Hundreds of fridges to cool the heart of ATLAS

The detectors used in the LHC experiments are packed with electronics that will register thousands of particles produced in the collisions. All this hard work will generate a lot of heat, but there are systems in place to help the electronics to cool down, not shut down.

Members of the DC section team in USA15 of the ATLAS cavern, standing behind four of the compressors used in the cooling system. The inlet and outlet pipes that carry the refrigerant to the experimental hall can be seen on the left. Left to right: M. Battistin, P. Bonneau, C. Houd, P. Feraudet, F. Corbaz, J. Lethinen, P. Guglielmini, M. Ciclet, P. Tropea, S. Berry (M. Pimenta absent).

An unconfirmed member of the DC section :-) in charge of a part of the perfluoropropane distribution network for the ATLAS evaporative cooling system.

The next time your desktop computer crashes from overheating, spare a thought for Pierre Feraudet, a member of the Detector Cooling section (TS/CV/DC). He is in charge of constructing the cooling systems for the detectors in five of the LHC experiments. The DC section comprises a small team of 11 staff. Together, they are responsible for the design and implementation of 27 different installations used to cool the detectors; each one was uniquely designed to serve specific needs.

Reliable cooling systems are essential for the smooth running of the experiments, as Michele Battistin (section leader) explained: 'if you have all the electronics embedded in a narrow place with no cooling, then after a while the heat they produce will burn them out. In your PC, where everything is enclosed in a box, there is a fan that ventilates the air to remove the heat. Inside a detector, it is similar to having the electronics of hundreds of PCs in a very confined space. In certain places, if there is no cooling, the temperature can rise by as much as 5°C per second and burn out the parts.'

The 27 systems differ in operating pressure (over or under atmospheric pressure), working processes (monophase or evaporative), and circulatory fluid (water or perfluorocarbon). The cooling systems for ALICE, ATLAS, CMS and LHCb detectors have been realised and are being installed underground and/or tested. Meanwhile, the system for TOTEM is in its component purchasing phase.

One of the most complex is the ATLAS evaporative cooling system for its inner detector. However, this actually works on the same principle as your household refrigerator - using the evaporation of a liquid to absorb heat in a liquid-vapour compression cycle. Evaporative cooling is very efficient. An example of this is perspiration; as the sweat on your skin evaporates, it absorbs heat thereby cooling the body. In a refrigerator, a circulating refrigerant draws heat from one area (inside the fridge) to another area (outside the fridge). During this process, the refrigerant undergoes a cyclic phase change from a liquid to a gas, then back to a liquid again.

The inner detector of ATLAS is equipped with hundreds of silicon detectors and their electronics. They are grouped in 204 cooling loops. Every loop has its own cooling system - its own 'fridge', which can be individually regulated. The heat exchange pipes are positioned right next to the electronics pads to cool them down. Perfluoropropane is used as the refrigerant because it is dielectric and does not damage electronic components in the event of a leak. Unfortunately, there is a catch, as Michele revealed: 'the disadvantage with this fluid is that it is very expensive - like a very refined wine.' The LHC experiments will need about ten thousands litres of perfluorocarbons in total.

Just as there is a small compressor in the back of your fridge, the ATLAS service cavern (USA15) houses the compressors of all the giant 'ATLAS fridges'. The ATLAS inner detector cooling system needs to set the cooling temperature within a range of + 20ºC to - 25ºC, with a precision of ± 0.3ºC. The technical challenge of controlling all 204 cooling units separately is the domain of Stephane Berry, a member of the DC section. In the loud hum of the compressor room, you may find him working on a laptop in his 'office'. Stephane cheerfully shrugged off the racket: 'Oh, it's nothing, I'm used to it. I sit here so I can tell by the sound whether there is something wrong with the system.' He ordered all the PLC (Programmable Logic Controller) hardware, designed and programmed the software with a visual interface, in effect giving the 'fridges' their 'brains'.

The tests for this particular cooling system started in July, carried out in collaboration with physicists. In the final configuration, seven large compressors will send the pressurised refrigerant to four pipes to carry the fluid into the experimental cavern. Here, they diverge into a myriad of small copper pipes the width of your finger, neatly weaving their ways to the centre of the detector and back again. The copper pipes are still being connected to the centre as the detector is being built, so the tests for this cooling system are not expected to be completed until Spring 2007.