18th March Richard Hawkings Humidity control in the ATLAS ID Richard Hawkings (CERN) JCOV meeting 18/3/04  Overview of humidity and associated gas flushing systems  Introduction to ATLAS ID  Gas volumes and thermal management  Subdetector humidity requirements  N 2 and CO 2 environmental gas systems  Humidity monitoring sensors

18th March Richard Hawkings The ATLAS inner detector - introduction TRT endcap A+B TRT endcap C TRT barrel SCT barrel SCT endcap Pixels  Whole ID sits inside bore of LAr calorimeter cryostat (6m long, 1.1 m radius)  TRT straw tube tracker, silicon strips (SCT) and pixels  Services run along cryostat bore and out along cryostat flange  Installed in sections: barrel (SCT+TRT), inner endcap (TRT A+B,SCT), outer endcap (TRT C) and pixels (inside pixel support tube PST)

18th March Richard Hawkings Thermal management - introduction  Different thermal/gas environments for the different subdetectors:  SCT and pixels run cold (-7 to 0 o C) in dry N 2  TRT and global ID volume warm (+20 o C) and in dry CO 2  Thermal screens to separate them (designs being revised – remove active cooling)  In normal operation, around 120 kW heat load in ID volume, balanced by cooling

18th March Richard Hawkings Environmental gas overview  TRT: ‘Envelope’ gas around straws is CO 2  Used for cooling (endcap) and ventilation (remove Xe which absorbs TR photons)  Keep concentration of N 2 in envelope below 1%, water vapour below 500 ppm  In high radiation environment, water vapour causes ageing affects on TRT straw wires  Need to keep active gas moisture below 500 ppm, straw walls permeable to water  Gas-tightness of TRT modules/wheels is unknown – requirements apply to ID vol.  SCT and pixels: Dry N 2 inside gas-tight thermal enclosures  Keep water vapour below 600 ppm (dew point –25 o C) inside enclosures  Critical to avoid condensation on cold components (modules, cooling pipes,…)  Avoid significant CO 2 contamination in SCT/pixel volume (acid formation, FSI…)  Avoid significant moisture around services feedthroughs to ID volume  Particularly important for pixel PP1, where cold cooling pipe joints cannot be insulated  Solution for ID global volume:  Flush ID global volume with up to 10 m 3 /hour of dry CO 2  Remove any N 2 leaking from SCT thermal enclosures, and keep area around TRT dry  Requires good overall sealing of ID volume (cryostat flange, beampipe)

18th March Richard Hawkings N 2 system for SCT and pixels  Maintain a dry environment for the silicon detectors  Remove initial moisture, outgassing and any ingress  Flow of ~ 1 exchange/hour (1-2 m 3 /hour) in initial operation, may be reduced later  Remove cupfulls of water outgassed by SCT carbon-fibre structures, then stabalises  Four independent but identical gas systems  SCT barrel, endcap A/C, pixels – volume inside pixel support tube  Each enclosure designed for leak rate of < 6 mbar.l/hour  Design overpressure 4 mbar, operating target 1 mbar  System controlled from gas rack in USA15 service cavern  Mass flow controller feeding long (~150 m) 6mm line to ID volume  Gas exhausts to UX15 cavern at detector platforms through purge valve or metered valve (switch remotely, adjust flow in cavern)  Differential pressure sensor used to measure over-pressure wrt global ID volume  Active control needed to cope with sudden temperature changes induced by power on/off on detector modules  Safety bubbler in case of control malfunction

18th March Richard Hawkings N 2 system details  Design/construction collaboration between ST-CV and ATLAS ID (Nikhef/CERN)

18th March Richard Hawkings CO 2 system for ID volume  Flush global ID volume with up to 10 m 3 /hour CO 2  3 m 3 /hour from barrel TRT ventil n  Up to ~3 m 3 /hour leaked from endcap TRT  Rest from series of CO 2 inlets spread around ID volume  CO 2 exhaust to cavern:  40 mm diameter vertical chimney pipe at each end of ID volume  6m high, 0.4 mbar static pressure head due to CO 2 heavier than air  Possible leaks from services feedthroughs at ends of ID volume  Safety and access implications  Switch to dry air during access, TRT will take time to recover  System design similar to N 2 system, but passive; all control hardware in USA15

18th March Richard Hawkings Humidity monitoring  Humidity monitored at various places inside the ID  Inside the SCT and pixel volumes – O(10) sensors per enclosure  For SCT, knowledge of humidity level is also important for internatl laser-based alignment system – need to know optical properties of gas very precisely  At most critical locations inside global ID volume: 4 locations per end with sensors at  =90 o and 270 o  Between barrel and endcap SCT close to services feedthroughs  Behind endcap SCT, close to endcap services feedthroughs  Close to end of pixel support tube and associated feedthroughs  At outer corner of ID volume, close to cryostat and patch panel PP1  Sensor choice:  Die-packaged Xeritron sensors from Hygrometrix (2cm long, few mm diameter)  3 wires readout to signal conditioning board and standard ELMB readout to ATLAS DCS system  Radiation-hard enough for ATLAS ID, temperature compensated  Sensor used for information – not direct control of gas system  Target flow rates are set manually in response to humidity information