LHC Report: Rocky Recovery

The last technical stop finished on Friday 8 July, but the machine returned to its pre-stop performance level over a week later.

Efficiency of LHC fills between 16 July and 20 July, 2011.

The cryogenics team had the entire ring cold by Saturday morning and the usual post-technical stop tests with circulating beams started soon after. Unfortunately, they were interrupted by a major perturbation to CERN’s electrical network caused by an impressive thunderstorm that swept over the Pays de Gex. There were major knock-on effects, including the loss of cooling to the cryogenics and an inevitable recovery period once normal service had been re-established. The beams were circulating again by Tuesday afternoon and the post-technical stop checks continued, beefed up with further tests to address a number of issues related to the power cut. 

Before the stop, the LHC had managed to get 1380 bunches per beam into collisions and the plan was to ramp back up relatively quickly to this level via fills with 48, 264, and 840 bunches per beam. 48 and 264 bunches passed off reasonably smoothly. However, 840 bunches proved more difficult with three consecutive fills lost at high energy due to UFOs.

UFOs are a regular feature of operation and they sometimes appear in bursts around one hour after the injection process. Usually these UFOs generate losses below the dump threshold, but following the stop they were stronger than usual. Operations opted to wait at 450 GeV for an hour to let the UFOs pass before ramping and this allowed for three fills - one with 840 bunches and two with 1092 bunches - that led the way back to physics with 1380 bunches. The evening of Monday 18 June saw a 1380 bunch fill go into 'Stable beams' with an initial luminosity of between 1.4 and 1.5 x10³³ cm^-2s^-1 – another new record. The UFOs have since quieted down.

With 1380 nominal bunches, the LHC is now dealing with around half design intensity. There are ongoing issues with UFOs, the effects of radiation on electronics, vacuum activity and so on. Mitigating these effects and learning to live with high intensity is all part of the process, and the current aim is to re-establish stable conditions and realize the potential of the machine to deliver some serious integrated luminosity.

by Mike Lamont for the LHC Team