lunedì 13 luglio 2009

CERN,LHC: ATLAS 'til beam

“We start to run eight weeks before the day of first beam,” says Christophe Clement, Run Coordinator. Originally planned to start on August first, he now foresees this in early September.The first four weeks resemble the spring’s slice runs. “We test various functionalities,” he explains, “make sure everything is running as it should.” This time, the muon system starts the run, joined by the calorimeters, then the inner and forward detectors. The fourth week is devoted to the high level trigger.“Then we start running continuously,” says Christophe. ATLAS will run without beam for four weeks. Then, we’ll see more beam splash events. And after a few more weeks, with luck, the very first LHC collisions.This week, Run Coordination makes its call for shifters during the start-up period. Before those shifts begin, much work has been scheduled in the cavern. The muon system will see one new TGC chamber and a few EE chambers. Also, access structures are to be installed, and the cabling will be better organised. LUCID is set to receive its LUMAT electronics, luminosity and monitor trigger cards that also serve for readout, moved from May to July. “There were some repairs on PMTs and fixing some gas leaks too,” says Marzio Nessi, Technical Coordinator. The evaporative cooling distribution racks can expect an upgrade to boost the detail with which the system can be controlled and increase its reliability. The extra weeks before start up bought this project time to finish as some hardware arrived late, and a few unexpected problems arose during assembly. Carbon dioxide flushes out the Transition Radiation Tracker, but some of it may be lost in the detector, “escaping the ID front plates and entering the muon detector,” according to Marzio. Although even this leakage is well within safety range, the safety team is adding extraction pipes for the excess carbon dioxide to the lower part of the detector. “This is part of the consolidation plan and is part of a plan to minimize unnecessary risks,” he says. And, as usual, the cavern will receive a final cleaning to get rid of any loose magnetic material, during the first half of August, and forward shielding will be installed. New octagonal shielding has been added this year, to be placed around the previous forward shielding.The rest of the work is concerned with how ATLAS will handle data. The operating system, DCS, has just been upgraded to its final version. Christophe notes: “It’s never final because they keep improving the control system all the time, but at least there shouldn’t be major patches unless there is a serious problem.”The slice tests were a bit nerve-wracking, since the systems were integrating with new software for the first time. However, come August, the only all-new version will be the high level trigger (HLT). One aspect under improvement is the trigger menus, which list criteria on which collision or cosmic events are recorded.The front-end and read-out electronics subsystems will be facing high rate tests before beam, inundated with “fake” randomly generated triggers to beef up the real data from cosmic rays. In particular, each community needs to ensure that its subsystem can handle a data rate of up to 50 kilohertz. Also, ATLAS is still undergoing clock tests. Most systems have the high reliability required for these tests, but new readout drivers have recently been installed for the Cathode Strip Chambers (CSCs) of the muon system. The CSCs have been left out for now but will join later, during the summer. “Some systems are completely immune, and some systems are not,” Christophe says. The central Level 1 trigger team has found ways to recover or reset smoothly when the LHC’s clock stops ticking.For Run Coordination, one of the most critical aspects of preparing for beam is the combined run. “We find various problems which are not seen when subsystems run standalone,” says Christophe. And what seems an acceptable reliability for a subsystem on its own is not enough for the 12-system ATLAS detector: “Even if a subsystem breaks the run every day or second day, it’s six to twelve breaks a day for ATLAS.” But with four weeks set aside to get rid of any lingering difficulties with running ATLAS all together, the detector should be fully prepared for beam when it arrives.
Katie McAlpine
ATLAS e-News

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