Brief summary of the progress on PS215/CLOUD during 200

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CERN-SPSC-2008-015/SPSC-SR-03218/04/2008EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHApril 18, 2008cloud2007 PROGRESS REPORT ON PS215/CLOUDCLOUD Collaboration1 INTRODUCTIONIn its Fourth Assessment Report, February 2007, the Intergovernmental Panel on Climate Change (IPCC)attributes more than 90% of the observed climate warming since 1900 to the rise of anthropogenic green-house gases in the atmosphere [1]. Aerosols and clouds are recognised as representing the largest uncer-tainty in the current understanding of climate change. The IPCC estimates that changes of solar irradi-ance (direct solar forcing) have made only a small (7%) contribution to the observed warming. However,large uncertainties remain on possible other solar-related contributions, such as the effects of changes ofultra violet radiation or galactic cosmic rays on aerosols and clouds. So far, no quantitative estimates ofgalactic cosmic ray-induced changes in aerosol and cloud formation have been reached. Experiments areplanned for the CERN PS215/CLOUD facility to quantitatively address this discrepancy.During 2007 the CLOUD activities have mainly involved 1) analysis of the 2006 beam data fromthe CLOUD Mk1 prototype, 2) preparation of the engineering design for the new CLOUD Mk2 prototypeand 3) submission of funding proposals to the EU FP7 and national agencies. These activities are brieflysummarised in this document, together with a request for CERN PS T11 beam time to set up and takephysics data with the CLOUD Mk2 prototype.2 ANALYSIS OF Mk1 PROTOTYPE BEAM DATADuring autumn 2006, the CLOUD Mk1 prototype was operated at the CERN PS both to provide technicaldesign input and also to study the effect of ionising particle radiation on aerosol formation and earlygrowth, in the presence of UV radiation and under atmospheric conditions. The aerosol chamber involveda 2×2×2 m3 stainless steel chamber built by the Danish National Space Center (DNSC). The ions weredetected using an air ion spectrometer (AIS) from U. Helsinki, and the aerosol particles using an SMPSand a CPC battery from PSI and U. Mainz. Gas-phase H2SO4 was measured by a mass spectrometer(CIMS) of MPIK-Heidelberg. The chamber was filled with ultra-pure air obtained from the evaporationof cryogenic N2 and cryogenic O2 liquids, provided by CERN and U. Helsinki, to which were addedwater vapour and trace amounts of O3 and SO2. The trace gas concentrations and chamber conditions(T, RH, UV) were continuously measured during the run.After extensive flushing and cleaning cycles with elevated O3 concentrations (few × 100 ppb),extremely clean conditions could be achieved in the aerosol chamber, in terms of low particle backgroundconditions (<0.1 particle/cm3 above the 3nm detection threshold) and low pre-existing concentrationsof H2SO4 (less than the detection limit of about 0.03 ppt). Aerosol nucleation bursts were generated andtheir characteristics analysed. Typical formation rates were about 10 cm−3s−1, and growth rates of about10 nm/h. The corresponding H2SO4 concentration was around 106 cm−3. The observed nucleation ratesare somewhat higher than observed at atmospheric conditions. However, the slope of the nucleation ratevs. H2SO4 concentration is similar to that observed at atmospheric conditions, supporting the idea thatexisting clusters are activated, probably by sulphuric acid. However, sulphuric acid is not able to explainFig. 1: On-line plot from the Mk1 2006 beam data showing indications of ion-induced nucleation events. Thefigure shows the progression of various aerosol chamber parameters over a 13-hour period (x axis). Each param-eter is indicated by its corresponding colour-coded y axis label. The red/black curve shows the aerosol numberconcentration (3 nm threshold size). The cyan curve shows the conduction current of air samples extracted fromthe aerosol chamber and passed through a Gerdien counter (note inverted scale). This directly measures the ion pairconcentration due to varying beam intensities, which produce the observed steps. The beam was first turned on at00:00 but, due to a charging