Measurement of the CMS Magnetic Field

TL;DR

This paper details the precise measurement of the CMS detector's magnetic field using a specialized fieldmapper with 3-D sensors, providing critical data for accurate modeling and operation of the superconducting magnet system.

Contribution

It introduces a high-precision fieldmapper and comprehensive measurement methodology for the CMS magnetic field, including steel yoke flux measurements and comparison with 3D modeling.

Findings

Measured magnetic flux density at 4 T with 0.05% sensor calibration.

Achieved measurement accuracy of steel flux density within a few percent.

Validated the TOSCA 3D model against empirical measurements.

Abstract

The measurement of the magnetic field in the tracking volume inside the superconducting coil of the Compact Muon Solenoid (CMS) detector under construction at CERN is done with a fieldmapper designed and produced at Fermilab. The fieldmapper uses 10 3-D B-sensors (Hall probes) developed at NIKHEF and calibrated at CERN to precision 0.05% for a nominal 4 T field. The precise fieldmapper measurements are done in 33840 points inside a cylinder of 1.724 m radius and 7 m long at central fields of 2, 3, 3.5, 3.8, and 4 T. Three components of the magnetic flux density at the CMS coil maximum excitation and the remanent fields on the steel-air interface after discharge of the coil are measured in check-points with 95 3-D B-sensors located near the magnetic flux return yoke elements. Voltages induced in 22 flux-loops made of 405-turn installed on selected segments of the yoke are sampled online during the entire fast discharge (190 s time-constant) of the CMS coil and integrated offline to provide a measurement of the initial magnetic flux density in steel at the maximum field to an accuracy of a few percent. The results of the measurements made at 4 T are reported and compared with a three-dimensional model of the CMS magnet system calculated with TOSCA.