State of the magnets

23 April 2009

End-cap and ends of the toroid coils



Over the last few months, the ATLAS magnet crew have been using the winter shutdown period to tie up a few loose ends and polish an already well-functioning system.

“We took the opportunity to invest and make things a bit more modern,” says Magnet System Project Leader Herman Ten Kate, referring to the upgrade work that has been done on the vacuum system. The few piston pumps that remained have now been replaced with modern screw-line apparatus that is effectively maintenance-free.

“When you were on the floor of the cavern a few weeks ago, you could still hear them: chump-chump-chunk,” says Herman. “Screw-line is nicely rotating so it’s almost noiseless. It’s more reliable, has no oil inside and is practically vibration-free.”

Elsewhere, the team fixed a few hotspots that had arisen in the magnet bus-bar system during running, and undertook a general review of the computer control system. “We took all the sensors and modernised them and harmonised the layouts on the synoptic controls panels, made it even more perfect than it already is…” smiles Herman.

After the LHC incident on September 19th, the magnets continued running for a few weeks before going into an early maintenance period on October 20th. The cryogenic system – comprising a main refrigerator and a shield refrigerator – was shut down in stages, in order to maintain the magnets in their cooled state.

“The system runs at four Kelvin, but when it is not in use, we switch off the main refrigerator and keep everything cold at about 50 Kelvin with the shield refrigerator. It’s important for the superconductive system,” Herman explains. Huge amounts of energy would be required to re-cool the system from ambient temperature if it were allowed to heat up fully, and the expansion and contraction associated with the heating and cooling would also introduce additional risk to the system. “Once it’s cold, you want to keep it cold for 15 years,” says Herman. “But still you have to do the maintenance on the two cooling machines.”

To that end, the main refrigerator – which is responsible for making liquid Helium at four Kelvin – underwent its standard maintenance before Christmas. During this time, the shield refrigerator – which operates as an intermediate layer at 40-50 Kelvin and takes the main cryogenic load of the system – remained in operation until weeks seven to nine, when all the detector services were shutdown. For both systems, the maintenance occurred on the surface in the compressor room (building SH1, the room to the left of the elevator shaft), the first floor in the services cavern where the refrigerators are housed, and in the main cavern in the proximity cryogenics area  and. The detector itself was therefore undisturbed.

The cryogenic plant was restarted before Easter, and it is now on the long slog down to four Kelvin, in preparation for the whole detector continuous cosmic run planned for the second half of June. “Right now it’s foreseen that the detector is closed in the second week of June, then we can start to play with the magnets,” says Herman. “This means the magnets are cold and we can energise them partly.” The time will be used to take measurements mapping local distortions in magnetic field, which is crucial for the muon system to be able to reconstruct the positions of the magnets accurately.

There will be a brief interruption to the continuous running in July, when the plan is for a few  missing muon chambers to be installed in the forward direction. “We’re removing three of our diffusion pumps on the Barrel Toroid because they’re in the way,” explains Herman, “but removing the pumps means that people are touching our vacuum system ... If you break the vacuum, you have a big disaster, so we have to be very careful.” In August, the magnet team hopes to go into continuous running right up until beam.

Concurrent to the physical maintenance, members of the magnet group have, for the first time, had time to study the magnet testing data gathered over the last few years. They’re addressing questions such as where, in detail, energy is dissipated to within the cold mass during a magnet quench. This, and investigations into other interesting questions, is ongoing.

 

 

 

Ceri Perkins

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