Thermo-dynamical measurements for ATLAS Inner Detector (evaporative cooling system)

During the construction, installation and initial operation of the Evaporative Cooling System for the ATLAS Inner Detector SCT Barrel Sub-detector, some performance characteristics were observed to be inconsistent with the original design specifications, therefore the assumptions made in the ATLAS Inner Detector TDR were revisited. The main concern arose because of unexpected pressure drops in the piping system from the end of the detector structure to the distribution racks. The author of this theses made a series of measurements of these pressure drops and the thermal behavior of SCT-Barrel cooling Stave. Tests were performed on the installed detector in the pit, and using a specially assembled full scale replica in the SR1 laboratory at CERN. This test setup has been used to perform extensive tests of the cooling performance of the system including measurements of pressure drops in different parts of system, studies of the thermal profile along the stave pipe for different running conditions / parameters and coolant flow measurements in the system. The pressure drops in the system and the associated temperatures in the barrel cooling loops have been studied as a function of the system variables, for example; input liquid pressure, vapour back pressure, module power load and input liquid temperature. Measurements were performed with 10, 11, 12, 13 barabs inlet liquid pressure in system, 1.2, 1.6, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0 barabs vapour back pressure in system, and 0 W, 3 W, 6 W, 9 W, 10.5W power applied per silicon module. The measurements clearly show that the cooling system can not achieve the design evaporation temperature of -25C in every part of the detector (SCT Barrel loops) in case of 13 barabs nominal inlet liquid pressure, 1.2 barabs minimum possible back pressure and 6W nominal power per SCT Barrel silicon module and especially at the end of the ATLAS ID operation period when modules will work on full power of 10.5 W. This will lead to the problem of thermal run-away of the ATLAS SCT, especially near the end of the operational period after significant radiation exposure has occurred. The LHC luminosity profile, depletion voltage and leakage current values and the total power dissipated from the modules were revised. Thermal runaway limits for the ATLAS SCT sub-detector were also revised. Results show that coolants evaporation temperature necessary for the sub-detector's safe operation over the full lifetime (10 years) is -15C with a safety factor of 2. Laboratory measurements clearly show that the cooling system can not achieve even this necessary evaporation temperature of -15C. It is now impossible to make mechanical modifications to the cooling system, for example; changing the diameter of the cooling pipes, or the thermal performance of the in-system heat exchanger or reducing the vapour back pressure. It was therefore decided to investigate changes to the cooling fluid and to test mixtures of Hexafluoroethane (R116) C2F6 and Octafluoropropane(R218) C3F8 at differing ratios instead of just pure C3F8 coolant presently used. For this purpose, a new "blending" machine was assembled in the SR1 laboratory, with a new device an "on-line acoustic flow meter and fluorocarbon coolant mixture analyzer" (Sonar Analyzer) attached to it. The Machines were connected to the already existing laboratory test station and new extensive tests were performed to investigate different proportion of C3F8/C2F6 blends to find the mixture ratio which resulted in the best operational performance as measured by: the temperature distribution, pressure drops and flow parameters over the system, to ensure best cooling performance of SCT Barrel cooling loops for long term ATLAS SCT operation. Measurements were performed with different percentage of C2F6 (1%, 2%, 3%, 5%, 10%, 20%, 25%) coolant in the C3F8/C2F6 mixture, for different power (0 W, 3 W, 6 W, 9 W, 10.5W) applied to dummy modules on the SCT cooling stave, with 13 barabs inlet liquid pressure and for different vapour back pressures (1.2, 1.6, 2.0, 2.5, 3.0 barabs) in the system. Results prove that with 25% of C2F6 in the blend mixture, it is possible to lower the evaporation temperature by ~10C in the case of nominal operation parameters of the system. The ATLAS Inner Detector Evaporative Cooling System can therefore reach the necessary evaporation temperature and therefore can guarantee thermal stability of the SCT, even at the end of the operation period.