CO2 cooling for HEP experiments

Abstract

The new generation silicon detectors require more efficient cooling of the front-end electronics and the silicon sensors themselves. To minimize reverse annealing of the silicon sensors the cooling temperatures need to be reduced. Other important requirements of the new generation cooling systems are a reduced mass and a maintenance free operation of the hardware inside the detector. Evaporative CO2 cooling systems are ideal for this purpose as they need smaller tubes than conventional systems. The heat transfer capability of evaporative CO2 is high. CO2 is used as cooling fluid for the LHCb-VELO and the AMS-Tracker cooling systems. A special method for the fluid circulation is developed at Nikhef to get a very stable temperature of both detectors without any active components like valves or heaters inside. This method is called 2-phase Accumulator Controlled Loop (2PACL) and is a good candidate technology for the design of the future cooling systems for the Atlas and CMS upgrades. I. EVAPORATIVE CO2 COOLING In detector applications it is crucial to minimize the hardware needed for the cooling inside the detectors. It is known that two-phase cooling is more efficient than single phase cooling. Less flow is needed and a tube can become almost isothermal when the pressure drop remains low. The smallest diameter evaporator tubes can be achieved with fluids which are evaporating under high pressure. For these fluids, the created vapor can not expand to a large volume. Therefore the pipe volume hence the diameter can stay low. A smaller diameter pipe contains less fluid mass. A high pressure fluid needs a thicker tube wall, but since the pipe diameter is smaller, the increase in mass of the tube wall is compensated by the diameter decrease. Hence, the total mass (tube+fluid) is lower when using a high pressure fluid as compared to a low pressure fluid. Another good feature of high pressure fluids is that larger pressure drops can be allowed. The influence of the pressure drop is related to the absolute pressure, and therefore less significant for high pressure fluids. This means that even smaller diameter tubes can be used. Currently used radiation hard fluids are fluor-carbons and CO2. Figure 1 shows the saturation pressure curves of some these fluids in the temperature range between +10 and -40°C. As can be seen, CO2 is the best candidate of the three and C3F8 the least interesting candidate fluid. A calculation later in this paper will show the superiority of CO2 compared to the two fluor-carbons.