Thermal measurements and characterizations for the CMS phase-1 barrel pixel detector and the CMS phase-2 upgrade tracker 2S module with evaporative CO2 cooling systems

Abstract

The CMS Phase-1 pixel detector and the CMS Phase-2 tracker are upgrade detectors for the silicon tracker of the CMS experiment at LHC, CERN. The upgrades are inevitable for the efficient performance of the CMS tracker at higher instantaneous and integrated luminosities of the LHC and the HL-LHC, respectively. The Phase-1 pixel detector was installed in winter2016/2017 and the Phase-2 upgrade tracker will be installed from 2025 to 2027.The detectors will experience radiation damage at levels which are nearly an order of magnitude higher than the values the original detectors were designed for. The higher radiation damage leads to higher leakage currents in the silicon sensors, because of induced defects in the crystal lattice. Their values depend approximately exponentially on the sensor temperature, with roughly a factor 2 at a temperature increase of 7 K. The effect of thermal runaway, which denotes the non-linear self-heating of the silicon sensors because of their leakage current, will eventually make the detectors inoperative. By cooling the detectors to low temperatures, the leakage current scan be drastically mitigated and their longevity enhanced. Hence, the thermal performance of the cooling structure is crucial for the operation. Coolant temperatures (a few 10 K below 0 °C) are necessary. The mechanics of the detectors have to be optimized for small temperature gradients in the order of 10 K between the silicon sensors and the coolant. Both detectors use the concept of evaporative CO 2 cooling systems at nominal coolant temperatures of −22 °C and −35 °C, respectively. Thermal measurements with test prototypes, so-called thermal dummies or mock-ups, are an important part of the detector design and construction. This thesis presents measurements and characterizations of the thermal properties for the Phase-1 barrel pixel detector and for the 2S modules of the Phase-2 Upgrade tracker with evaporative CO2 cooling systems. Within the CMS Tracker collaboration, the results of both measurement projects contributed substantially to the understanding of the thermal properties of these detectors. For the Phase-2 part, a detailed characterization of the thermal properties of the 2S module is presented. The 2S module is a silicon strip module with two silicon sensors and three electronic hybrids. Typical power values are 5.4 W for the hybrids and 0.3 to 0.5 W for each sensor. Dummy2S modules were built and systematic thermal measurements were made. A dedicated test setup was developed, constructed, and commissioned. A custom CO2 system with a cooling temperature of −30 °C was used. The thermal resistance of the sensors to the cooling system was measured with bare modules with no hybrids attached and the thermal coupling of the sensors to the ambient was estimated. The thermal performance of a fully assembled 2S module with hybrids and wire bonds is studied. The effect of thermal runaway is demonstrated. All test results are directly compared to predictions from Finite Element simulations which have been developed. The mechanical assumptions made in the Finite Element model could be confirmed with measurements. For the Phase-1 part, thermal measurements with a thermal mock-up of one Layer 2 barrel pixel detector half-shell and 114 thermal pixel dummy modules were conducted. The measurements were made with a LUKASZ CO2 cooling system. At the full heat load of 200 W temperature drops of 4 to 5 K along one cooling line could be observed because of pressure drops in the two-phase CO2 in the pipes. The temperature gradient in the detector leads to a systematic temperature distribution in the detector. This effect directly affects the temperature dependent leakage currents of the silicon pixel sensors. The observed leakage current distributions with relative factors of 1.4 between power sectors in the real BPIX detector are compatible with the predictions from the measurements with the mock-up. It was predicted from the measurements and shown in the real detector that changing the CO2 mass flow reduces the temperature gradients in the detector.