Electron clouds are the bane of particle accelerators: a few stray electrons in a vacuum chamber, when stirred by a proton beam, can bounce off the walls of the beam screen (the metallic inner surface of the vacuum chamber), multiplying and whizzing around the beam. The resulting “cloud” can lead to a heat load being deposited on the cryogenic circuit and to a significant decrease in the beam quality, especially in areas where bunches are denser, such as inside the focusing triplet magnets surrounding the collision points of the LHC. The production of denser beams is precisely the goal of future accelerator projects such as the High-Luminosity LHC, which aims to achieve a ten-fold increase in integrated luminosity across the collider – making the issue of electron clouds even more pressing. And yet, crippling electron clouds may soon be a thing of the past thanks to a new method for coating beam screens with a layer of carbon.
While the copper surface of the LHC magnet beam screens can give back up to two electrons for any single one it receives, a carbon surface will yield only one particle at most. With that in mind, spraying LHC magnets with carbon seems like a no-brainer to thwart electron clouds. But, in practice, this is not easily achieved: engineers in TE-VSC-SCC (Surfaces, Chemistry and Coatings) must coat the beam screen with a carbon layer that is fine enough to preserve the resistivity properties of the copper surface without disturbing the fragile environment of the LHC magnet. They thus resort to a physical vapour deposition technique called sputtering. A graphite rod, inserted inside the vacuum chamber, is bombarded with argon ions produced in a plasma. As the ions hit the rod, carbon atoms on its surface are sprayed out, scattering towards the beam screen, on which they settle: a carbon layer forms on the copper surface of the screen.
Implementing the principle of sputtering carbon onto a beam screen poses a number of physical challenges, forcing engineers to jump through many hoops. To increase the adhesion of the carbon on the copper (i.e. to make it stick), the native copper oxide must first be removed by bombarding the beam screen with argon ions before coating it with an intermediate titanium layer, which adheres well to both the copper and the carbon. In addition, the titanium removes hydrogen impurities in the plasma, which would have caused the carbon to lose its valuable electronic properties.
“Beyond these physical challenges, we are also dealing with significant spatial constraints, working inside the LHC tunnel, on magnets that cannot be taken out of the collider. This made us develop creative ways of treating the surface from a distance”, explains Pedro Costa Pinto, the project leader. To combat these constraints, a modular sputtering device has been designed, composed of a titanium rod and a carbon rod enclosing small permanent magnets. This plasma cell can be pulled by a cable along the LHC magnet. The device has already proved its worth on the LHC’s Q5L8 quadrupole, which received the carbon treatment before the LHC restart as a first test. The first results are unequivocal: the standalone magnet has received minimal heat load (damage from the electron clouds) compared to all other magnets.
The logical next step will be to apply this technology where it is needed most: on the new triplet magnets surrounding the ATLAS and CMS collision points, where the luminosity is particularly high. In parallel, the first HL-LHC magnet beam screens will undergo the same treatment. “The brand-new HL-LHC screens haven’t been placed in the accelerator yet, which obviously makes things easier for us, since the sputtering can be carried out in the workshop, in a controlled environment. However, we need to update both our method and our tools to adapt for the larger, innovative beam screens”, says Spyros Fiotakis, who has worked on the carbon-coating method since its inception.
“When we presented the project in 2015, few believed we could make carbon-coating work on a magnet in the LHC tunnel. Seven years later, we are ready to apply this technology to more and more machines, with the hope of lifting a long-running limitation on the performance of particle accelerators”, adds Pedro. Time will tell whether carbon coating will save accelerators from themselves, but the technology will, without a doubt, be part of the answer.