Detectors: unique superconducting magnets

The enormous toroidal superconducting magnet of ATLAS during its installation. Each of its eight coils, the last of which is being assembled in this photo, is 25 metres long. (Image: ATLAS/CERN)

Even before they were used widely in particle accelerators, superconducting magnets were adopted for the detectors used to analyse collisions. A magnetic field is essential for identifying the particles emerging from collisions: it curves their trajectory allowing physicists to calculate their momentum and to establish whether they have a positive or negative charge. The stronger the field and the larger the volume on which it acts, the higher the resolution of the detector.

As early as the 1960s, physicists saw the potential benefits of using superconducting magnets in their detectors. In the early 1970s, experiments in the United States and at CERN were developing large superconducting magnets capable of generating fields of up to 3.5 Tesla. This development work was all the more daring since the technology was still in its infancy. But contrary to the magnets for accelerators, which need to be produced in their dozens, the magnets in detectors are unique.

One of the trailblazers of these detectors was CERN’s Big European Bubble Chamber (BEBC), which entered service in 1973 and in which the superconducting magnet generated a field of 3.5 Tesla. Its stored energy was almost 800 megajoules, a level of performance that wouldn’t be bettered until the late 1990s.

The superconducting coil of CMS, the biggest superconducting solenoid magnet ever built, being inserted in its cryostat. (Image: Maximilien Brice/CERN)

In the 1980s, significant progress was made on improving the magnets’ performance and making them more “transparent”, so that they didn’t interact with the particles and change their characteristics. Increasingly larger magnets were constructed and the work culminated in the 2000s with the giant superconducting magnets of the landmark CMS and ATLAS experiments at the Large Hadron Collider (LHC). The first of these is a huge solenoid that generates a field of 4 Tesla and is able to store 2.7 gigajoules, enough energy to melt 18 tonnes of gold. The second is an enormous and completely novel toroidal magnet formed of eight superconducting coils, which also generate a magnetic field of 4 Tesla, surrounding a smaller solenoid.

The next generation of superconducting magnets for detectors, which will be even bigger and more powerful, is being developed in the context of preparations for major accelerator projects at CERN and elsewhere.

This text is published on the occasion of the conference EUCAS 2017 on superconductors and their applications​. It is based on the article entitled “Unique magnets”, which appeared in the September issue of the CERN Courier.