Around 1700 magnet circuits are needed to circulate beams in the LHC. Come 2015, each and every one of these circuits will have to be able to accept their 7 TeV equivalent current. For the LHC’s 24 main dipole and quadrupole circuits, this will mean the consolidation of all their interconnections. But what about the rest of the LHC’s circuits that had been mostly operating at around 60% of the nominal value? How will they handle the ramp-up to design energy? Those questions were asked and answered during the recently completed series of powering tests.
“We looked for any weaknesses in the circuits while they are still at cryogenic temperature,” explains Mirko Pojer, Engineer in Charge of the LHC. “In 2008, we noted a few difficulties in individual magnets at 7 TeV. The recent tests allowed us to identify which of these issues should be addressed during the Long Shutdown 1, which could require the entire magnet to be replaced, and where we need to intervene on the splices of the small-current circuits.”
The tests also gave the team a chance to identify the limits of all the circuits. “Not every circuit needs to be operated at nominal current in order for the machine to run at 7 TeV,” continues Mirko. “We found a few circuits that cannot operate at that level, and now we know how far we can push them without a loss of performance or a current trip.”
The powering tests were carried out from the CERN Control Centre (CCC), where operators were able to bring the superconducting circuits up to 7 TeV just as they would during operation. The team began by powering the entire machine, and then went sector by sector once issues were identified. In 10 days, more than a thousand tests were performed on 540 circuits. “We did come across a few surprises,” says Mirko. “We were able to resolve all of the smaller issues – for example, a failing power converter – through immediate intervention. We also identified a number of circuits that will require more detailed analysis and possibly an intervention during LS1.”
Once the circuits in a sector had been signed off on, the Electrical Quality Assurance team started testing the electrical insulation of each magnet with respect to earth. For these tests, engineers apply a high voltage between the magnet coil and earth to ensure it is perfectly electrically insulated. This second type of electrical test began on Friday 22 February, in parallel with the powering tests, and will take 8-9 days per sector. Once completed, the warming-up of the magnets will begin at long last.