A vacuum as empty as interstellar space

Cross-section of LHC prototype beam pipes showing "beam screens". Slits in the screens allow residual gas molecules to be pumped out to maintain the vacuum (Image: CERN)

With the first start-up of beams in 2008, the Large Hadron Collider (LHC) became the biggest operational vacuum system in the world. It operates at a variety of levels of pressure and uses an impressive array of vacuum technologies.

A three-in-one vacuum system

The LHC is unusual in that it has three separate vacuum systems: one for the beam pipes, one for insulating the cryogenically cooled magnets and one for insulating the helium distribution line.

To avoid colliding with gas molecules inside the accelerator, the beams of particles in the LHC must travel in a vacuum as empty as interstellar space. In the cryomagnets and the helium distribution line, the vacuum serves a different purpose. Here, it acts as a thermal insulator, to reduce the amount of heat that seeps from the surrounding room-temperature environment into the cryogenic parts which are kept at at 1.9 K (-271.3°C).

The largest vacuum system in the world

With a total of 104 kilometres of piping under vacuum, the vacuum system of the LHC is among the largest in the world. The insulating vacuum, equivalent to some 10-6 mbar, is made up of an impressive 50 km of piping, with a combined volume of 15,000 cubic metres, more than enough to fill the nave of a cathedral. Building this vacuum system required more than 250,000 welded joints and 18,000 vacuum seals. The remaining 54 km of pipes under vacuum are the beam pipes, through which the LHC's two beams travel. The pressure in these pipes is in the order of 10-10 to 10-11 mbar, a vacuum almost as rarefied as that found on the surface of the Moon. The LHC’s vacuum systems are fitted with 170 Bayard-Alpert ionisation gauges and 1084 Pirani and Penning gauges to monitor the vacuum pressure.

A vacuum thinner than the interstellar void

Ultra-high vacuum is needed for the pipes in which particle beams travel. This includes 48 km of arc sections, kept at 1.9 K, and 6 km of straight sections, kept at room temperature, where beam-control systems and the insertion regions for the experiments are located.

In the arcs, the ultra-high vacuum is maintained by cryogenic pumping of 9000 cubic metres of gas. As the beam pipes are cooled to extremely low temperatures, the gases condense and adhere to the walls of the beam pipe by adsorption. Just under two weeks of pumping are required to bring the pressures down below 1.013 × 10-10 mbar (or 10-13 atmospheres).

Two important design features maintain the ultra-high vacuum in the room-temperature sections. Firstly, these sections make widespread use of a non-evaporable "getter coating" – developed and industrialized at CERN – that absorbs residual molecules when heated. The coating consists of a thin liner of titanium-zirconium-vanadium alloy deposited inside the beam pipes. It acts as a distributed pumping system, effective for removing all gases except methane and the noble gases. These residual gases are removed by the 780 ion pumps.

Secondly, the room-temperature sections allow "bakeout" of all components at 300°C. Bakeout is a procedure in which the vacuum chambers are heated from the outside in order to improve the quality of the vacuum. This operation needs to be performed at regular intervals to keep the vacuum at the desired low pressure.

Though these technologies were developed for fundamental research, they have found everyday uses: ultra-high vacuum technology made possible a major improvement in the performance of solar thermal collector panels, for example.

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