effect in the Gerdien counter, it did not record a reliable reading until about 02:00. Noother chamber parameters were changed during this period, although temperature variations of the chamber due tovariations of the East Hall temperature can be seen in the purple curve. Variations of the aerosol number—bothincreases and decreases—appear to be initiated by changes of the beam intensity. Note that the increase of thechamber temperature after 10:00 hours indicates that the data were unreliable after that time.the observed growth, suggesting the possible presence of additional trace gases in the chamber that werenot measured.Interestingly we were able to observe different kinds of new particle formation events. Someof the events are related to ion induced nucleation and ion-ion recombination to form neutral clusters.In these cases, up to as much as half of new particle formation can be explained by ion processes (anon-line example is shown in Fig. 1). However, during some nucleation events less than 1% of newparticle formation can be explained by ion processes. These results are being prepared for conferencepresentation.In summary, the Mk1 tests confirmed the feasibility of the CLOUD experiment and providedimportant input for the technical design, as well as providing initial indications of ion-induced nucleationevents.3 Mk2 DESIGNA new 3m aerosol chamber—the CLOUD Mk2 prototype (Figs. 2–4)—has been designed for the nextCLOUD run at the CERN PS. The chamber is constructed from stainless steel and will operate near2Fig. 2: Vertical section through the CLOUD Mk2 prototype showing the 3m aerosol chamber and its thermalcontrol housing. Also shown are the field cage, fibre-optic UV illumination (through the roof plate), circulationfans, sampling probes, racks housing the analyser instruments and a typical man (1.75m).1 atm pressure, but can accommodate modest (around 10%) adiabatic pressure reductions to allow fordroplet activation. It will be operated between about -50◦C and 40◦C. In comparison, the Mk1 chamberoperated strictly at 1 atm pressure and room temperature. The Mk2 prototype incorporates many designimprovements that were learnt during the Mk1 beam run in 2006, among which are:• Temperature stability: Reliable nucleation measurements require a temperature stability for thechamber of around 0.1◦C. This will be achieved with a thermally-insulated housing through whichprecisely-temperature-controlled air will circulate. All thermal sources (electronics, etc.) are lo-cated outside this enclosure. The temperature of the inlet gas will be equalised with the chamber,before entry. Furthermore, a new design for the UV system has been developed based on opticalquartz fibres which eliminates the large thermal load present in the Mk1 prototype from the bankof UV lamps. This should allow UV burst experiments to be carried out without any appreciabletemperature rise in the chamber• Cleanliness: CLOUD is measuring processes involving ppt concentrations of trace condensablegases. This requires extreme attention to inner surface cleanliness, to the absence of leaks in thedetector and to the strict avoidance of unsuitable materials. The inner surface of the Mk2 chamberwill be electropolished, as will the stainless steel sampling probes and the stainless steel pipes thatsupply the chamber gases. The successful Mk1 ultra-pure air system from cryogenic liquids willbe re-used for the Mk2. A completely new gas system is being designed to humidify the ultra-pureair and add the trace gases. The homogeneity of the chamber contents has been optimised by theaddition of two internal, magnetically-coupled, fans located at the top and bottom of the chamber(Fig. 2). No plastic materials will be used in the chamber or gas supply system. The chamberseams are welded, and metal seals are used on all access ports, windows and feedthroughs. All the3Fig. 3: Horizontal section through the 3m CLOUD Mk2 prototype. The EastHall T11 experimental area is shown after modifications for the 2009 beamtests (notably, suppression of the gas alcove).Fig. 4: The bare 3m aerosolchamber, showing the ringof 16 sampling ports in themid plane and the accesshatches at the top and bot-tom.Control room(accessible during beam)gas rack /electronics area(accessible during beam)T11 beam area(inaccessible whenbeam on)T10 beam area(inaccessible whenbeam on)Fig. 5: East Hall T11 experimental area for the 2006 beam tests of the 2m CLOUD Mk1 prototype.4sampling ports/probes will be equipped with isolation valves (there were none for Mk1). The Mk2chamber will be bakable to about 100◦C and has the facility to be internally cleaned in situ.