A Distribution Feed-Box (DFB) brings power to the LHC magnets and maintains the stability of the current in the superconducting circuits.
The LHC was the last machine to be handed back to operators after the completion of maintenance work carried out during the Year-End Technical Stop (YETS) that had started on 14 December, 2015.
During the eleven weeks of scheduled maintenance activities, several operations took place in all the accelerators and beam lines. They included the maintenance in several points of the cryogenic system, the replacement of 18 magnets in the Super Proton Synchrotron; an extensive campaign to identify and remove thousands of obsolete cables; the replacement of the LHC beam absorbers for injection (TDIs) that are used to absorb the SPS beam if a problem occurs, providing vital protection for the LHC; the dismantling and reinstallation of twelve LHC collimators in order to modify the vacuum chambers, which restricted their movement; upgrades to the beam instrumentation, including various beam monitors, with further upgrades scheduled for the next Extended Year-End Technical Stop (EYETS) starting in December; and several electrical maintenance operations to ensure a stable operation of the machines.
The YETS also gave the experiments the opportunity to carry out repairs and maintenance work on their detectors. In particular, for ATLAS, this meant fixing the vacuum chamber bellow and installing new cables for triggering and controlling, as well as new water-cooling cables; at CMS, the cold box, which had caused problems for the experiment’s magnet during 2015, was cleaned and various water leaks on the site were fixed.
Bringing back beams to the machine after a technical stop of a few weeks is no trivial thing. The Electrical Quality Assurance (ELQA) team needed to test the electrical circuits of the superconducting magnets, certifying their readiness for operation – that is, their capability to withstand the high voltages that might occur during powering. All specified circuits were successfully validated after fixing some very minor non-conformities. The teams also checked the correct functioning of the redundant powering that is used in the event of an electrical power cut to protect critical systems in the machine. During the checks, some critical problems appeared; they will be fixed before the machine receives beam.
On 4 March, after completing all the preparatory activities, the powering tests of the superconducting circuits were able to start. These are joint effort of many groups across three different departments – including experts on magnet and power converters, protection and interlock, planning and operation – and this requires good collaboration, preparation and coordination of the various activities. In less than two weeks, over 7000 tests are being performed on the 1600 circuits. Even though the tests are executed automatically, the experts in charge of running and analysing them need to pay careful attention to the thousands of multi-coloured signals on their screens.
At present, more than three quarters of the tests have been performed, proving the capability of the circuits to reach the values needed for operation during Run 2. The target is close, but attention has to remain high to be able to start the final checks in just a few days before accepting beams in the machine. Soon, yet another commissioning period will be over with the beams expected back in the LHC in a couple of weeks.