Over two months since its stop in December, the LHC is slowly coming out of hibernation - even though the temperature of magnets in the LHC tunnel has been decreasing in recent days. The tunnel has been crowded with hundreds of people busy with maintenance activities and preparations for the restart. The end of most maintenance (and the access to the tunnel) is scheduled for 21 February, when the Operations team will take back ownership of the machine from the Programmed Stops Coordination team and push beam preparations forward.
The cool-down of all LHC sectors (left floating at around 80K during the Christmas break) restarted three weeks ago. At present, more than half the machine is at nominal cryogenic temperature and the completion of the cool-down is expected by 27 February. As soon as a sector is cold, the Electrical Quality Assurance (ElQA) team starts the high-voltage qualification of the superconducting circuits, which checks insulation and instrumentation integrity. These qualifications were started on the first Sector available (Sector 23) during the Chamonix workshop week, and have since been carried out on three more Sectors (56, 67 and 78) with no non-conformity observed.
Once a circuit has undergone high-voltage qualification, powering tests of the superconducting circuits can began. These tests officially started on 10 February, after the validation and preparation of the first sector. To minimize the impact on activities requiring tunnel access (which will continue for a few more days), the powering tests only happen during the evening and at night, to cope with safety constraints. The aim of the tests is to push performance of all LHC circuits to their operational level. The tests involve injecting current through the superconducting circuits while checking their protection mechanism – which are essential for the safe operation of the machine. After operating at 3.5 TeV for two years, the LHC is entering a new domain, with the main dipole and quadrupole circuits powered at a different current level for operation at 4 TeV.
Apart from small issues - and the need to debug tools we are using this year to improve test execution - the tests are progressing well. All superconducting circuits should be commissioned during the first week of March. A few days of machine check-out will then drive us to the first beam, planned for 14 March.