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LHC is incredebly cold and its operated at very strong vacuum. But, in reality we could not attain the perfect vacuum, so, inside LHC there are tiny number of small number of gas molucules left in LHC and many of them remain frozen to the beam pipe. We have to clean it as much as possible. This cleaning process of LHC is known as “scrubbing”. This is so important for the stable operation of LHC such that there is an entire team (they are called scrubbin team), who is responsible for it.

Why the scrubbing is so important?

As we know, inside LHC proton beams are moving at its fullest speed near to the speed of light. So, it will collide to the molecule and liberate the electrons from them further the liberated electron may hit another proton or to the LHC beam pipe, where it will collide to the frozen molecules and realease more electrons and in this process it is able to create the avalance which weakens the vacuum and sooner disrupts the beam.

But, the cloud of buzzing electrons inside the beam pipe posseses an interesting self-healing feature. When the chamber wall is under intense electron bombardment the probability of it creating the secondary electrons decreases and the avlanche is gradually mitigated.

Before, ramping up the LHC up to its full intensity, LHC team runs the machine for several days with as many low-energy protons as they can safely manage and intentionally produce electron coluds. The effect is that we have fewer loose electrons during the LHC physics runs.

In other words, accelerator engineers clean the inside of the LHC a little like they would unclog a shower drain. They gradually pump the LHC full of more and more sluggish (slow moving or inactive) protons, which act like a scrub brush and knock off the microscopic grime ( dirt ingrained on the surface of something, especially clothing, a building or the skin ) clinging (fitting closely to the body and showing its shape) to the inside of the beam pipe. This loose debris is flushed out by the vacuum system. In addition, the bombardment of electrons transforms simple carbon molecules, which are still clinging to the beam pipe’s walls, into an inert and protective coating of graphite.

Ref: http://www.symmetrymagazine.org/article/how-to-clean-inside-the-lhc