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Accelerator Report: Overcoming setbacks, antiprotons return as LHC recovers luminescent brilliance


The Accelerator Report published on 10 May highlighted that the 2023 antiproton physics run was delayed by 50 days (reducing the run to 122 days instead of the 172 initially scheduled) due to a broken magnet in the injection region of the Antiproton Decelerator (AD). Consequently, the beam commissioning of the AD was due to start on 12 June and the delivery of antiprotons to the AD-ELENA experiments on 30 June.

Following the hard work of many experts, the AD operations team received on Friday, 1 June at 11:58 – 12 days earlier than rescheduled – the green light from the AD injection kicker expert: beam could be injected again in the AD ring, signalling the start of the beam commissioning. The AD operations team and the equipment experts rescheduled their activities to focus on the AD beam commissioning in order to bring forward the rescheduled start of physics, reducing the number of lost physics days from 50 to about 40. This is, of course, much welcomed by the AD-ELENA experimental users, who are eagerly awaiting low-energy antiprotons to perform their experiments.

The RF finger module (right-hand side) ensures a low-impedance (low-resistivity) electrical connectivity between the LHC vacuum chambers. When this electrical connection is not good enough, it affects the circulating beam by making it unstable, deteriorating its quality or creating losses that can lead to a beam dump. (Image: CERN)

The rest of the LHC injector chain is running well for the LHC and the fixed-target experiments. However, the LHC suffered a temporary dip in luminosity production due to the replacement last week of a radiofrequency (RF) finger module in the machine section located near the ATLAS experiment (Point 1).

In the early evening of Thursday, 25 May, the LHC beam was dumped during acceleration on two consecutive fills. Both beam dumps were triggered by slow local losses* left of Point 1. X-ray imaging investigations and beam-loss studies led to the conclusion that one of the RF finger modules in a warm section was heating up or arcing, degrading the vacuum in that area and causing the slow local beam losses. The luminosity production was interrupted and, during the long Whitsun weekend, various teams intervened in the LHC tunnel to replace the RF finger module (with subsequent vacuum pumping). Already in the morning of Tuesday, 30 May, beams were injected and circulated to check the vacuum conditions. In the evening, a first fill with only 700 bunches per beam, followed by a second fill with 1200 bunches, were used for physics while also conditioning the area of the newly installed RF finger module. In the following days, the intensity ramped up to 2400 bunches and, in a second stage, the intensity per bunch was increased from 1.3x1011 to 1.6x1011 protons per bunch.

Until further notice and pending greater understanding of the cause of the RF finger module fault, the bunch intensity will be limited to 1.6x1011 protons per bunch. By now, the LHC is again filled with the default 2400 bunch filling scheme with 1.55x1011 protons per bunch, producing close to 1 fb-1 per day. As I write, the integrated luminosities for ATLAS and CMS are each 16 fb-1, which is about 5 fb-1 below the target value, but we’re catching up.


* “Slow local losses” happen when some beam particles get lost in specific parts of the ring when interacting with the gas molecules in the degraded vacuum. This process takes some time before the threshold to dump the beam is reached.