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.