A thermosiphon for ATLAS

A new thermosiphon cooling system, designed for the ATLAS silicon detectors by CERN’s EN-CV team in collaboration with the experiment, will replace the current system in the next LHC run in 2015. Using the basic properties of density difference and making gravity do the hard work, the thermosiphon promises to be a very reliable solution that will ensure the long-term stability of the whole system.


Former compressor-based cooling system of the ATLAS inner detectors. The system is currently being replaced by the innovative thermosiphon. (Photo courtesy of Olivier Crespo-Lopez).

Reliability is the major issue for the present cooling system of the ATLAS silicon detectors. The system was designed 13 years ago using a compressor-based cooling cycle. “The current cooling system uses oil-free compressors to avoid fluid pollution in the delicate parts of the silicon detectors,” says Michele Battistin, EN-CV-PJ section leader and project leader of the ATLAS thermosiphon. “After a few years of operation, the compressors started suffering fatigue problems that caused important failures and frequent leaks. Significant resources were then devoted to investigating the problem and in 2009 we found a solution to reduce the effects of the fatigue but we were not able to fix the causes. The system has allowed the ATLAS inner detectors to be cooled to -20°C since then, but it was always evident that these compressors would not be a long-term solution.”

Since 2010 the CERN and ATLAS experts have been exploring alternative solutions to the problem. “We looked for new types of oil-free compressors available on the market but without success. In parallel, we tried to find alternative solutions to cool the detector, and we eventually decided to design a thermosiphon system,” explains Battistin.

The new thermosiphon system (see box for a technical description) will allow the cooling fluid to circulate naturally without any mechanical pumping component in the circuit. Instead, the system uses gravity – the ATLAS experiment is situated 92 metres below ground – to create the pressure difference that drives the movement of the fluid. “This system will require less maintenance, and the absence of vibration will prevent the development of fluid leaks,” says Battistin. “Therefore, the circuit is oil-free by definition and allows us to adopt the most standard industrial solutions to condense the fluid. Moreover, once condensed, the large quantity of cooling fluid will act as an important energy buffer that will allow the system to work for several minutes in the event of a power cut.”

The coolant currently used in the thermosiphon is perfluoropropane (C3F8), which evaporates at -20°C in the inner parts of the silicon detectors, thereby taking the heat away from the delicate silicon sensors and related electronics. “The thermosiphon solution will allow lighter fluid mixtures like C3F8-C2F6, to be used,” says Michele Battistin. “This will reduce the coolant evaporation temperature to -30°C if so required by the ATLAS inner detector groups. Lowering the temperature of the detector, in fact, helps the silicon sensors to sustain the effects of radiation damage, and increases the lifetime of the detector.”

“The project was launched in 2010, mainly relying on CERN and ATLAS resources for the design, integration, purchase, assembly, installation and commissioning of the system, and has made use of a wide range of our colleagues' skills, and of the competences of a number of CERN and ATLAS colleagues,” says Battistin.

The ATLAS thermosiphon system is now entering the commissioning phase. It will be operational at the start of the next LHC run in 2015. From then on, the previous cooling system will be used as a back-up system in the event of a breakdown or maintenance work.

Technical description

Thermosiphon diagram.

The thermosiphon system provides warm (20°C), high-pressure liquid perfluoropropane (C3F8) at the detector’s distribution point. The liquid expands inside capillaries and evaporates at –25°C and 1.67 bar on the silicon detector structures. The warm (20°C) gas collected at the detector exhaust is taken to the thermosiphon condenser located on the roof of the SH1 building at Point 1. Here, the gas is liquefied at 0.3 bar (-60°C). The 92-m-high liquid column creates up to 16.5 bars of hydrostatic pressure at the detector’s liquid distribution point.

The thermosiphon system will circulate 1.2 kg of perfluoropropane per second to remove up to 62.4 kW of heat dissipated by the ATLAS inner silicon detector electronics. This fluid is radiation-resistant and dielectric; therefore, in the event of an accidental leak, it would not damage the silicon detector.


by Rosaria Marraffino