The main CERN vacuum systems explained

December 14, 2018

The study of particle physics is conducted in machines known as particle accelerators (or particle colliders). These machines use huge electromagnetic fields to accelerate proton particles to velocities approaching the speed of light, focus them into a fine beam, and then monitor the matter that results from their collision with other particles.

The world’s largest and most powerful particle accelerator

The world’s largest and most powerful particle accelerator, the Large Hadron Collider (LHC) began operation in 2008, and is run by CERN (Conseil Européen pour la Recherche Nucléaire), the world’s leading centre for the international collaboration of nuclear research.

One of CERN’s major activities is to conduct such particle collisions within a series of tunnels that border France and Switzerland.

These tunnels (and the associated equipment and plant), act as a full-scale nuclear research laboratory, wherein protons are accelerators around a 27 km loop using cryogenically cooled, and superconducting magnets held at temperatures colder than that in outer space.

These high-speed beams of protons are channeled into a detection chamber where they collide with a proton “cloud” in an ultra-high vacuum. The resulting “exotic matter” that spills out of this collision is short-lived, but nevertheless, the decay products can tell us about the sub-atomic sized building blocks from which matter is formed, and thus the fundamental physics that makes up and controls almost everything in our universe. 

Over the years, CERN has evolved via numerous upgrades. Each evolution building upon the successes of previous stages. Each iteration reflects CERN’s more ambitious aims, as previous goals are achieved and the scientists gain still further insight into the fascinating world of sub-atomic particles.

The most recent upgrade of the CERN story is the High-Luminosity LHC (HL-LHC), which hones in (amongst other things) on the principle that more collisions can be achieved when particle beams impact head-on, rather than at an angle. This change of orientation will yield more particle collisions, and thus more data to help scientists unravel even more mysteries of the universe.

Whilst the LHC can manage a billion proton collisions per second, the upgraded HL-LHC aims at seven times this number, thereby yielding a ten-fold increase in data collected. This will be achieved by focusing the beam of circulating protons even tighter using a new bank of 120 magnets, including 24 superconducting quadrupoles and four superconducting dipoles. These new magnets raise the field strength from 8.1 to 11.5 tesla. This work will be completed by 2026 at the earliest.

The LHC, along with the detectors and other key experiments at CERN, requires ultra-high vacuum conditions while being operated. The main CERN vacuum systems are the beam vacuum and the insulation vacuum for the powerful superconducting magnets. 

The beam vacuum needs to be at ultra-high vacuum level to provide a good beam lifetime and low background for the experiments. Both cryogenic pumping (where residual gas molecules are physiosorbed on the cold bore surface at 1.9 K) and non-evaporable getter (NEG) pumping mechanisms (where residual gas molecules are chemically adsorbed on the surface of the beam pipes) are being used.

The insulation vacuum of the superconducting magnets, cooled down with liquid helium to 1.9 K (approx. -271 °C), needs to ensure a good thermal insulation of the cooling system in order to maintain the low temperatures.

Main high and ultra-high vacuum pump technologies being used within the LHC are ion pumps and turbomolecular pumps, which also have to cope with specific operational challenges such as tolerance to high radiation and magnetic fields levels.

In order to operate such large vacuum systems in a reliable manner, leak tightness also needs to be ensured. Leak detection during assembly of the LHC was definitely the biggest challenge for leak detector developers and operators. Never were so many joints examined in one machine! 

Upgrades from the LHC to HL-LHC will lead to a 20-30% increase in new particle discovery, as well as ensuring the viability of the whole LHC project up to 2040.

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