CO2 cooling challenges at CERN for the future phase 2 upgrade program

At CERN, evaporative CO2 is the baseline cooling solution for the thermal management of the phase 2 silicon detectors. Since 2008 CO2 cooling is used in 3 detectors with capacities ranging 1 to 15 kW at -30°C. A special pumped cycle was developed to guarantee accurate temperature control under all operational conditions. The challenges for the upgrade are: the large increase of the cooling power (250 - 450 kW), the large number of parallel operating evaporators (~1000x), the low evaporative temperature (-45°C). and the implementation of a primary R744 transcritical system. Implementing this R744 primary cooling solution will make the future detector cooling to work fully with natural working fluids (R744 and R718). This paper describes the design and prototyping phase of the system with surface storage and the new primary R744 system, which bridges the industrial applications of R744 refrigeration to the highly demanding performances of high energy physics experiments.