Out of the clouds

A large fraction of the LHC beam-time over the last two weeks has been devoted to the second phase of the scrubbing of the vacuum chambers. This was aimed at reducing the formation of electron clouds in the beam pipes, this time performed with 25-nanosecond spaced bunches. This operation is designed to prepare the machine for a smooth intensity ramp-up for physics with this type of beam.

The scrubbing of the accelerator beam pipes is done by running the machine under an intense electron cloud regime while respecting beam stability constraints. When electron cloud production becomes sufficiently intense, the probability of creating secondary electrons at the chamber walls decreases and this inhibits the whole process. In this way, the scrubbing operation eventually reduces the formation of electron clouds, which would otherwise generate instabilities in the colliding beams.

The second phase of LHC scrubbing started on Saturday, 25 July, when 25ns beams were circulated again in the LHC. Trains of 72 bunches from the SPS, which were used to fill the LHC at the beginning, still showed typical signs of beam degradation induced by the electron cloud. The transition to trains of 144 bunches took place only when the quality of these beams had visibly improved. Over the following days, the beam quality continued to steadily improve over time, proving that the scrubbing of the beam chambers was successfully having the desired effect.

By the end of the first week, the number of bunches in the LHC had reached about 2400 per beam, which is the maximum number that can be injected into the LHC in trains of 144 bunches. The quality of these beams at the end of the injection process, however, was still significantly degraded by the electron cloud phenomenon, in part due to the long time needed to complete the injection process. Further scrubbing was performed during the second week, leading to beam lifetimes of 10 to 20 hours at injection energy, in spite of the continued presence of a dense electron cloud in the machine that was evidenced by the heat load measured by the cryogenics system in the arcs.

During this second scrubbing run, the machine experts also made the first attempts to boost the scrubbing process by injecting into the LHC the so-called “doublet” beams, i.e. pairs of bunches (each bunch is 5ns apart from the other), spaced by 25ns from one another. The idea is based on the expected increase of electron cloud production in the LHC using this beam scheme, as predicted by simulations and proved by experiments in the SPS.

Initially, a few machine operation shifts were devoted to the operational development needed to enable the LHC to “accept” beams with this unusual beam structure. Subsequently, after the main issues had been resolved, trains of doublets (12, 24, 36) were injected into the LHC in the middle of the second scrubbing week. An entire day was devoted to tests to assess the scrubbing potential of this type of beam.

As expected, the doublet beams created much more electron cloud than the 25ns beam. However, they also produced severe instabilities caused by the electron cloud they produce. Although it was possible to store several trains of 24 widely spaced doublets in the LHC, they were affected by electron cloud instabilities and large losses at the end of the trains that were injected later in the process. To allow doublet beams to be used effectively in the future, a necessary condition is to first reach a more advanced degree of scrubbing with 25ns beams. While their use turned out to be premature at the present stage, doublet beams will remain an important weapon to be kept in store for future scrubbing sessions, should these become necessary at a later stage.

The final step of the second scrubbing run consisted of validating the physics filling schemes with 1176 bunches with 25ns spacing and assessing the machine settings needed to provide these beams. This was successfully completed in the morning of last Saturday, 8 August. The LHC is now ready to take trains of 1176 bunches with 25ns spacing for physics, and this will potentially increase as the limitations to the speed of the injection process are overcome and further scrubbing is achieved with physics fills.