Even the most cutting-edge machines require moments of respite. That’s why, in the early morning of Monday, 19 June, LHC operation was paused for one week to allow the technical teams to carry out preventive and corrective maintenance on the machine and its subsystems.
One week earlier, in the afternoon of Tuesday, 13 June, the beams were dumped, marking a break in a successful period of luminosity production to switch to a busy and tightly scheduled machine development (MD) programme, with no fewer than 14 different topics to cover, including studies on operating crystal collimators during the energy ramp and beam response, analyses of the diamond-detector-based beam-loss monitors, beam dynamics to better understand slow beam degradation from electron cloud effects, and beam instability measurements with different bunch intensities. In addition, time was allocated to set up cycles and beams for future physics runs that are scheduled after the technical stop.
For each MD, a procedure is drawn up by the proponents, detailing the goals of the MD and the required beam parameters and machine settings. These procedures are then scrutinised by the LHC Studies Working Group (LSWG) and restricted Machine Protection Panel (rMPP) before the topics are selected and scheduled in the MD slot planned in the yearly LHC schedule. Final approval is then given by the LHC Machine Committee (LMC). Although these MDs take time out of potential physics time, they are very valuable in order to better understand the machine and beam behaviour with a view to increasing beam performance not only during Run 3, but also post-LS3, for the HL-LHC era. One could say that they are a worthwhile investment for future luminosity production.
As I write, the machine is in the hands of the Technical Coordination team (EN department) to carry out the many technical stop (TS) activities on the entire LHC and in the experimental caverns. Of the many planned activities, two were already mentioned in the Accelerator Report of 6 April. The first is the reinstallation of the crystal collimator that broke and had to be removed from the ring during the last phase of hardware commissioning at the end of March. This activity is the reason why the TS was extended by a day – to complete the bake-out, pump-down and hardware tests of the crystal collimator. The second activity is the preventive replacement of the two rupture discs installed in April by two discs that have passed the recent pressure test.
The plan is that cryogenic conditions will be restored in the afternoon of Friday, 23 June; the LHC will then be handed back to the Operations team (BE department) at 16.00 to restart all the subsystems. However, the beam will remain off until the end of the afternoon of Saturday, 24 June in order to finalise the crystal collimator reinstallation. Once that is completed, beam operation will be re-established, initially for special physics runs and later for luminosity production.
The (preliminary) outcome of the MD studies will be presented at the LSWG on Tuesday, 27 June – an opportunity to discuss and gain a greater understanding of the machine and beam dynamics with a view to keeping the LHC’s performance strong and increasingly efficient.