• Analysing instruments: The Mk2 will allow up to 16 external analysers to be attached to thechamber via sampling ports (Figs. 3 and 4). The chamber will also be equipped with optical portsto view droplets and ice particles created inside the chamber during activation studies (adiabatic ex-pansions). A more advanced range of analysers will be employed compared with the Mk1. Amongthe new instruments will be a proton-transfer-reaction mass spectrometer (PTR-MS) developed byU. Innsbruck for ultra-sensitive detection (ppt concentrations) of volatile organic compounds. Inaddition, new instruments are being developed in the collaboration to detect aerosols below the 3nm size threshold of current commercial particle counters.• Space for analysing instruments and local electronics: In order to avoid large transmissionlosses, the sampling lines into the analysing instruments must be not more than a few tens ofcentimetres in length. This implies that large instruments must be efficiently located in close prox-imity to the chamber. By rotating the chamber relative to the Mk1 orientation, and by suppressingthe gas alcove in T11 (Figs. 3 and 5), a greatly-improved layout can be achieved for the samplinginstruments. Discussions have taken place with the PS and East Hall coordinators on the T11 areamodifications, which are planned to take place during 2008.• Beam size: For the 2006 run, the T11 beam dimensions (1× 1.2 m2) were significantly less thanthe chamber (2 × 2 m2). This should be much improved for future operations since the apertureof the final dipole has been increased by 50%, and CLOUD will be located further downstream,allowing the beam to diverge further. However, further development of the T11 beam optics isdesirable to produce the largest possible beam size for CLOUD. For the next run, CLOUD willinstall a new counter hodoscope to measure the beam intensity and x/y profile, and also a cosmicray counter to monitor the hard and soft galactic cosmic ray components in the T11 experimentalarea.4 PLANNING4.1 Collaboration aspectsFull CLOUD collaboration meetings are held approximately twice each year. During the past year, thecollaboration has met in Helsinki (2–4 April 2007), Salzburg (12 September 2007) and Frankfurt (7–8 February 2008). The experiment is led by the CLOUD Steering Committee, comprising six seniorpartners and chaired by the spokesperson. All major policy decisions are made by the CollaborationBoard, which comprises each of the institute leaders within the collaboration. During 2007, and with theassistance of CERN management, a Collaboration Agreement was prepared and adopted for CLOUD. Itprovides the constitutional framework for the experiment and is based on similar documents for the LHCexperiments.Two new groups have recently joined the CLOUD collaboration: the University of Lisbon, Por-tugal (A. Amorim) and the Institute for Nuclear Research and Nuclear Energy (INRNE), Sofia, Bulgaria(J. Stamenov). Both groups are taking responsibilities in the Mk2 design and construction, namely in theUV system and DAQ (Lisbon), and in the Mk2 chamber and infrastructure (INRNE).4.2 ResourcesEU FP7 proposals: Two EU Framework 7 proposals were submitted in 2007 to support CLOUD ac-tivities: CLOUD-DS (CLOUD design study in the Capacities–Research Infrastructures–Design Studiesframework, 2.8 MC) and CLOUD-ITN (network of 10 Marie Curie fellows, 2.4 MC). The CLOUD-DSproposal was eventually rejected as being outside the scope of the call (CLOUD is considered by the EC5as an experiment, not a research infrastructure facility). On the other hand, the CLOUD-ITN proposalwas approved and the project will start in June 2008. It involves 8 Ph.D. students who will carry out theirthesis research on the CLOUD Mk2 prototype and