CO₂ Cooling Developments for HEP Detectors

Two phase (evaporative) CO₂ detector cooling is gaining significant interest as a demonstrated alternative to traditional cooling techniques in High Energy Physics (HEP). The upgrade programs of the inner detectors of ATLAS and CMS are investigating evaporative CO₂ cooling as an alternative to the current fluorocarbon cooling systems. CO₂ as coolant has superior thermo dynamical properties compared to other coolants used in HEP detectors leading to significant smaller diameter cooling pipes and hence lower mass inside the detector. Two operational HEP detectors use evaporative CO₂ cooling as part of the thermal management of their silicon detectors; the LHCb-VELO and the AMS02-Tracker. Both CO₂ systems use the 2- Phase Accumulator Controlled Loop (2PACL) technique developed at Nikhef. The 2PACL method is an ideal method for HEP detectors as it is very stable, uses only passive components inside the detector volume and it is easy to control. This paper describes the experiences gained with CO₂ cooling in the past and looks at possible future applications within HEP experiments. Special attention is paid to the amount of material needed inside the active detector volume when CO₂ is used.