CO 2 COOLING FOR THE LHCB-VELO EXPERIMENT AT CERN.

CO 2 as evaporative coolant has gained interest for the use in high energy particle physics experiments. It is applied as coolant in the Vertex Locator (Velo) of the LHCb detector, which is an experiment at the Large Hadron Collider at CERN in Geneva. Silicon particle detectors like the Velo have special requirements on cooling. The sensors which are spread over a large volume must be kept at a stable cold temperature (-7 oC) at all times while the attached electronics generate a substantial amount of waste heat which has to be taken by the cooling system. The cooling infrastructure in the detector needs to be of low mass and the construction materials including the coolant need to be radiation resistant. CO 2 as coolant is a good option for this application as it can withstand a large amount of radiation and has excellent thermal behavior in small diameter tubes. The CO 2 cooling system for the Velo uses the 2-Phase Accumulator Controlled Loop (2PACL) method. This method supplies low quality CO 2 into the evaporator at a constant pressure and requires no active components inside the detector. This paper describes the design of the Velo detector and the Velo Thermal Control System VTCS). It explains the 2PACL method and test results of the VTCS are presented and experience gained during VTCS commissioning are described.