Snapshot of the typical star-like distribution of the electron cloud on the beam screen of a sextupole magnet.
Despite the ultra-high vacuum of the LHC beam pipe, residual gas molecules and electrons remain trapped on the walls of the vacuum chamber. When the beam circulates, they are liberated from the surface of the walls and they eventually destabilise the beams. This phenomenon is called “electron cloud”.
The LHC beam-time of week 23 was fully devoted to the “scrubbing” of the vacuum chamber walls to prevent the formation of electron clouds. In this mode of operation, the LHC is repeatedly filled with as many closely spaced bunches as possible, which are able to provoke intense electron clouds in the pipes. In a beneficial loop, when the pipe walls are under a strong electron bombardment, they gradually become less prone to produce further electrons and this, in turn, inhibits the electron cloud formation. In this way, the scrubbing operation reduces formation of electron clouds, which would otherwise heat up the walls, degrade the vacuum in the beam pipes and eventually generate instabilities or degradation of colliding beams.
Scrubbing the LHC this year was deemed even more necessary because of the opening of Sector 1–2 during the Extended Year End Technical Stop (EYETS) 2016–17 and the consequent contamination of the pipe inner walls.
The LHC scrubbing began on Tuesday, 6 June. Long trains of 288 bunches from the SPS were used to fill the LHC. The cells of Sector 1–2 exhibited, as expected, a far larger heat load at the beginning with respect to those of all the other sectors. Over the following days, however, the heat load in Sector 1–2 gradually decreased and the beam quality was seen to be steadily improving, demonstrating that the scrubbing of the beam chambers was being successful.
By Friday night, the number of bunches in the LHC reached 2820 per beam, which is the maximum number that can be presently achieved. The injection process was quite fast thanks to both the injection of long trains from the SPS and to the LHC cryogenic control system, which could efficiently react to the rapidly changing heat loads on the cold walls while more and more beam got injected. In spite of the continued presence of a dense electron cloud in the machine, the full beam was circulating stably and with little degradation in LHC by Saturday night, while the heat load in Sector 1–2 went down to the value it had reached at the end of 2016.
The final day of this year scrubbing run was devoted to dedicated beam tests aiming to address long-standing questions, as well as (yet) unexplored operational aspects, such as the difference in electron cloud production between the two beams and the machine settings necessary to guarantee beam stability with the current amount of electron cloud in the machine. All tests were successfully completed as of Monday, 12 June, early in the morning. The LHC is now ready to continue the intensity ramp-up with 25-nanosecond beams for physics, which will eventually lead to 2556 high-brightness bunches per beam stored in the machine, which is the objective for 2017.