A new type of coating to chase the clouds away

The electron cloud problem needs to be addressed with innovative solutions, particularly in view of the rapidly approaching HL-LHC upgrade. CERN’s Vacuum, Surfaces and Coatings group has greatly improved its amorphous carbon coating technique, which is an alternative to the scrubbing process used so far. This technique is now fully mature and is being used for the vacuum chambers of the SPS magnets and the delicate beam screens of the LHC’s quadrupole triplets.

The violet light is produced by the argon plasma used when sputtering the amorphous carbon. The beam screen is coated in this case using the magnetic field of the quadrupole itself. (Image: Pedro Costa Pinto)

We know that conditioning (or “scrubbing”) the beam pipe reduces the avalanche-like creation of secondary electrons from the tube’s walls, thus preventing the formation of unwanted electron clouds. But it has also been observed that scrubbing naturally leads to an increase in the concentration of carbon on the pipe’s surfaces. “This gave us the idea that applying a thin film of carbon to the walls of the vacuum chamber could provide an alternative solution to beam tube scrubbing,” says Paolo Chiggiato, head of the Vacuum Surfaces and Coatings group of the Technology department (TE-VSC).

At the end of 2014, despite the promising results obtained through coating the inner walls of the vacuum chambers of 16 SPS magnets with a fine layer of amorphous carbon (a-C), there were still some issues to be resolved, including the mysterious origin of some scattering in the secondary electron yield (SEY) rate – i.e. the number of secondary electrons produced on average per incident electron. In the meantime, the management of the HL-LHC project made a request for a study of the feasibility of coating the LHC triplets with carbon to cope with the future HL-LHC beams and the known maximum thermal load of the cryogenic system.

It took the TE-VSC group months of intensive R&D to realise that the presence of even small residual fractions of hydrogen caused an increase in the SEY rate. “The whole operation needs to be carried out in extremely good ultra-high vacuum (UHV) conditions to avoid any impurities,” explains Mauro Taborelli, a member of the Surfaces, Chemistry and Coatings (SCC) section of the TE-VSC group.

In parallel, Pedro Costa Pinto, together with the TE-VSC-SCC section coating team, was working on improving the hollow cathode previously used to coat the 16 SPS magnets. The new design provides for a train of short cathode modules instead of a single, longer coating device. The shorter modules are assembled on site during insertion, allowing the experts to coat two adjacent SPS dipoles in situ each time, without having to remove the vacuum chambers from their position in the tunnel.

An even more complex solution was designed to address the coating of the LHC quadrupole triplets ready for the HL-LHC era. The electron cloud problem will be of particular concern in these magnets as there will be two beams in the same beam screen, with higher intensity and higher brightness than in the present LHC. While the beam screens at CMS and ATLAS won’t pose any problems since they will be completely replaced, those at ALICE and LHCb will not be replaced. Therefore, almost 35 metres of the triplets’ beam screen need to be treated on site on both sides of ALICE and LHCb, where an aperture only 15 cm long is available to insert a coating device.

For these reasons, the TE-VSC group has developed two additional coating techniques, specifically for the LHC triplets. The first takes advantage of the magnetic field of the quadrupole itself to spray carbon on the beam screens and enhance the deposition efficiency. Should this not be sufficient, another solution involves the integration of permanent magnets in the cathode in order to be able to also coat the interconnection between the magnets, where there is no magnetic field to leverage.

Currently, the work programme is to progressively coat one sextant of the type-B dipoles and half of the SPS quadrupoles in situ during the next extended Year-End Technical Stop (EYETS) and during LS2, in the framework of the LIU (the LHC Injectors Upgrade) project. For the LHC triplets the feasibility study will be concluded at the end of this year, and it may be possible to start the coating process in the triplet regions around LHCb and ALICE as soon as during LS2.

by Stefania Pandolfi