CMS magnet
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| CMS magnet | |
|---|---|
| Name | CMS magnet |
| Location | CERN |
| Type | Solenoid |
| Field strength | 3.8 tesla |
| Operational since | 2008 |
| Operator | CERN |
CMS magnet
The CMS magnet is the large superconducting solenoid that provides the central magnetic field for the Compact Muon Solenoid detector at CERN's Large Hadron Collider. It produces a uniform axial field used to bend charged-particle trajectories for momentum measurement in experiments such as those that led to the discovery of the Higgs boson and precision studies involving the Top quark, W boson, and Z boson. The magnet integrates with detector systems developed by collaborations including CMS Collaboration and interfaces with infrastructures like the Large Hadron Collider tunnel and the Worldwide LHC Computing Grid.
Introduction
The CMS magnet is a key subsystem of the Compact Muon Solenoid experiment installed in the CERN underground hall adjacent to the Large Hadron Collider ring and near the CMS detector. Conceived during the design phase in the 1990s alongside competing proposals such as the ATLAS toroid system, it was built to enable accurate momentum reconstruction for charged particles produced in proton–proton collisions at multi-TeV center-of-mass energy. Project stakeholders included national laboratories and institutions such as Fermilab, DESY, INFN, Brookhaven National Laboratory, and industrial partners in Germany, Italy, and Switzerland.
Design and Specifications
The CMS solenoid is a large-bore superconducting coil designed to generate a nominal central field of 3.8 tesla within a free bore of about 6 meters and a length of roughly 12.5 meters. Its design parameters derive from performance requirements defined by collaborations including CMS Collaboration and engineering standards from organizations such as European Organization for Nuclear Research working with contractors like Alstom and Areva. The coil windings are constructed from stabilized Niobium–Titanium cable embedded in aluminum, assembled into multilayer modules, and cooled by liquid helium circuits similar to systems used at LEP and contemporary accelerator magnets. The solenoid is enclosed in a massive return yoke that serves both magnetic and structural roles and integrates with muon detectors like Drift tube chambers, Cathode strip chambers, and Resistive plate chambers.
Construction and Installation
Manufacturing of the coil and cryostat involved industrial partners and research centers such as CERN, Fermilab, CEA Saclay, and INFN Milano, with components fabricated across Europe and North America. Assembly phases replicated techniques proven in projects like the Tevatron superconducting magnets and relied on metrology tools from institutions like Max Planck Society laboratories. The magnet was transported to the CERN site and lowered into the CMS experimental cavern using specialized gantries, cranes, and systems comparable to those used for LEP detector installation. Integration required coordination with the CMS Collaboration detector integration team, cryogenics groups at CERN Directorate, and safety oversight by agencies including national regulatory bodies.
Operational Performance
In operation, the CMS magnet provides a stable axial field of approximately 3.8 tesla with homogeneity tailored to tracker systems including the CMS Tracker based on silicon sensors and pixel detectors. Field mapping campaigns employed equipment and analysis methods similar to those used in ATLAS and in precision experiments at DESY and KEK, producing field maps used in track reconstruction by collaborations using software frameworks like GEANT4, ROOT, and CMSSW. The magnet has supported key measurements by the CMS Collaboration including Higgs boson property determinations, searches for physics beyond the Standard Model, and studies of heavy-flavor hadrons from CERN collisions.
Safety and Quench Protection
Safety systems for the CMS solenoid include active quench protection, energy extraction circuits, instrumentation from vendors linked to projects at Brookhaven National Laboratory and Fermilab, and procedures coordinated with CERN safety authorities. The magnet stores large magnetic energy similar to other large accelerator magnets such as those at the Large Hadron Collider and the Relativistic Heavy Ion Collider, requiring robust interlock systems, cryogenic pressure relief, and emergency protocols shaped by incidents in accelerator history and standards from organizations like the International Electrotechnical Commission. Quench detection and mitigation use fast-acting dump resistors and superconducting busbar monitors to route stored energy safely to external circuits.
Scientific Role and Applications
The CMS magnet enables precise momentum measurement in charged-particle tracking, critical for analyses by the CMS Collaboration including Higgs boson discovery publications and precision electroweak measurements involving the W boson and Z boson. Its field facilitates muon bending for identification and alignment with muon systems that connect to experiments at the LHCb and ATLAS collaborations in comparative studies. Beyond primary particle physics goals, magnet technology and operational experience inform magnet design in projects such as ITER, future collider proposals like the Future Circular Collider, and magnetic resonance developments in applied research at institutions like MIT and Stanford University.
Maintenance, Upgrades, and Decommissioning
Maintenance of the solenoid involves periodic cryogenic campaigns managed by CERN cryogenics teams, component replacement coordinated with groups such as Fermilab technicians and detector subsystem teams within the CMS Collaboration. Upgrade pathways have been evaluated in the context of High-Luminosity Large Hadron Collider upgrade plans, aligning with tracker and muon system replacements coordinated with funding agencies like European Commission programs and national science foundations. Decommissioning procedures follow international best practices and intergovernmental agreements overseen by CERN governance and partner institutions to manage radioactive activation, material recycling, and legacy data preservation in archives used by collaborations including CMS Collaboration and analysis centers on the Worldwide LHC Computing Grid.