On 29 April, the LHC team received the green light for the final step in the intensity ramp-up and added the last 141 bunches to obtain a full machine with 2352 bunches in each beam.
The 2024 beam commissioning and subsequent intensity ramp-up to about 1900 bunches per beam went very smoothly and, as mentioned in my last report, we were well ahead of schedule. Today, we are still on schedule, but some of the margin has been consumed by two main challenges that were encountered in the last two weeks, which have led to some changes to the filling scheme.
The first challenge occurred on 17 April, when the machine was filled with 1791 bunches per beam. Abnormal beam losses were observed in the collimation region (Point 7) during the final stage of the “squeeze process”, where the beam size in the experiments is reduced to increase the number of collisions.
The collimation system is designed to absorb particles that stray from their trajectory and could hit sensitive components of the accelerator, such as the superconducting magnets, and interfere with their operation. To avoid this happening, Point 7 is equipped with primary and secondary collimators.
The primary collimators, which are situated close to the beam, intercept the deviating beam particles (also called the primary particles), absorb part of their energy and redirect them to the secondary collimators. The secondary collimators, which are further away from the beam, then absorb these particles.
On 17 April, a breaking of the collimation hierarchy was observed: a secondary collimator started playing the role of a primary collimator for certain particles. This can damage the secondary collimator, as it is not designed to intercept the primary particles. Many studies are ongoing to understand the issue, especially as this effect is not observed when the machine is filled with only a few bunches, which is the case during the beam commissioning, when the LHC team validates the collimation hierarchy. In the meantime, the “squeeze” has been limited, sacrificing a few percentage points of luminosity, but avoiding potential damage to the machine parts.
The second challenge arose on 22 April, when the 1.9-Kelvin refrigeration unit A (QUARC A) at Point 8 stopped working due to a faulty cold compressor. Consequently, the cryogenics team switched to the spare unit B (QUARC B), which was in cold standby. Unfortunately, the QUARC B is less efficient, resulting in a loss of cooling capacity. A good cooling capacity is needed in sector 7-8 to extract the heat load induced by the electron cloud. Therefore, the number of bunches per beam was reduced from 1983 to 1215 the day after. On 24 April, the cryogenics team was ready to switch back to QUARC A and the cooling capacity was recovered by 25 April. A first fill with 1419 bunches was put in collision, successfully followed by a 1959-bunch fill during the night.
However, to reduce the heat load and leave the possibility open to further increase the total number of bunches, the injectors switched from bunch trains containing three batches of 48 bunches each to bunch trains with three batches of 36 bunches each. With this bunch pattern, the number of gaps in the bunch trains increases and the electron cloud production in the LHC decreases, hence the heat load to the cryogenics system.
On 26 April, the next intensity step was made, increasing the number of bunches from 1959 to 2211 bunches per beam. Over the following weekend, these 2211 bunches per beam, together with an excellent machine availability of 70% and beams in collision close to 58% of the time, resulted in an accumulation of 2.5 fb-1, which corresponds to a very encouraging 0.83 fb-1 per 24 hours.
On 29 April, after careful analysis, the cryogenics team concluded that there was indeed margin in the cryogenic cooling system to perform the final intensity step and increase to 2352 bunches per beam, still using the three batches of 36 bunches from the SPS.
Collisions with 2352 bunches per beam and a limited “squeeze” of the beam will be the default running mode for the time being, until the collimation hierarchy issue is understood and resolved. Despite this, the luminosity production is very good and looks promising for the remainder of the year.