Measuring the Magnetic Flux Density with Flux Loops and Hall Probes in the CMS Magnet Flux Return Yoke

TL;DR

This paper details the measurement of magnetic flux density in the CMS detector's steel yoke using flux loops and Hall probes, supported by 3D modeling, to ensure accurate magnetic field mapping for particle detection.

Contribution

It introduces a comprehensive measurement system combining flux loops, Hall sensors, and 3D modeling to accurately map the magnetic field in the CMS magnet yoke.

Findings

Flux loop measurements during magnet ramps validate the magnetic field model.

The system successfully reconstructs flux density at operational current.

Measurements support precise calibration of the CMS magnetic environment.

Abstract

The Compact Muon Solenoid (CMS) 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-length free bore, enclosed inside a 10,000-ton return yoke made of construction steel. The flux return yoke consists of five dodecagonal three-layered barrel wheels and four end-cap disks at each end comprised of steel blocks up to 620 mm thick, which serve as the absorber plates of the muon detection system. To measure the field in and around the steel, a system of 22 flux loops and 82 3-D Hall sensors is installed on the return yoke blocks. A TOSCA 3-D model of the CMS magnet is developed to describe the magnetic field everywhere outside the tracking volume that was measured with the field-mapping machine. The voltages induced in the flux loops by the magnetic flux changing during the CMS magnet standard ramps down are measured with six 16-bit DAQ modules. The off-line integration of the induced voltages reconstructs the magnetic flux density in the yoke steel blocks at the operational magnet current of 18.164 kA. The results of the flux loop measurements during three magnet ramps down are presented and discussed.