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Saturday, October 13, 2007

LHC( Large Hadron Colider) UPDATES

ATLAS big wheel triumph

ATLAS celebrates installing the last of the eight big wheels. These wheels form the end-cap muon spectrometer of the detector.

The team and guests at the celebration stand between two of the eight 25-metre diameter big wheels.

The installation of the last sector of the ATLAS muon big wheels.

The ATLAS end-cap muon detectors, the 'big wheels', have been compared to many things: flowers, orange halves, clock faces and works of art. But, more importantly, each is an incredible feat of engineering. On Friday last week the team celebrated the completion of the last wheel, and moved into the final stages of installation.

"I must admit that at the end of last year I would not have believed that we would manage to install these eight big wheels essentially on schedule," admitted Peter Jenni, ATLAS spokesperson. The first of the wheels took a long time to install, but the last one took just a couple of weeks. "This is really a great achievement."

The big wheels harbour ATLAS's middle layer of muon chambers in the forward region and are one of the last large pieces to be installed. Each is 25m across, weighs between 40 and 50 tonnes and contains around 80 precision chambers or 200 trigger chambers. The support structure itself is just one third of the weight of the total wheel.

Because of their sheer size, each wheel had to be made in 12 pieces for the trigger planes and 16 pieces for the precision-measurement planes, or "petals", of aluminium, the last of which was installed on Friday. Each was assembled at CERN using components from all over the world before being fitted together, piece by piece like a jigsaw.

Because of the need for space for the chambers, designing a suitable structure presented a unique challenge, one that project engineers, Raphaël Vuillermet, Dimitar Mladenov and Giancarlo Spigo were happy to take on.

"The detectors themselves have been on drawings for 15 years; everyone knew where they would go but no one knew about the structure," explains Dimitar. "There were chambers everywhere so our design had to build around them and in the small spaces in between them."

The result after 3 years of calculation, design and sleepless nights was a uniquely thin and light structure that is precise to less than a millimetre.

The 100-member collaboration from Israel, Japan, the US, China, Russia, Europe and Pakistan began assembly of components in 2005 and installation in 2006. "Because the pieces are so delicate we had to be careful throughout the whole process," explains Raphaël. "I was very afraid about something happening to the chambers and also to the people, because you are working 30 metres up. But we didn't have any problems."

The completion of the big wheels is symbolic for ATLAS because, as technical coordinator Marzio Nessi explains, "the big wheels were always seen as something we would do at the end. And now we have done them."

For Dimitar the biggest challenge was the timing. "I feel proud, but not for myself, for everyone. It was the result of hard work. The only thing that we were lucky about was the weather; if there had been a single day of heavy rain we might have been delayed. But Marzio said not to worry and to leave the weather to him, and the weather was great. I don't know how he did it!"

Now just two smaller scale wheels and the end-wall chambers remain to be installed, and the big wheels have already begun to give read-outs as part of test runs using cosmic ray data that ATLAS performs every six weeks.

With their striking symmetry and aesthetic appeal the big wheels are likely to become icons of the experiment. But to Marzio, all pieces of ATLAS are beautiful. "This piece just happens to be 25m high."

A word from the DG: LHC commissionning 
enters the home straight

In an age of blogs there are seemingly no secrets, so by the time Lyn Evans gave his talk on the status of LHC commissioning on 13 September, everyone seemed to know about plug-in modules, beam position monitors and transmitters embedded in ping-pong balls. All the on-line speculation made for interesting reading, and is a clear sign of the growing interest there is in CERN as we approach LHC start-up. We are now entering the final phase of commissioning, and things are going well given the unprecedented complexity of the task in hand.

Following the cool-down, powering and warm-up of Sector 7-8 earlier this year, we have learned a great deal about what it means to commission the LHC. There have inevitably been hitches, including the plug-in modules, or PIMs. When the LHC is cooled down, each sector shrinks by about 10 metres in length, and this has to be absorbed by bellows between components and a system of sliding copper fingers (PIM) that ensure electrical connectivity around the ring. When warming up Sector 7-8, a small number of fingers buckled as the machine expanded and are being repaired. The problem is understood, and concerns only a small percentage of the PIMs. To identify precisely where the problem occurs, an ingenious system involving blowing an object like a ping-pong ball with a 40 MHz transmitter (the frequency of the beam bunches seen by the position monitors) along the beam pipe has been devised.

Lessons learned from Sector 7-8 are being put into practice in other sectors. A second sector has been rapidly cooled to 80 degrees above absolute zero, a third is undergoing pressure tests, and testing of the remaining five sectors will now start at the rate of one every two weeks. In the sectors currently under test, vacuum leaks have been isolated and are also being repaired.

Meanwhile, the repair of the LHC's inner triplet magnets is complete. A team from CERN, Fermilab, KEK and the Lawrence Berkeley National Laboratory has successfully completed the repairs. So far, three of the eight triplets have been installed and successfully pressure-tested in the tunnel. The remaining triplets are in the process of installation and pressure testing.

All of this is business as usual when bringing a new particle accelerator on-line. There are inevitably hurdles to be overcome, but so far there have been no show stoppers. We can all look forward to the LHC producing its first physics in 2008.

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