that can be integrated to measure the flux in the steel at full excitation of the solenoid. The Hall probes will give supplementary information on axial component of magnetic field and will permit to estimate the remanent field in steel after the fast discharge. An experimental R&D program has been done, using the test flux-loop, two Hall probes, and small steel plugs between the pole tips of a laboratory dipole electromagnet with a controlled power supply, to measure the behavior of the test flux-loop and Hall probes during excitation of steel from full saturation to zero field. The results of the measurements are presented and discussed. (6 refs) .