iτiτi AiAi 1(1.2±0.2)×10 6 min0.42±0.11 2(4.1±0.6)×10 4 min0.10±0.01 3(3.7±0.3)×10 3 min0.23± ±25 min0.21± ±5 min0.04±0.03 Donor removal & stable acceptor Unstable acceptor Reverse annealing Evaporative cooling in ATLAS – present and future Georg Viehhauser for the ATLAS ID collaboration The ATLAS evaporative cooling systemRadiation damage controlA simple model of thermal stability Cooling for the ATLAS sLHC UpgradeWhat can be done to improve this system? Can the present system achieve what is required? QHQH Q RsRs RcRc TCTC TSTS a)b) Q RtRt TSTS T0T0 Removes heat from Pixel and SCT (barrel and endcap) sub-systems. Coolant: C 3 F 8. Single stage cycle with warm transfer pipes Condensation at 20°C and 17bar a. Target evaporation at -25°C (1.6bar a ). Minimum compressor suction pressure 0.8 bar a. Large compression ratio (~1:20) achieved by two-stage, oil-free compressors. Fixed mass flow to accommodate load fluctuations → heater (immersion type heaters). Internal HEX for sub-cooling of liquid to reduce mass flow. 60kW in 204 circuits. Largest evaporative cooling system in HEP to-date (by more than factor 10). The commissioning of this system has been challenging: Components for this system fall into different ATLAS ID subsystem responsibilities. Specifications were not always consistent. Leak checking such a large system needs to be well prepared in the design. Leaks have developed in inaccessible locations. The heaters developed a series of technical faults (leaks, electrical shorts) requiring several reworks. Once the heaters have been identified as problematic they have been relocated to a less inaccessible location. Since the rework no heaters have failed. The heaters need to boil off and heat up the return fluid over a very short length. The high uniform power density required makes control of the heaters difficult due to the very different heat transfer of single-phase and two-phase flow. The high compression ratio introduces high compressor stress resulting in fatigue cracks and requiring shorter than anticipated maintenance intervals. Late modifications to pipe runs to overcome problems during the commissioning have resulted in higher return line pressure drops. An inaccessible failure in the thermal shield requires to operate the outermost SCT layers at a higher temperature (5°C) to maintain thermal neutrality with the next ATLAS sub-detector, the TRT. This system is now operating reliably. The total operational period (time × number of circuits) is xxx y. Temperature-dependent annealing affects development of depletion voltage (V dep ) and leakage current (I leak ). Predictions for their development for different temperature operational scenarios have been made based on models in the literature. These predictions were made for the innermost SCT barrel (B3) on the assumption of 342d/y with cooling (216d/y with electronics on), and 23d/y of maintenance without cooling. Different cooling temperatures have been studied. Depletion voltage prediction Based on: M.Moll, Radiation Damage in Silicon Particle Detectors, Dissertation, Hamburg G.Lindström et al., Radiation hard silicon detectors – developments by the RD48 (ROSE) collaboration, NIM A466 (2001) Based on: S.J.Bates, The effects of proton and neutron irradiations on silicon detectors for the LHC, Ph.D. thesis, Darwin College A Silicon detector is thermally stable if coolant temperature and detector thermal resistance are low enough to remove heat from electronics and sensor leakage. This is a 3-dim problem, usually solved by FEA. However such results are specific to the modelled structure and the input parameter set. We have developed an analytic approach which offers some general (if approximate) results that allow relatively simple extrapolation of the performance. Assume zero sensor resistance: Then the thermal balance is given by Differentiation at the critical point (stability limit) yields with And for the critical coolant temperature Thermal resistance values can be obtained from comparison with FEA: Conclusion: Need coolant temperature of -25°C to maintain thermal stability of innermost barrel SCT layer with a factor 2 headroom at end of ATLAS. The outermost barrel SCT layer (B6) needs to be cooled above -8°C at the start, dropping to -17°C at the end of its lifetime, which is reached when the leakage power at 0°C is 125μW/mm 2. Beyond this B6 could not be operated stably at a sensor temperature of 5°C. This is not an issue for the pixel and endcap SCT subsystems due to lower pressure drops (smaller load per circuit, smaller line impedance) and different heat path design on the detector modules. Options considered for the future ATLAS evaporative cooling system: Fluorocarbons: In this scenario we would build on the long experience we will have accumulated on the present system. The necessary modifications outlined for the present system would be essential stepping stones towards a phase II cooling system. Further changes would include: Pressure drops and coolant temperatures were measured in a test setup representing a cooling loop on the ATLAS barrel SCT, and cooling system components (transfer pipes, heaters, HEXs, etc.) as used in ATLAS and arranged in a similar geometry. Conclusion: The pressure drops in the barrel SCT circuits and their return pipes are larger than anticipated, and