Abstract
This review article describes the performance of the magnetic field measuring and monitoring systems for the Compact Muon Solenoid (CMS) detector. To cross-check the magnetic flux distribution obtained with the CMS magnet model, four systems for measuring the magnetic flux density in the detector volume were used. The magnetic induction inside the 6 m diameter superconducting solenoid was measured and is currently monitored by four nuclear magnetic resonance (NMR) probes installed using special tubes at a radius of 2.9148 m outside the barrel hadron calorimeter at ±0.006 m from the coil median XY-plane. Two more NRM probes were installed at the faces of the tracking system at Z-coordinates of −2.835 and +2.831 m and a radius of 0.651 m from the solenoid axis. The field inside the superconducting solenoid was precisely measured in 2006 in a cylindrical volume of 3.448 m in diameter and 7 m in length using ten three-dimensional (3D) B-sensors based on the Hall effect (Hall probes). These B-sensors were installed on each of the two propeller arms of an automated field-mapping machine. In addition to these measurement systems, a system for monitoring the magnetic field during the CMS detector operation has been developed. Inside the solenoid in the horizontal plane, four 3D B-sensors were installed at the faces of the tracking detector at distances X = ±0.959 m and Z-coordinates of −2.899 and +2.895 m. Twelve 3D B-sensors were installed on the surfaces of the flux-return yoke nose disks. Seventy 3D B-sensors were installed in the air gaps of the CMS magnet yoke in 11 XY-planes of the azimuthal sector at 270°. A specially developed flux loop technique was used for the most complex measurements of the magnetic flux density inside the steel blocks of the CMS magnet yoke. The flux loops are installed in 22 sections of the flux-return yoke blocks in grooves of 30 mm wide and 12–13 mm deep and consist of 7–10 turns of 45 wire flat ribbon cable. The areas enclosed by these coils varied from 0.3 to 1.59 m2 in the blocks of the barrel wheels and from 0.5 to 1.12 m2 in the blocks of the yoke endcap disks. The development of these systems and the results of the magnetic flux density measurements across the CMS magnet are presented and discussed in this review article.
Highlights
The main difficulty in large magnetic systems having an extensive flux-return yoke is to characterize the magnetic flux distribution in the steel blocks of the yoke
The Compact Muon Solenoid (CMS) magnetic field is provided by a wide-aperture superconducting thin solenoid [4] with a diameter of 6 m and a length of 12.5 m, where a central magnetic flux density of 3.81 T is created by an operational direct current of 18.164 kA [5,6,7]
There are three types of sensors used in the system: one probe (B) has an active volume made of a solid material containing a large amount of hydrogen to measure the magnetic induction in the range from 0.7 to 2.1 T; one probe (F) has a sealed glass tube containing heavy water (D2O) to measure the magnetic field in the range from 1.5 to 3.4 T; four probes (A, C, D, E) have a similar nuclear magnetic resonance (NMR) sample filled with D2O to measure the magnetic flux density in the range from 3 to 6.8 T
Summary
The main difficulty in large magnetic systems having an extensive flux-return yoke is to characterize the magnetic flux distribution in the steel blocks of the yoke. Continuous measurements of the magnetic flux density in the return yoke are not possible; in common practice, software modelling of the magnetic system using special three-dimensional (3D) computer programs is applied [1]. The steel yoke of the magnet is used as magnetized layers that wrap the muons, which allows them to be identified and their momenta to be measured in a muon spectrometer. The CMS magnetic field is provided by a wide-aperture superconducting thin solenoid [4] with a diameter of 6 m and a length of 12.5 m, where a central magnetic flux density of 3.81 T is created by an operational direct current of 18.164 kA [5,6,7]
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