subsequent upgrades, plus 2 postdocs, for a period of3 years (i.e. 30 person-years total). One of the postdocs will be permanently based at CERN.National funding: All of the analysing instruments for the Mk2 prototype will be provided by thepartner institutes, supported by their national funding agencies. The Lebedev Physical Institute has beenawarded a research grant to support CLOUD activities by the Russian Foundation for Basic Research(RFBR) under the CERN-RFBR agreement on scientific cooperation. The design manpower for theCLOUD Mk2 facility (aerosol chamber, thermal housing, field cage, UV system, gas system and infras-tructure) is being provided by PSI (10 person months engineering), CERN (engineering support and 6person months designer) and, for the UV system, U. Lisbon. The construction costs of the CLOUD Mk2facility will be paid from a common fund shared among the partner institutes and by in-kind contribu-tions.4.3 Beam request and scheduleAt the Frankfurt CLOUD collaboration meeting, 7-8 February 2008, the design, construction and plan-ning for the next CLOUD run in T11 was discussed. One of the key analysers is a CIMS (Chemical IonMass Spectrometer) that can measure H2SO4 concentrations at the sub-0.1 ppt level. There are only 3groups in the world at present that are operating such CIMS (of which one is in CLOUD). Unfortunately,the CIMS will not be available for the previously-planned T11 run in September–November 2008. Sincethis device is critical for CLOUD, we decided at the Frankfurt meeting to postpone the fall 2008 runin T11 until PS startup in 2009. Since this run will be a major data collection period for the 10 MarieCurie fellows who will join CLOUD in the second half of 2008, a spring 2009 run will also better matchtheir schedules. Furthermore, after discussions with several potential companies for construction andelectropolishing of the Mk2 chamber, the new requested schedule of spring 2009 also well matches theconstruction schedule.Our beam request is therefore as follows:• Postponement of the PS215/CLOUD 10-wk run from Sep-Nov 2008 until PS startup in 2009. It isour understanding that the 2009 accelerator schedule is not yet defined but, if it is similar to thisyear, this would indicate a beam request of May–July 2009.5 CONCLUSIONSCLOUD made further good progress in 2007. The 2006 beam data from the CLOUD Mk1 prototype hasbeen analysed and the first physics results from the experiment are ready for conference presentation.The engineering design for the Mk2 prototype has been prepared. The new detector incorporates manydesign improvements that were learnt during the Mk1 beam run in 2006, and represents a very largeadvance in performance relative to the first experiment. The financial and manpower resources for theexperiment have developed significantly—in particular with the successful EU FP7 award of a networkof 10 Marie Curie fellows who will carry out their thesis research on the CLOUD Mk2 prototype.It is foreseen that the 3m CLOUD Mk2 prototype will be capable of a broad range of importantphysics measurements over a period of 2–3 years, and its performance and physics reach can be sub-stantially developed over this period. After this time, it will be replaced with the final CLOUD facility,which will include an aerosol pressure chamber of around 4.5m diameter. This chamber will be too largefor the T11 area and will require installation in the downstream half of an extended T9 or T10 beamline.Preliminary discussions have begun with the PS and East Hall coordinators on the long-term plans forthe East Hall beamlines.6References[1] IPCC Fourth Assessment Report - Working Group 1 Report, “The Physical Science Basis” (2007).http://www.ipcc.ch/ipccreports/ar4-wg1.htm[2] CLOUD Collaboration, A study of the link between cosmic rays and clouds with a cloud chamberat the CERN PS, CERN proposal SPSC-P317, SPSC-2000-021 (2000); CERN SPSC-P317 Add.1,SPSC-2000-030 (2000); CERN SPSC-P317 Add.2, SPSC-2000-041 (2000); CERN-SPSC-2004-023, SPSC-M-721 (2004); CERN SPSC-P317 Add.3, SPSC-2006-004 (2006); CERN SPSC-SR-019, SPSC-2007-014 (2007).7

2007 Progress Report on PS215/CLOUD

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