Instrumentation to measure beam parameters is essential for delivering high-quality beams, not only in the LHC, but throughout the whole injector chain. In many cases, redundancy is built in so that measurements can continue even if one device fails. Recently, however, the Proton Synchrotron (PS) reached the point where a broken wire scanner had to be replaced to ensure continued monitoring of beam quality.
On 20 August, attention also turned to Linac4, where the ion source required an intervention to replace a malfunctioning valve controlling the hydrogen gas inlet. Such work typically takes only a couple of hours, but is followed by 8 to 10 hours of source reconditioning before normal operation can resume. The timing offered a nice opportunity to carry out the PS wire-scanner replacement.
While this work was taking place in the injectors, the LHC aimed for uninterrupted collisions. At 8.05 a.m., the machine was fully filled, and by 8.37 a.m. beams were colliding. At that moment, the Linac4 source was taken offline to begin the gas-valve replacement. The wire-scanner exchange in the PS was also under way by 8.45 a.m. and, thanks to excellent preparation, was completed by 11.00 a.m., with vacuum pumping immediately following. Meanwhile, the Linac4 source team also finished the gas-valve exchange efficiently, allowing reconditioning to start.
By the early evening, beams were back in the injector chain. At around 6.20 p.m., a slightly lower-current beam (35 mA instead of 40 mA out of the source) was injected into the PS Booster, sufficient to re-establish all operational beams. Just five minutes later, the PS itself was receiving beam again. Not long after, the SPS also received beam. The injector complex was back in business…
Unfortunately, at 9.35 a.m., the beams in the LHC were dumped due to an issue with the quench-protection system in one of the magnet circuits. With no beam expected from the injectors before the evening, access was granted to carry out outstanding activities. By 12.30 a.m. on 21 August, beams were once again colliding in the LHC, and luminosity production resumed.
On 1 September at 8.00 a.m., the LHC beams were dumped, marking the transition from physics operation to the second machine development (MD) block of the year. For the experiments, this means a pause in luminosity production, but the dedicated MD period is essential for testing future operational scenarios and preparing the way for the High-Luminosity LHC (HL-LHC).
The switch came at a moment when the LHC was performing strongly. In the weeks leading up to the MD block, the LHC managed to catch up impressively well with its luminosity forecast curve, thanks to very good machine availability and rapid turnaround times, i.e. the interval between dumping one fill and colliding the next. By the morning of 1 September, the LHC had delivered 70 fb⁻¹ of integrated luminosity to both ATLAS and CMS, just 3 fb⁻¹ shy of the forecast 73 fb⁻¹. This stands in sharp contrast to mid-July, when the LHC was behind on the target by about 6 fb⁻¹.
The MD programme that is due to end at 8.00 a.m. on 5 September is particularly interesting. Accelerator physicists are refining optics control at 6.8 TeV, exploring new ways to streamline the ramp-and-squeeze sequence, and testing improved cleaning strategies around the collimation regions. Other studies are pushing the boundaries of beam stability, probing novel collimation with bent crystals, and benchmarking how high-intensity beams interact with the machine environment. Several fills will also be dedicated to HL-LHC-like high-intensity bunch trains, assessing how they behave through injection, acceleration and collisions.
From 5 September, the LHC will return to luminosity production, beginning a new four-week period of proton collisions before the third four-day MD block starts in early October. This upcoming physics period offers the chance not only to maintain the excellent progress of recent weeks, but also to catch up fully, and possibly even surpass, the forecast luminosity curve, as we work steadily toward the 2025 target of 120 fb⁻¹.
