Today, almost the whole accelerator complex is operational and providing beams to all the experimental facilities as scheduled. The LHC experiments are taking data, while the LHC is nearing the end of the intensity ramp-up phase; already last weekend it was colliding beams with 1200 bunches per beam, one week earlier than initially scheduled. As I write, beams with ~1800 bunches are in collision – the last step before the full machine is filled with ~2400 bunches per beam.
The only physics that has not yet started is at the Antiproton Decelerator (AD), which was initially scheduled to start on 11 May, but unfortunately had to be delayed due to a technical problem. On 14 March, during the hardware recommissioning of the antiproton complex concluding the year-end technical stop (YETS), a water leak appeared in a special quadrupole magnet in the AD machine. The leak, situated at the entry of the insulated magnet coils, could not be repaired in situ, which meant that the roof of the AD tunnel had to be opened and the magnet removed for repair in the magnet workshop. The coils were exchanged, the magnet was tested and the magnet field maps were measured. After full validation, the repaired magnet was re-installed in the AD tunnel on 28 April, which was followed by electrical and vacuum reconnection. After the initial vacuum pump down, vacuum leak detections were performed with success. Since the vacuum in the AD machine needs to be of very high quality, the vacuum chamber and associated equipment had to be baked-out. This is a more than two-week-long process that started last week: the vacuum chamber and associated equipment for the whole vacuum sector concerned are heated up to evacuate and pump the residual gas molecules, including those from the surface layer of the vacuum chamber. Upon completion of the bake-out, cooldown is needed, equipment needs to be removed and the machine made ready to be commissioning with beam.
While various teams were busy repairing and validating the magnet and its reconnection, the AD-ELENA operations team continued the hardware recommissioning of the other parts of the AD, while also performing ELENA beam commissioning with H- ions from a local ion source with the aim of minimising the time lost and being as efficient as possible for the recommissioning of the antiproton beams for the experiments.
This means that the beam commissioning of the AD will start on 12 June and be compressed to aim for delivery of antiprotons from ELENA to the eagerly waiting AD-ELENA experiments on 30 June.
All the technical teams at CERN work hard during the YETS to execute the huge task of corrective and preventive maintenance in addition to consolidation and upgrade activities. Thanks to their efforts and high-quality work, most of the accelerator complex was recommissioned efficiently and delivered nearly all the required beams on – and in some cases ahead of – schedule. Sometimes, nevertheless, a single component that had previously functioned well and showed no signs of weakness can cause problems that force us to change some of our plans.