The height of cool

The first LHC sector has been cooled to below 2 K (-271°C), a triumph of technology achieved in several stages over the last two months.

The first LHC sector has been cooled to below 2 K (-271°C), a triumph of technology achieved in several stages over the last two months.

Tension mounted in the cryogenics control room as the eagerly awaited feat drew nearer and nearer. On Thursday, 5 April, the first LHC sector was cooled to below 2 K (-271°C), colder than outer space! Sector 7-8 (an eighth of the accelerator) thus becomes the world's largest superconducting installation cooled by superfluid helium.

This remarkable achievement marks the end of more than two months of commissioning work. The cooling method used was first validated on the 107-metre-long LHC test cells (Strings 1 and 2). This is the first time that the principle has been applied on such a large scale. The 3.3-kilometre-long sector is made up of about thirty cells and comprises over 200 dipole magnets and short straight sections (which contain the quadrupole magnets).

The cooling process began at the end of January and was carried out in three stages, with a host of tests and detailed checks between them. During the first stage, the 4700 tonne sector was cooled to 80 K, just above the temperature of liquid nitrogen. Over 1200 tonnes of liquid nitrogen were used in total over the period concerned. At this temperature, the material reaches 90% of its final thermal contraction, representing a 3-millimetre shrinkage per metre of the steel structures. To achieve this, compensation devices (bellows and expansion loops) were fitted in the interconnections between the magnets. The total contraction over the sector as a whole is close to 10 metres!

On 5 March, the teams began work on the second stage, which involved cooling the sector to 4.5 K using gigantic refrigerators. Around 15 tonnes of helium were needed to fill the sector. Each sector has its own refrigerator, and each of the main magnets is filled with liquid helium.

The final stage, which began in mid-March, required a sophisticated pumping system to bring the pressure down and cool the magnets and the 10 tonnes of helium they contain to 1.9 K. To achieve a pressure of 15 millibars, the system uses a combination of hydrodynamic centrifugal compressors operating at low temperature and positive-displacement compressors operating at room temperature. At 1.9 K, helium is superfluid, flowing with virtually no viscosity and allowing greater heat transfer capacity.

The complexity and large number of sub-systems to be commissioned for the first time, together with various interface conditions to be managed, account for the time needed to cool the sector. The control system of one sector has to manage approximately 4000 inputs/ouputs and 500 regulation loops that need to be adjusted. In addition, the teams have carried out extensive checks to make sure that the cooling is done with all the necessary caution. This learning phase, which was long but vital, has also enabled the teams to prepare for cooling the other sectors. 'This achievement vindicates over 15 years of work and I wish to congratulate all the teams who have been involved in it', underlines Laurent Tavian, AT/ACR Group Leader.

Hats off to the various teams who contributed to the success of the project, the AT-ACR Group, all the support groups and the industrial partners involved in the operation and maintenance activities!