Electron Cloud and Scrubbing Studies for the LHC

Electron cloud build-up resulting from beam-induced multipacting is one of the major limitations for the operation of the LHC with beams with close bunch spacing. Electron clouds induce unwanted pressure rise, heat loads on the beam screens of the superconducting magnets and beam instabilities. Operation with bunch spacing of 50 ns in 2011 and 2012 has required decreasing the Secondary Electron Yield of the beam screens below the multipacting threshold for beams with this bunch spacing. This was achieved by continuous electron bombardment induced by operating the machine with high intensity beams with 50 and 25 ns spacing during dedicated periods at injection energy (450 GeV) and at top energy (3.5 and 4 TeV). The evolution of the Secondary Electron Yield during these periods, in different sections of the machine, can be estimated by pressure rise, heat load and by bunch-by-bunch RF stable phase measurements. The experimental information on the scrubbing process will be discussed and a possible “scrubbing strategy” to allow the operation with 50 ns and 25 ns beams after the Long Shutdown in 2013-2014 will be presented.