Electron cloud and scrubbing in 2012 in the LHC

During 2011, the scrubbing dose accumulated in the LHC during the tests with 25 ns beams could decrease the SEY of the chambers well below the multipacting threshold for 50 ns beams. During the Winter shutdown , the conditioning was preserved to such an extent as to guarantee smooth electron cloud free operation with 50 ns beams since the beginning of the 2012 run. However, the 25 ns injection tests that took place in July 2012 revealed that the chambers had slightly deconditioned since the last 25 ns test of 2011. Although this was not sufficient to cause electron cloud formation with 50 ns beams, this effect could noticeably impact on the quality of the 25 ns beams. A more extensive run with 25 ns beams (with stores for studies at both 450 GeV and 4 TeV and including a brief physics run) took place during December 2012 to gain experience in this mode of operation and gather more experimental information on the scrubbing process in the LHC. The outcome of this run is used to provide the guidelines to define a scrubbing strategy for operation after Long Shutdown 1 (LS1) with both 50 ns and 25 ns beams. A more general goal of the 25 ns run was also the identification of possible bottlenecks , which could prevent the safe injection and storage of the number of bunches needed to perform an efficient scrubbing process first and physics later.