What is happening in ALICE during the shutdown
In order for the LHC and all the experiments to work properly throughout the year, certain changes need to be done. The right time for that is the year-end technical stop, or YETS. It usually begins in the last working week in December, right after the heavy-ion run, and lasts for about 10 weeks.
Nearly all the detectors in ALICE are carrying out maintenance at the moment, but amongst all this work there are three crucial activities: on the time projection chamber (TPC) readout electronics, on the low voltage power supplies and on the inner tracking system (ITS) ventilator.
The most critical upgrade is the replacement of the TPC readout control units (RCU boards) with new ones. “These are basically printed circuit boards with an FPGA, pin grid array and many other components. They were produced and tested last year. They are a different kind of cards in the sense that they implement a new readout scheme, which will allow to read out the TPC at a much faster rate” – says Arturo Tauro, deputy technical coordinator of ALICE.
As it turns out, this intervention is quite tricky. There are more than 200 cards, located in 36 sectors. Since the space in the TPC is limited, only two people can work on each side of the detector. First they need to open all sectors one by one, then to access the cards by disassembling the electronics, removing the fibers and the cables, and finally – to replace them. And, of course, before closing the sector, every card in it has to be tested.
Another important change, on which technicians have been working, includes the low voltage power supplies. Almost all of the detectors, not only in ALICE, but in the other experiments as well, use such supplies. A rupture in the copper connectors of the cooling parts in some of them has been observed, which is due to cracks in the material they are made of. Since the water pressure in them is a few bars, an eventual failure of these connectors could have severe consequences. If one of the top breaks, the water spilling can damage a series of power supplies. And if that happens during the run, it means a loss of important data.
That is why a replacement of the fittings is needed. To do that, every power supply is being removed from its rack and brought to the surface, where a pool of trained technicians does the repair.
The silicon strip detector (SSD) in the inner tracking system of ALICE is generally cooled by water. In addition to that there is an air flow, which is used to keep the humidity level in the detector low. If it goes above a certain value, it would simply damage it. That is why the cooling group of CERN has redone the ITS ventilator, so that it can give a better stability of the humidity. “The new machine is now installed, but there is still a lot of work on the equipment that needs to be done” – says Arturo.
In the meantime different CERN groups – electrical, IT, general infrastructure services and many more, are carrying out interventions in mutual agreement with the collaboration.
As you can imagine, proper coordination is crucial for a project of such scale. As Arturo says: “Some of the interventions can be done in parallel. It’s important that we know when we can access the pit and when we cannot. A part of the shutdown is to do maintenance on all the parts that people do not see during the operation of the experiment, but which are vital. When they are not seen, that means that they are working well. And they are working well only because people are taking care of them during the YETS.”