Development and test of the CO2 evaporative cooling system for the LHCb UT Detector

The upgrade of the LHCb detector, which will take place during the Long Shutdown 2 from mid 2018 to the end of 2019, will extend significantly the physics reach of the experiment by allowing it to run at higher instantaneous luminosity with increased trigger efficiency for a wide range of decay channels. The LHCb upgrade relies on two major changes. Firstly, the full read-out of the front-end electronics, currently limited by a Level-0 trigger to 1 MHz, will be replaced with a 40 MHz trigger system. Secondly, the upgraded LHCb detector will be designed to cope with an increase of the nominal operational luminosity by a factor five compared to the current detector. Compared to the current experiment several subsystems need to be partially rebuilt. Among these the 4 TT planes will be replaced by new high granularity silicon micro-strip planes with an improved coverage of the LHCb acceptance.The new system is called the Upstream Tracker. The radiation length of each UT plane should not exceed the value of 1 % X0. The cooling system has to maintain the temperature of the sensors at -5 °C by removing the heat generated in the ASICs, in the silicon sensors due to self-heating, and in the cables that provide the power to the front-end electronics. The acceptable temperature excursion over the sensor is in the range of 5 °C. The temperature of the ASICs should be kept under 40°C for optimal functioning. The cooling power of the UT detector is rated at 5 kW. An efficient cooling system is necessary for maintaining the temperature of the sensors below - 5 °C in order to reduce the leakage current and prevent thermal runaway in presence of radiation damage. CO2 bi-phase cooling systems have successfully been built and operated for the LHCb VELO particle detectors, which pioneered the use of evaporative CO2 cooling in high energy physics, for the AMS tracker, and recently for the ATLAS Pixel Ineer B-Layer (IBL). They have proved to be very efficient and reliable, providing effective cooling with reduced impact on the material budget. In the UT detector the heat load is dominated by the power dissipation of the read-out ASICs, that are bonded directly to the sensor and positioned close to it in the active tracking volume. Simulation studies based on finite element analysis (FEA), has proved that evaporative CO2 cooling is the optimal choice in terms of cooling effciency and material budget. The CO2 evaporation around - 30 °C take place in cooling pipes embedded in the local support structures: 68 vertical staves, 1.8 m long. High thermally conductive carbon foam in an optimized sandwich structure design, provide a good heat transfer from the sensor and front-end electronics to the cooling pipe. A "snake pipe" design with bent tubes passing underneath the ASICs is currently considered as the baseline solution, providing the maximal heat transfer and the lower and uniform detector temperatures. The material for the pipe is titanium with 2 mm inner diameter and 0.1 mm thickness.The use of a vertical 3 m long "snake pipe" gives the best thermal performance for the detector, but R&D for a system with this geometry was mandatory. R&D activities and real scale test on prototypes have been done, and are in progress, to prove and finalize the design concept.