Chilling a giant

The CMS solenoid has been cooled down to its operating temperature of -269°C after a three-week cooling campaign.

The CMS solenoid is cold - and that's an understatement. At -269° C, the gigantic coil is chilling out at its operating temperature and is superconducting, an important milestone for the AT/ECR cryogenics group, the CMS coil collaboration (which includes CERN, ETH Zürich, Fermilab, INFN Genova, ITEP Moscow and Saclay) and the CMS collaboration at large.

The coil consists of 14.5 tonnes of superconducting cables, which are embedded into 74 tonnes of pure aluminum and reinforced with 126 tonnes of high mechanical-strength aluminum alloy and 9 tonnes of insulation. Added together, that's a 223.5-tonne magnet. Cooling a component of such massive proportions is anything but easy.

'Superconductivity so far, despite it's great potential, has not yet made a mark in the commercial world because there is the complication of cooling down to liquid helium temperature, which is sophisticated, complicated and expensive,'said CMS Magnet and Integration Group Leader Domenico Campi. 'Nevertheless, for such a huge experiment like CMS, we cannot avoid the use of superconductors and thus of cryogenics. It is simply inconceivable to make a magnet of such power with a resistive conductor within acceptable parameters.'

After completion, the coil was suspended from the vacuum tank using large titanium tie-bars, under the supervision of Bruno Levesy from Saclay. The coil's vacuum vessel was then welded closed by the firm DWE under the supervision of Hubert Gerwig, Andrea Gaddi and collaborators. The cool-down process began in early February. Cryogenics engineers had to carefully cool the magnet evenly and slowly to protect it from deformation and damage.

The cool-down is achieved by circulating helium gas through a series of pipes surrounding the coil. In the first phase, the helium is steadily refrigerated to ever lower temperatures by the means of -196° C liquid nitrogen. Then, to achieve the lowest temperatures, the helium is injected into small turbines, cooled to liquefaction temperature and sent to fill up the coil circuits.

After the three-week cooling campaign, the superconducting magnet is now resting at its operating temperature.

The service and operation of the helium refrigeration plant is led by a team of two: project leader Goran Perinic and technical engineer Thierry Dupont. They are supported by a team of operators and their colleagues from the technical support sections led by Marco Pezzetti and Olivier Pirotte of the AT-ECR cryogenics group.

The cool-down system is fully automated, but Perinic and Dupont have spent the last three weeks close to the plant, in the control room or in front of a computer at home, closely watching the slow refrigeration process and ready to intervene at the smallest sign of trouble. They've both worked on the project for six years.

'It is nice to add a tiny milestone to a technology whose roots are in Geneva,' Perinic said. 'It is just more than 128 years ago that Raoul Pictet managed to refrigerate a gas to a cryogenic temperature and to liquefy it for the first time at the University of Geneva. I doubt that he had imagined an application like this.'

For the nominal current of 20,000 Amps, the solenoid is designed to reach a magnetic field of 4 Tesla, almost 100 000 times the Earth's magnetic field. It will have enough stored energy (2.7 GigaJoule) to melt 18 tonnes of gold. The superconducting coil will be kept at operating temperature for magnetic testing starting in May. It will be lowered into the CMS cavern, 90 metres underground, before the end of the year.