On Thursday, 13 April, the extended year-end technical stop (EYETS) was officially declared complete and the machine handed over from the team coordinating the technical stop to the Operations team.
However, before starting operations with beam, the hardware had to be re-commissioned. This operation included cooling down the sectors and performing the electrical quality assurance and powering tests.
To do that, all sectors have been progressively cooled down to their nominal operating temperature, 1.9 Kelvin, over the last few weeks, whereas they had been kept at about 20 K during most of the EYETS.
As soon as the sectors were at nominal operating conditions, the electrical quality assurance (ElQA) test could take place, to check the integrity of the electrical insulation of the magnets. This test concerned only the three main (13 kA) circuits for all sectors except for sector 1-2, where all circuits had to be qualified because a magnet has been replaced. It was right in this sector that a defective instrumentation cable for magnet protection was found in the warm part, possibly damaged during the heavy activities in this sector. The problem was fixed and this last sector finally released.
Once the ElQA test had been completed, the power converters and quench protection systems could be unlocked. The circuits were then ready to be powered.
The powering tests of the LHC superconducting circuits, taking place in the second half of April, are a big part of the re-commissioning, since almost 1600 circuits have to be switched on and tested in just few weeks.
These tests aim to verify the correct behavior of the protection functionality of each circuit and confirming the capacity of the magnets to reach the currents at which they will be operated along the year. This is a crucial phase in the preparation of the machine, as a protection failure could result in a catastrophic event; if a circuit fails to reach the desired current, it could limit the performance of the machine during the year. About 10 000 test steps are executed, powering circuits at currents rating from 60 A for the small orbit correctors to 13 kA for the 3-km-long dipole and quadrupole chains.
An incredible amount of work has been done in the last few years to continuously optimise the powering tests and to automate their execution and analysis. This effort has resulted in a reduction in the time needed to qualify the entire machine from several months down to few weeks. Nevertheless, a large number of tests have to be performed in less than three weeks and lots of information has to be processed and analysed by the experts. That’s why, for the execution of the tests, we rely strongly on several software applications and many equipment experts. After heavy software debugging, due to the numerous modifications done during the EYETS, the powering tests were able to start and are now progressing at a good pace: seven sectors have already been qualified and the tests in sector 1-2 (delayed due to the fault investigation) are at a good stage. All superconducting circuits will be ready before the end of the week and the machine check-out (the last phase of verification of all systems working together) will finalise the preparation for the beam.
After four months of frenetic work at every point of the machine, all activities are close to completion and the LHC is now getting ready for a new, long year of physics.