LHC Report: Out of the clouds

In order for the LHC to deliver intense proton beams to the experiments, operators have to perform “scrubbing” of the beam pipes. This operation is necessary to reduce the formation of electron clouds, which would generate instabilities in the colliding beams.


Electron clouds are generated in accelerators running with positively charged particles when electrons - produced by the ionisation of residual molecules in the vacuum or by the photoelectric effect from synchrotron radiation - are accelerated by the beam field and hit the surface of the vacuum chamber producing other electrons. This avalanche-like process can result in the formation of clouds of electrons. Electron clouds are detrimental to the beam for a few reasons. First, the electrons impacting the walls desorb molecules and degrade the ultra-high vacuum in the beam chamber. Furthermore, they interact electromagnetically with the beam, leading to the oscillation and expansion of the particle bunches. This increases the probability of quenches in the superconducting magnets and entails a luminosity reduction. Fortunately, when electron cloud production becomes sufficiently intense, after a while, it reduces the probability of creating secondary electrons at the chamber walls and inhibits the avalanche. Running a machine under an intense electron cloud regime is the definition of machine scrubbing.

The scrubbing process in the LHC started gently on Wednesday, 24 June, when the first long trains of bunches spaced 50-nanoseconds (ns) apart were circulated in the LHC. The strong electron cloud initially created caused the first beams injected with this bunch spacing to become unstable and lose particles. Then, however, this electron cloud started “scrubbing” the beam pipes, thus reducing the number of electrons emitted.

This beneficial effect meant that more bunches could then be injected, while the beam stability and quality improved at the same time. Within about 36 hours of the beginning of scrubbing, it was possible to circulate stable trains of 600 bunches in the machine with very limited electron cloud generation and, consequently, without beam degradation.

At this point, the machine experts judged that the scrubbing process could only be efficiently continued by packing bunches closer together. It was therefore decided to fill the LHC with beams of many bunches with 25 ns spacing, which is also the target beam configuration for luminosity production in Run 2.

While this switch made the beam operation more challenging and all available weapons to stabilise the beams had to be used, the amount of electron clouds produced around the LHC quickly increased and the scrubbing progressed well. It took about four additional days to reach the point of filling the LHC with 1200 bunches in tightly packed trains of bunches separated by 25 ns. The beam stability, initially difficult to handle, also gradually improved as the electron cloud became less dense in all the machine sectors.

The next step consisted of performing a test run with 50 ns spaced beams filling the whole LHC, which was expected to be electron cloud free after the scrubbing period. This was successfully finished this Friday morning (3 July). The machine is now fully validated for 50 ns operation, and very good progress has been made towards 25 ns spacing. However, some work is still needed to prepare the LHC for operation with the full complement of 25 ns spaced bunches later in this year. Another scrubbing run will be performed later in the summer.

by Giovanni Rumolo, Giovanni Iadarola and Hannes Bartosik for the LHC team