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Why the LHC magnets are blue – and other colourful accelerator questions answered

Are all LHC magnets blue? Who decides the colour of a magnet and on what basis? What does the small purple one do? See our answers below


LHC tunnel Pictures during LS2
The Large Hadron Collider (LHC), CERN's flagship accelerator. (Image: CERN)

Springtime has arrived in Geneva, where CERN is located, bringing with it colourful blossoms and the whir and buzz of nature awakening. A few dozen metres beneath the fertile soil, another equally buzzing ecosystem is springing back to life: CERN’s accelerator system, whose rings are gradually entering their recommissioning phase. Whether the beauty of our metal machines resembles that of mother nature is open to debate, but one thing is certain: when it comes to colourfulness, our accelerators can compete with most blossoming meadows.

Magnets are systematically painted to protect them from rust, except in the case of superconducting magnets (like those of the LHC), where the vacuum vessels containing the equipment are painted instead. Besides the blue of the LHC dipole magnets, which bend particle beams to preserve the particles’ circular trajectory, CERN’s accelerators are painted in colours ranging from red to green, purple, orange and various shades of silver. How are these colours chosen and why? The short answer is that CERN’s top physicists and engineers decide which ones they like the best. Indeed, unlike other pieces of equipment whose colour code is strictly regulated for safety reasons, the teams developing the magnets have free reign over the colour of their creations.

Certain unwritten rules do influence their decision-making, however, as Vittorio Parma, formerly in charge of the LHC cryostats, explains: “Working in accelerator tunnels can be quite gloomy as the lighting is poor. To offset this, we tend to go for the brighter, more luminous colours that make working around the magnets easier.” This swayed Vittorio’s team towards the choice of a gleaming white for the vacuum vessels containing the LHC’s quadrupole magnets, which focus the particles in tighter bunches, when they designed the LHC superconducting magnets in the 1990s. The white alternates with the more familiar blue of the dipoles and the deep red of the triplet quadrupole magnets, which further focus the beam around the collision points. They will be joined in a few years’ time by the dark blue of the future High-Luminosity LHC’s 11 Tesla magnets, which are currently undergoing tests. The darker shade is intended to reflect the magnet’s stronger magnetic field than that of the regular LHC dipoles, which are lighter in colour.

The LHC with its blue dipole magnets and white quadrupole magnets (top left), a red LHC triplet quadrupole magnet before installation (top right) and a prototype of the dark blue 11 Tesla HL-LHC dipole magnet (bottom). (Image: CERN)

This gaudy picture is completed by the magnets of CERN’s other accelerators (LINAC 4, the Proton Synchrotron (PS) and its Booster (PSB), the Super Proton Synchrotron (SPS) and the antimatter decelerating (AD) rings, to name but a few). “Each machine was built at a different point in time, by different people with different mindsets. Each team chose the hues of their magnets without following any strict code and, as a result, each machine is a unique, colourful artwork. This showcases the diversity and the creativity of the work done here at CERN”, explains Davide Tommasini, who led the development of the superconducting magnets for the LHC.

Consequently, a bending dipole magnet in the PS Booster is green, while its SPS counterpart is red, and a blue magnet may be a dipole in the LHC or a quadrupole in the SPS or LEIR. This somewhat messy patchwork contributes to the strong visual identity of CERN’s accelerators, from the green and orange of the PS Booster to the red and dark blue of the SPS – not to mention the magnets of the transfer lines, which boast their own specific colours, such as the mint and lavender of the superb dipole magnets we see below.

The PS Booster very nearly took a different path, recalls Giorgio Brianti, Division Leader at the time the machine was built. “I thought it would be nice to hold a competition for a colour scheme.” Coming at the tail end of the flower-power era, though, this was maybe not such a good idea. “The winning entry was kind of psychedelic, with lots of bands of colour all over the place. I didn’t like the result at all, so I presented the prize of a few bottles of champagne to the winner, but I chose the colours myself.”

The PS Booster with its orange quadrupole and green dipole magnets (top left), dipole magnets used in the PS Booster transfer lines (top right, bottom left), and the Super Proton Synchrotron with its red dipole and blue quadrupole magnets (bottom right). (Image: CERN)

So, which is your favourite?