Electron cloud effects in the LHC in 2011

The LHC began operation with 50ns beams in early April, 2011. The observation of pressure rise, heat load in the arcs, beam quality degradation and synchronous phase shift during the first days with 50ns beams clearly revealed the development of an electron cloud inside the machine. However, a 5-day dedicated scrubbing run with 50ns beams was sufficient to mitigate this effect and allow for subsequent physics production with this bunch spacing. Further- more, from the end of June to the end of October, 2011, five Machine Development (MD) sessions with 25ns beams took place, during which all the aforementioned electron cloud indicators again made an appearance and further scrubbing could be achieved. In this paper, we will first briefly explain how the electron cloud can be detected in the LHC and give a summary of the electron cloud observations in the LHC during 2011. Then, we will quantify the scrubbing process in terms of evolution of secondary electron yield (SEY or δ max ) of the chamber surface by comparing the measured data — pressure, heat load, synchronous phase shift — with the simulation results, obtained with newly developed and optimized tools. Finally, we will discuss the influence of the electron cloud on the evolution of some beam observables.