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LHC Report: Investing in the future

The latest Machine Development period ended on early Monday morning, 18 June, after six days of intensive studies of the LHC accelerator


LHC tunnel pt1 various angle
LHC tunnel pt1 various angle (Image: CERN)

On Tuesday, 12 June at 7.00 a.m., a six-day Machine Development (MD) period started, during which the machine was available for accelerator physicists and the machine equipment groups to study beam behaviour and the operation of the accelerator equipment. With the present peak LHC luminosity being twice the design luminosity, we could say that our mission is accomplished and that we don’t need to study the details of LHC beam operation any further. However, following the LHC Injector Upgrade (LIU) programme, after Long Shutdown 2, the beam intensity will be double the present level. After Long Shutdown 3, the HL-LHC project will then allow us to obtain even smaller beam sizes at the collision points. Our knowledge of how to keep these high intensity beams stable in the LHC is not complete and many study programmes are ongoing, in which theories are being compared to reality.

Research to understand the beam instabilities represented a large fraction of the studies over this Machine Development week. In one of the studies beam instabilities were induced by applying excitation noise to the beam. It was observed that in some cases it took a surprising 10 minutes for the transverse beam size to increase after applying the noise. 

Techniques related to new hardware were also tested, like the long-range beam-beam compensation wires embedded in the collimators. Newly installed crystal collimators were also put to the test. The Radio Frequency group assessed the use of lower accelerating voltages than normally used during the beam injection process to reduce the longitudinal oscillations of the beam.

Another focal point was the test of the ATS optics. Beam size variations in the arcs of the accelerator are used to squeeze the beam to a small focus at the centre of the experiments. A variation of these ‘Telescopic Optics’ was studied, where the beams are not round in the centre of the experiments as is normally the case but have different beam sizes in the horizontal and vertical plane as the collision point by a factor two. 

The heat load generated by the beam and transferred to the cryogenic system was also studied. This heat load will become more significant at the higher beam intensities planned for the future. At present, the heat load in the eight arcs of the LHC can differ by more than a factor of two in identical conditions and so far the reason for this difference is not understood. Tests were carried out to vary the currents of the different corrector magnet circuits in the different arcs, and to apply ‘bumps’ to the beam in the arcs. At first glance, no large effects were observed, but careful analysis might give a hint of the physics processes behind these differences around the machine. 

Overall, it was an intensive week with a strict schedule: 24 hours a day for six days for the different groups to each perform their few hours of research. In total, 15 different studies were performed. An intensive week of investment in the future, increasing our understanding of the LHC and thus improving its future performance. The next Machine Development period, which will last 5 days, is scheduled for the end of July.