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

Abstract

At CERN, evaporative CO2 is the baseline cooling solution for the thermal management of the phaseII silicon detectors which will be installed in 2025 in the ATLAS and CMS experiments. 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 (300 - 550 kW), the large number of parallel operating evaporators (~1000x), the low evaporative temperature (-45°C) and the implementation of a primary R744 trans-critical cooling 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.