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Situated deep beneath the Franco-Swiss border is a massive ring of metal and magnets: the Large Hadron Collider (LHC), the world's largest and most powerful particle collider.
Housed 100 meters underground at CERN (Conseil Européen pour la Recherche Nucléaire), the LHC is made up of a 27-kilometer ring of superconducting magnets. The collider’s accelerator allows two extremely high-energy particle beams to travel in opposite directions at speeds close to that of light before colliding with one another.
A successful and powerful collision requires that ultra-high vacuum levels be maintained while the LHC is being operated. The following is an overview of the primary vacuum systems employed in the collider.
A collision involves two particles that are "beamed" at high speeds in opposite directions, with each particle in its own tube. These two tubes must be kept at extremely high vacuum levels to maximize the duration of the beam and maintain low pressure for the experiments. This is achieved by what is known as the beam vacuum.
Two pumping mechanisms are used to achieve this:
Cryogenic pumping: residual gas molecules are physically adsorbed into the cold bore surface at a temperature of 1.9 K
Non-evaporable getter (NEG) pumping: residual gas molecules are chemically adsorbed onto the surface of the beam pipes
The LHC's superconducting magnets are cooled with liquid helium to a temperature of 1.9 K (approx. -271 °C). The LHC employs a powerful vacuum to thermally insulate the magnets and consequently maintain the extremely low temperatures needed for a successful collision.
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The primary pump technologies employed within the collider are:
An Ion pump is a type of vacuum pump that operates by sputtering a metal getter.
Turbomolecular pumps operate on the principle that gas molecules can be made to undergo repeated collisions with a moving solid surface to attain momentum in the desired direction.
Both pump systems must have the ability to tolerate high radiation and magnetic field levels.
A major challenge faced while assembling the LHC was leak detection. Leak tightness must be ensured to reliably operate large vacuum systems. A leak can cause base pressure to rise above or fall below the required levels, resulting in the need for significant — and expensive — re-working.
The LHC is currently being upgraded and its successor will be known as the High Luminosity Large Hadron Collider (HL-LHC). The upgrades began in June 2018 and are set to be completed in 2027. The changes are expected to increase the discovery range of new particles by around 20-30% over the current LHC and also extend the LHC's lifetime to 2040.
A perfect vacuum is essentially a region in space that has no particles. In reality, achieving this is impossible — just as achieving absolute zero temperature is impossible. Even the Large Hadron Collider operates at a pressure of 10-10 to 10-11 mbar, which means that there are still about 200,000 molecules per cubic centimeter.
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