lunedì 13 luglio 2009

CERN(LHC/ATLAS): Repairs and upgrades.

Steve Myers spoke to a full house about the LHC status on Thursday, July 2nd. “The last time I’ve ever seen the auditorium as full as this,” he joked, “it was a very animated discussion about the closure of LEP.” The main auditorium offered standing room only, but the presentation was available via webcast around CERN and the rest of the world.He opened with a recap of the progress in sector 3-4. Thirty-nine dipoles and 14 quadrupoles have been reinstalled, lowered down the shaft and delivered to their segments of the tunnel. Thirty-seven of these were replaced with spares while the accelerator team repaired the remaining 16 in SMI2. “This has been done without a single accident,” Dr Myers notes of the tight movements through the tunnel. The last one went underground on April 30th, and the final electrical connections were made in early June.During the incident last September, soot and debris of the multilayer “super insulation”, which protects the cryogenics, entered the beam pipe due to the electrical arc and helium explosion. Seventy-eight per cent of the pipe in sector 3-4 was contaminated. The worst of it, nearest the arc, was replaced with the magnets, and the rest of the beam pipe in sector 3-4 has received a careful cleaning. In total, 2.4 km of vacuum pipes were cleaned. The accelerator group is also upgrading the quench protection system (QPS) to monitor connections between and within magnets, with new functions layered over the existing system. They will proceed with caution in powering the magnets to test the upgrades.Two major efforts have been undertaken to prevent similar incidents: improving the detection of possible bad connections between magnets and adding pressure relief valves for increased protection.The superconducting cables run along copper stabilisers in the bus bar connections. These bus bars need to touch at the interconnects between magnets, copper as well as the superconductor. If the copper stabilisers aren’t connected, then they can’t do their job during a quench: taking on the current from the no-longer-superconducting cable. At a bad connection, the current would dodge from the copper back into the superconductor, causing melting or electrical arcing – as we learned the hard way last autumn. The accelerator team has been making sure the copper is properly soldered at all interconnects.The LHC group has also prepared the magnets in the event that helium gas is suddenly released into the isolation vacuum once more. “We decided at Chamonix this year that we would install these new pressure release valves,” said Dr Myers. The accelerator group has placed valves on the top sides of the dipoles and quadrupoles in sectors 1-2, 3-4, 5-6, and 6-7. Valves on the bottoms of the inner triplets and main cryogenic boxes will protect them from a gas overpressure as well.Several tests were conducted on all warm sectors. They revealed ten dipole interconnects with resistances higher than 35 micro Ohms. These have been repaired, along with ten quadrupoles with resistances higher than 80 μΩ.Sector 4-5 was warmed to 80 K to be tested as well, and this investigation revealed a splice with a suspiciously high resistance. The team decided to warm sector 4-5 to 300 K for further testing. Previously, noise had made the measurements difficult – problem splices with under 40 μΩ of resistance couldn’t be spotted. Opened two weeks ago, two splices were repaired on the first of the month. They ran a second set of tests at 300 K with better temperature stability, to control the noise and started closing last week.Sectors 1-2 and 6-7 have seen dipole magnets replaced, while sector 5-6 had a connection cryostat repaired. Sector 1-2 had its cool-down delayed by two weeks in order to test some suspect interconnects while warm. This leaves three sectors that have yet to be tested. The outcomes of the warming of sector 4-5 and the tests to the remaining sectors will determine the energy at which the LHC can safely run this year. Once those results are in hand, in early August, the team will have to decide whether it’s worthwhile to take the time to repair – or whether it is best to run at a lower energy.But for now, we can still be optimistic as Director General Rolf Heuer advised. Once the situation is assessed in these last three sectors, the decision will be made on the “preferred scenario,” as Dr Myers put it: “the highest possible energy associated with the earliest date.”
Katie McAlpine
ATLAS e-News

Nessun commento: