Figure taken from the ATLAS reference paper 2008 JNST 3 S08003 , p18. Caption: Geometry of magnet windings and tile calorimeter steel. The eight barrel toroid coils, with the end-cap coils interleaved are visible. The solenoid winding lies inside the calorimeter volume. The tile calorimeter is modeled by four layers with different magnetic properties, plus an outside return yoke. For the sake of clarity the forward shielding disk is not displayed.

The best possible summary, given by the project leader Herman ten Kate in October 2018, is in the file attached at the bottom of this page. 

ATLAS features a unique hybrid system of four large superconducting magnets.

After approximately 15 years of design, construction in industry and system integration at CERN, the barrel toroid was ramped up to nominal field in December 2006 [ news item ]

The ATLAS magnet system consists of :

• A solenoid, which provides a 2 T axial magnetic field for the inner detector, while minimizing the radiative thickness in front of the electromagnetic calorimeter. The flux of the solenoid is returned by the steel of the hadronic calorimeter.
• A barrel air toroid, which provides a field of 0.5 T. It consists of 8 coils, which were integrated in their cryostat and tested at CERN over a period of 3 years. The net Lorentz forces of 1400 tonnes per coil and the self weight of the toroid [ 830 tonnes + 400 tonnes for the muon chambers ] are counteracted by the mechanical structure, and the installation is precise within a few millimeters.
• Two Endcap Toroids, which consist each of a single cold mass. They were some of the heaviest objects lowered in the cavern, and can slide along the central rails to facilitate the opening of the detector for access and maintenance.

ATLAS barrel Toroid parameters [ taken from 2011 Fact Sheet & and on the public web site ]:

• 8 coils of 25.3 m length, 20.1 m outer radius   = a quite unique magnetic field volume
• Total weight is 830 tonnes (with 370 tonnes cold mass), with a stored energy of 1.08 GJ
• working point temperature 4.7 K, with 56 km of Ai/Nb Ti/Cu conductor and 100 km of superconducting wire
• Magnetic field on super conductor is 4 Tesla. For comparison an ordinary MRI magnet in a hospital has 1.5 Tesla

==>  A specific subsystem web site provides more links and photos

Detailed magnetic field modeling and novel instrumentation have been developed to allow a high-precision mapping of the magnetic field as well as a reconstruction strategy in the muon spectrometer, where the field is highly non uniform: 

• In the inner detector, the systematic error affecting the momentum measurement of charged tracks is dominated by the relative alignment of detector components.
• In the muon spectrometer, the expected sagitta is about 0.5 mm for a muon with momentum of 1 TeV. To match the required precision, the field measurement accuracy has to be better than 2 to 5 10-3 . The reading of 1840 field sensors is compared with simulation and used for reconstructing the field in space. The effect of the magnetized tile calorimeter and localized perturbations induced by other ferromagnetic structures has to be taken into account in the B-field modeling, as well as each coil fabrication tolerance and deformations under gravitational and magnetic loads.
• The exact location of each magnet coil, and the relative position of the endcap and barrel toroids in particular, are re-fitted after each power cycle or access period.

Stories:

• 1999 to 2007: the story of the construction in ATLAS & CERN news
• 2005 to now: integration & ramping up in the ATLAS public web site Updates

Images:

• 03.03.2002 The first coil casing for the toroid arrived in Building 180
• 30.01.2003 Photos of the ATLAS barrel toroid integration in Building 180
• Other photos with the tag Magnet in CDS collections

How much of it can we see @P1 ?

The 8 iconic barrel toroid elements as well as the endcap toroid are one of the "highlights" of the underground standard view. The magnet services include a vacuum and a cryogenic system, located in the USA15 side cavern and on the P1 surface. The toroids are cooled with a forced flow of helium: two large helium dewars can be seen on the side of the detector, in the main cavern, as well as the transfer lines some of which – obviously – have to be flexible to follow the Endcap toroid movements.

Latest update of this page: 09.11.2017 , Version 0