ShareICR2011 highlights: cooling particle detectors
Cooling systems for particle detectors attracted interest at the IIR’s Congress in Prague in August 2011. These highly specific applications have very strict requirements compared with standard cooling systems: refrigerants should be resistant to radiation and magnetic fields, chemically stable and also dielectric in the event of evaporator leakage.
Cooling systems for particle detectors were the subject of two different papers presented at the IIR Congress in Prague in August 2011. These highly specific applications have very strict requirements compared with standard cooling systems: refrigerants should be resistant to radiation and to magnetic fields, chemically stable and also dielectric in the event of evaporator leakage.
Fluorocarbons – both single and double phase – are often used for particle detectors, as normal HFC refrigerants are not radiation resistant, but evaporative CO2 cooling is also attracting a lot of interest for these applications, according to Verlaat, Colijn and Postema from CERN. In their paper (1) they describe two particle detectors equipped with CO2: the AMS on the International Space Station and the LCHb at the Large Hadron Collider at CERN. According to the authors, evaporative CO2 proves superior to fluorocarbons such as C3F8 in these detectors which require very low mass structures: mass tends to deviate particle tracks and CO2 cooling is superior in small channels – it can operate in channels five times smaller than for fluorocarbons.
The high pressures required by CO2 systems (typically 100 bar vs. 15 bar for a C3F8 system) have often been considered a disadvantage as they were thought to add mass to the detector because of thicker walls; however, this is no longer the case as very small channels can be used, due to the high vapour density, latent heat and low viscosity of CO2. Its application actually significantly reduces the mass in silicon detectors, while providing stable operation at temperatures ranging from +20 °C to -40 °C.
Experience gained from both systems was a source of knowledge, for instance concerning the evaporators, which are typically very different from those used in ordinary applications as the heat sources need to be spread over a large volume and connected in series to multiple parallel tube branches. This arrangement has a major impact on the overall behaviour: the parallel evaporator system is inherently unstable and this should be taken into account at the design stage.
Many detector designers are considering CO2 for their new or upgraded detectors, not only for reasons of radiation damage, but also to a large extent to benefit from the enhanced performance levels provided by technological progress.
However, during the same Congress, a team from the Czech Technical University in Prague (Vacek et al.) (2) presented an altogether different solution for the Roman Pots (RP) detectors that are being installed and used within the TOTEM project also at the LHC: a non-traditional vapour-cooling circuit for detectors capable of operating with fluorocarbon refrigerants (R218, R116, R610 or their mixtures).
The system in the Roman Pot (RP) insertion is designed to remove the heat load from the sensors and electronics and operated well below -10°C (the temperature limit of the hottest spot). Because of the high radiation environment of the LHC tunnel, the main part of the refrigeration system is placed in an underground and protected accessible service area. An evaporative fluorocarbon secondary cooling strategy makes it possible to transport fluid at ambient temperature over long distances (147 and 220 m) without heat losses but at low enough pressure drops for them to be offset by the compressors. A C3F8 dielectric refrigerant was selected because it is non-flammable, non-conductive and radiation resistant with a highly effective heat-transfer process, due to its two-phase flow-boiling regime. An evaporator consisting in 2.5 mm outer diameter thin-walled copper nickel tubes in an S-shape configuration is squeezed inside the frame structure of the Roman Pot per se. These evaporators are fed by capillary tubes that provide the throttling of the liquid and later evacuated in gas phase via larger diameter tubes. The cooling plan has proved its reliability. Among the total number of 24 Roman Pots commissioned prior to the installation in the underground area over the last 12 months, only 5 were defective, but all of them could be repaired on time.
(1) The future of CO2 cooling in particle physics detectors, Verlaat et al.
(2) Versatile refrigeration circuit developed for the particle detector commissioning, Vacek et al.
Fluorocarbons – both single and double phase – are often used for particle detectors, as normal HFC refrigerants are not radiation resistant, but evaporative CO2 cooling is also attracting a lot of interest for these applications, according to Verlaat, Colijn and Postema from CERN. In their paper (1) they describe two particle detectors equipped with CO2: the AMS on the International Space Station and the LCHb at the Large Hadron Collider at CERN. According to the authors, evaporative CO2 proves superior to fluorocarbons such as C3F8 in these detectors which require very low mass structures: mass tends to deviate particle tracks and CO2 cooling is superior in small channels – it can operate in channels five times smaller than for fluorocarbons.
The high pressures required by CO2 systems (typically 100 bar vs. 15 bar for a C3F8 system) have often been considered a disadvantage as they were thought to add mass to the detector because of thicker walls; however, this is no longer the case as very small channels can be used, due to the high vapour density, latent heat and low viscosity of CO2. Its application actually significantly reduces the mass in silicon detectors, while providing stable operation at temperatures ranging from +20 °C to -40 °C.
Experience gained from both systems was a source of knowledge, for instance concerning the evaporators, which are typically very different from those used in ordinary applications as the heat sources need to be spread over a large volume and connected in series to multiple parallel tube branches. This arrangement has a major impact on the overall behaviour: the parallel evaporator system is inherently unstable and this should be taken into account at the design stage.
Many detector designers are considering CO2 for their new or upgraded detectors, not only for reasons of radiation damage, but also to a large extent to benefit from the enhanced performance levels provided by technological progress.
However, during the same Congress, a team from the Czech Technical University in Prague (Vacek et al.) (2) presented an altogether different solution for the Roman Pots (RP) detectors that are being installed and used within the TOTEM project also at the LHC: a non-traditional vapour-cooling circuit for detectors capable of operating with fluorocarbon refrigerants (R218, R116, R610 or their mixtures).
The system in the Roman Pot (RP) insertion is designed to remove the heat load from the sensors and electronics and operated well below -10°C (the temperature limit of the hottest spot). Because of the high radiation environment of the LHC tunnel, the main part of the refrigeration system is placed in an underground and protected accessible service area. An evaporative fluorocarbon secondary cooling strategy makes it possible to transport fluid at ambient temperature over long distances (147 and 220 m) without heat losses but at low enough pressure drops for them to be offset by the compressors. A C3F8 dielectric refrigerant was selected because it is non-flammable, non-conductive and radiation resistant with a highly effective heat-transfer process, due to its two-phase flow-boiling regime. An evaporator consisting in 2.5 mm outer diameter thin-walled copper nickel tubes in an S-shape configuration is squeezed inside the frame structure of the Roman Pot per se. These evaporators are fed by capillary tubes that provide the throttling of the liquid and later evacuated in gas phase via larger diameter tubes. The cooling plan has proved its reliability. Among the total number of 24 Roman Pots commissioned prior to the installation in the underground area over the last 12 months, only 5 were defective, but all of them could be repaired on time.
(1) The future of CO2 cooling in particle physics detectors, Verlaat et al.
(2) Versatile refrigeration circuit developed for the particle detector commissioning, Vacek et al.