News from the Pit

5 May 2008

One of the end cap magnets

At the end of May, the final commissioning tests of the ATLAS magnet system will start. If everything goes to plan, the tests will run till the end of July and will leave the magnet ready for operation.

The purpose of the oncoming tests, explains Alexey Dudarev, senior magnet engineer with the ATLAS magnet team, is to prove that all three superconducting Toroids can simultaneously reach the nominal current of 20,500 A that will provide the required magnetic field to the ATLAS experiment.

"The current situation of the system is that the three Toroidal magnets are cooled down to liquid helium temperature and are waiting for current," Alexey says.

Prior to the oncoming tests, End Cap Toroid A (ECTA), End Cap Toroid C (ECTC) and Barrel Toroid (BT) magnets were tested separately. The test of the BT was carried out at the end of 2006, and it showed it could perform at nominal conditions. In November 2007, the ECTC and ECTA magnets were tested at lower currents, 15 kA and 10 kA respectively.

In the tests scheduled for the end of May, both End Caps will be tested separately at higher currents: "We are going to power each magnet to 21,000 A to make sure that there is some margin." The ATLAS magnet team will gradually ramp up the current in successive steps of a few kA each. At the end of each step, a fast current discharge will be initiated.

One of the reasons of the increase of current in steps is to make sure that the magnets safety systems work well: "In case a resistive zone appears somewhere within the superconducting coil, we have to test that the 0.25 GJ of stored energy in each Toroid magnet can be safely distributed within the magnets cold mass to prevent the system from high thermal stress." Alexey says.

After testing the two End Caps separately, the system will be ready for a combined test of the ECTA and ECTC magnets at 20,500 A. This step is also crucial in that it is designed to avoid damage to the magnets when they operate together: "As the two End Cap magnets are electrically connected in series, in the case of power failure, the stored energy which may go into one End Cap can be doubled," says Alexey.

After all the preparatory tests, the main test of the entire system will start in July, with all the ATLAS Toroid magnets connected together and a total stored energy of 1.6 GJ.

During the commissioning phase, the ATLAS magnet team has to deal not only with problems arising from high magnetic energy, but also from the associated mechanical forces working inside and between the magnets. Thus, these forces in the magnets should be continuously monitored and made uniform by proper shimming: "The interaction structure is supporting an axial force of 240 tons between the End Caps and the Barrel Toroid coils," Alexey says. "If this structure fails, the force can cause damage to the magnet.". Yet another good reason to charge the system to full load in many steps.

In addition to the technical difficulties inherent to the tests, the team will have to work mainly in night shifts, as magnetic field can only be applied during the night to allow non-stop works on other systems of the ATLAS experiment: "And if we want to have the system ready by the end of July, we have to work also weekends," Alexey says "But we expect to be successful."




Cristina Jimenez