ATLAS e-News
23 February 2011
News from the pit
29 November 2007
One of the end cap magnets
It has been another busy month in the pit. The ATLAS magnet team took a significant step towards finishing work on the ATLAS detector as testing of the End Cap Toroid magnets began, but an incident during the testing process has likely only slightly damaged an end-cap calorimeter, according to an on-going investigation.
The Barrel Toroid magnet was tested under optimal conditions in November of last year. The end cap magnets make up the rest of the ATLAS toroid magnet system. Each magnet is superconducting, and so both need to be cooled to 4.2 Kelvin and maintained in a good vacuum for optimal working conditions.
“It is the last magnet being tested on ATLAS before nominal operation, so this is a major milestone,” says Arnaud Foussat, magnet engineer from CERN PH-DT1.
The end cap toroid magnets were installed in the experimental cavern in the summer of 2007. AT-ECR, PH-DT1 and the ATLAS Magnet group worked hard last month to cool the magnets to 4.2 Kelvin. The cooling process took five weeks because the magnets are so large – the cold mass of each is in the region of 130 tons.
Now that the magnets are at optimal working conditions, testing has begun. “We had to make sure that all the instrumentation and magnet safety systems were operational before the power test,” says Arnaud.
Up to 200 megajoules of energy is stored in each magnet, and there are safety systems to help dissipate that energy away from the magnet in case a hotspot appears somewhere within the coil system. In the middle of November, the ATLAS magnet team fed a current of 4 Kiloamperes through one magnet, and they planned to gradually ramp up the current to the nominal value of 21 Kiloamperes.
Forces
“We expect large attraction forces between the two. We have to monitor those forces and make sure they are uniform so that there is no potential damage to the structure,” says Arnaud. On 22 November it became clear just how strong those forces could be. During testing, the magnet was not sufficiently braced, and moved several centimetres. “We believe what stopped it moving further was the calorimeter,” says Luis Hervas, LAr Installation Coordinator from CERN PH-ATA.
It is still unclear if the calorimeter was significantly damaged during the incident. The cryoline, a flexible line supplying the end-cap calorimeter with argon, appears to have borne the brunt of the magnet's impact. The cryoline comprises two concentric tubes, one inside the other, and the outer has been damaged. “If the inner line has also been damaged then the calorimeter will have to be re-warmed,” says Luis, a process that takes several weeks. “But from what we can see now, the electronics of the calorimeter crates are probably undamaged,” adds Luis.
Over the next few days the most important task is to move the magnet back without causing further damage. When the magnet is in its outer position, it will be easier to assess the damage to the calorimeter. More details will be given in ATLAS e-news next week. 