Although summer weather has finally arrived and many have left or will soon leave for holidays, the accelerator complex has no intention of letting up. In fact, the summer is often a very productive period for physics: in July, the LHC was producing collisions around 55% of the time.
During the summer, the activities scheduled are mainly aimed at physics production, leading to an increased number of cycles for the experiments and fewer interruptions. However, the warm weather, especially when coupled with high humidity, can pose challenges for the cooling of the accelerators. Many of the accelerator systems, including the cryogenics that keep the LHC magnets cold and superconducting, rely on cooling towers, but high outside temperatures and humidity reduce their cooling capacity.
High temperatures can also affect the buildings that house critical equipment, such as power converters, beam instrumentation electronics and radiofrequency electronics. For instance, a recent temperature increase of 2 to 3°C in the LHC surface building 2475, located above LHC Point 4, caused a noticeable shift in the collision point along the longitudinal axis of the beam in all four LHC detectors (see graphic for the ALICE experiment). This shift meant that collisions no longer occurred in the centre of the experiments.
Experts from the Cooling and Ventilation group (EN department) intervened, adjusting the temperature regulation to improve the situation. LHC operations experts established that for each degree celsius increase in the building, the collision point shifted by approximately 1.5 mm. This demonstrates the importance of stable temperatures in these buildings, especially during the hotter days of the summer.
Another summer challenge is the increased occurrence of thunderstorms. The lightning associated with these storms can cause electrical perturbations, potentially stopping some of the accelerator equipment and thus halting the beam. Interestingly, it is not usually the thunderstorms directly above CERN that cause these issues, but those affecting the areas around the high voltage electrical lines.
Over the years, the equipment groups have worked hard to make the accelerator complex more resilient to electrical perturbations, and their efforts have paid off. However, it is impossible to make all equipment completely insensitive to electrical glitches. So, when you see lights flickering in your house during a thunderstorm, operators and equipment experts at CERN are probably very busy re-establishing beam production in our accelerators.