Cleaning the LHC, Where Air Particles Count as Dirt

Inside the Large Hadron Collider, beams of particles sprint 17 miles around in opposite directions through a pair of evacuated beam pipes. The interior of the pipes needs to be spotless, but it’s not dirt or grime that clogs the LHC—it’s microscopic air molecules.

There’s a tiny number of simple atmospheric gas molecules loose in the LHC beam pipes since the machine doesn’t have a perfect vacuum, and even more frozen to the walls. Protons racing around the LHC crash into these floating air molecules, detaching their electrons. The liberated electrons jump after the positively charged protons but quickly crash into the beam pipe walls, depositing heat and liberating even more electrons from the frozen gas molecules there. This process quickly turns into an avalanche that weakens the vacuum, heats up the cryogenic system, disrupts the proton beam and dramatically lowers the efficiency and reliability of the LHC.

It sounds like a big mess, but the clouds of buzzing electrons inside the beam pipe possess an impressive self-healing feature, according to LHC accelerator physicist Giovanni Rumolo. “When the chamber wall is under intense electron bombardment, the probability of it creating secondary electrons decreases and the avalanche is gradually mitigated,” he says. “Before ramping the LHC up to its full intensity, we run the machine for several days with as many low-energy protons as we can safely manage and intentionally produce electron clouds. The effect is that we have fewer loose electrons during the LHC’s physics runs.”

In other words, accelerator engineers clean the inside of the LHC a little like they would unclog a shower drain. A group of experts, officially called the “Scrubbing Team,” handles the cleaning each year before the LHC’s summer operations program begins. The team gradually pumps the LHC full of more and more sluggish protons that act like a scrub brush and knock off the microscopic grime clinging to the inside of the beam pipe. This loose debris is flushed out by the vacuum system. In addition, the bombardment of electrons transforms the simple carbon molecules still clinging to the beam pipe’s walls into an inert and protective coating of graphite.

“Scrubbing is essential if we want to operate the LHC at its full potential,” Rumolo says. “It’s challenging because there is a fine line between thoroughly cleaning the machine and accidentally dumping the beam. When we’re scrubbing, we work around the clock in the CERN Control Center to make sure the accelerator is safe and the scrubbing is working properly.”