"Chasing clouds in the LHC" - Large Hadron Collider's summer cleaning
However, despite the ultra-high vacuum, residual gas molecules remain trapped on the surface of the walls of the beam pipes, which also contain electrons. When the beams circulate, these electrons are emitted from the walls and accelerated by the beam’s electrical field. The accelerated electrons then hit the walls with enough energy to release the trapped molecules, thereby compromising the vacuum. At the same time, they set off an avalanche of even more electrons, forming electron clouds that can be dense enough to destabilise the beam. The electron-cloud phenomenon is amplified the higher the number of proton bunches in the beam and the more closely spaced they are.