too large to achieve the required evaporation temperature of -25°C for the barrel SCT. This pressure drop is dominated by the pressure drop from inaccessible components. Surface condensers Surface condensers reduce the compression ratio and with it compressor stress. The gravitational head of the liquid from the surface condenser to the distribution racks in the pit (~80m) is >10bar for C 3 F 8, which can be used to reduce the compression ratio for the compressors to 1:10. The condensation temperature is reduced to 10°C. Thermosiphon If the surface condenser is operated very cold (-50°C) the saturation pressure in the condenser will be cold enough to extract the coolant from the detector without the need for any compressor or pump, thus reducing the number of active components in the system, at the cost of poor efficiency and a substantial external cooling plant. Conclusion: Major R&D and modifications to the ATLAS evaporative cooling system are needed to secure long-term operation of the cooling for the ATLAS ID. Fluorocarbon blends Adding a small amount of C 2 F 6 to C 3 F 8 (20%/80%) shifts the saturation curve to higher pressures. The margin from the evaporation pressure for pure vapour (2.1bar a ) to the compressor (or thermosiphon) suction pressure increases, allowing to reach an evaporation temperature of -25°C even in the barrel SCT. Drawbacks of mixtures are lower cooling pipe wall-coolant heat transfer coefficient, and evaporation isotherms are no longer isobaric, but have lower pressure for higher vapour quality. The pressure gradient in the pipe is smaller than this, resulting in a significant temperature non-uniformity along the pipe. Mixture control can be achieved through speed-of-sound measurements. Leakage current estimates from minimal model Short strips (r~xxcm) Long strips (r~xxcm) Outer pixels (r~15cm) Inner pixels (r~7cm) We have defined a set of requirements for the cooling of the upgraded ID. These differ from the requirements for the present system as the future ATLAS Silicon tracker will be ~1nb -1 Conclusion: The depletion voltage and the leakage current at a reference temperature after irradiation are only weakly depending on the exact cooling scenario, as long as the yearly maintenance period (at 20°C) is around 3 weeks and the detector is kept moderately cold (≤0°C) at all other times. The barrel SCT has been designed and tested on the inner barrels for up to 500V bias voltage. Leakage current prediction Pressure [bar a ] Enthalpy [J/kg] N eff,0 = 1.026×10 12 cm -3 N C0 = 0.7N eff,0 c = 0.075cm -1 /N C0 g a = cm -1 τ a = 55h (T R =20°C) E a = 1.09 eV g C = cm -1 g Y = cm -1 τ Y = 480d (T R =20°C) E Y = 1.33 eV inaccessible See: D.Attree et al., The evaporative cooling system for the ATLAS inner detector, 2008 JINST 3 P07003See: G.Beck, G Viehhauser, Analytic model of thermal runaway in Silicon Detectors, submitted to Nucl. Instr. Neth. Cooling loop temperature vs back pressure The detector temperature can be set by the back pressure at the distribution racks outside ATLAS. However, for a nominal detector load (6W) the coolant temperature never reaches -25°, even for lowest backpressures. C 3 F 8 saturation curve The reason for the increasing offset between the stave temperature and the C 3 F 8 saturation curve for lower back pressures is the pressure drop on the detector and from the detector to the distribution racks. This stems dominantly (~80%) from parts which are inaccessible (Detector loop & manifold and internal HEX). Warm surface condenser Thermosiphon Compressor Warm condenser Cold condenser 250K 280K 300K 320K See: ATLAS upgrade ID cooling system requirements, ATU-SYS-ES-0004, CERN EDMS ver. 5 larger and more finely segmented, although the total power scales weaker than the number of channels, due to new ASIC technologies. The total power expected is up to 180k, in ~1000 detector loops. colder, to maintain thermal stability at the larger radiation damage sustained at sLHC. The target maximum cooling temperature will be -35°C. This gives a large headroom for the strip sub-system, while it is probably sufficient for the pixel sub-system. External instead of internal pre-cooling by a HEX at accessible location. Transfer pipes from HEX to ID would be cold (requiring insulation). Alternative heater design (warm liquid HEX or low power density electrical heater ) at accessible location. CO 2 : CO 2 has many appealing properties (large latent heat, good heat transfer to cooling pipe wall). Such a system would be new, although we would attempt to build on experience with the LHCb VELO cooling system. A possible approach could be the scale-up of this system to up to 10 independent units of ~20kW (scale-up per unit by factor ~10). Sensor isotherms Lines of equal strip current Uncertainties about the future tracker have been included in these estimates as safety factors, which will need to get reduced during the further development of the sLHC ID. B3 10y expectation B6 10y expectation Capillary Compressor Condenser Backpressure regulator Internal HEX See: R.Bates et al., Reassessment of cooling requirements for the ATLAS barrel SCT, ATL-COM-INDET Note that this plot shows the leakage current at a reference temperature. To obtain the actual leakage current it needs to be scaled to the sensor temperature according to