A new coating technique for accelerating crab cavities
Research being done as part of the Future Circular Collider (FCC) study is already bearing fruit, as the development of a new sputtering method for manufacturing crab cavities shows. These cavities, which are located on either side of the collision points, tilt the particle bunches so that their overlap area is as large as possible when they cross each other, making it possible to increase and control the accelerator’s luminosity. This technique is in its infancy, as the first crab cavities are being developed for the High-Luminosity LHC (HL-LHC). They will be made of bulk niobium, a superconducting material that is traditionally used for radiofrequency cavities. However, bulk niobium is very expensive, which is why alternatives are being sought for use in colliders of the future. To reduce costs, the scientists intend to use copper coated with a thin layer of niobium instead of bulk niobium.
Copper has previously been coated with niobium for the LHC’s radiofrequency cavities, using a technique called magnetron sputtering. A magnet surrounded by a negatively polarised niobium cylinder (the “magnetron”) is inserted into the cavity in order to generate an argon plasma. The electrons present in the plasma, excited around the magnetic field lines, ionise the positively charged argon atoms, which are accelerated towards the niobium cathode. The argon ions hit the niobium, whose atoms are sprayed out and scatter around the cavity before settling on the copper walls.
The constant negative polarisation technique suits the LHC’s elliptical radiofrequency cavities, but the more complex inner shape of the crab cavities prevents a uniform layer from being deposited on the walls. This is where teams from the BE-RF, EN-MME and TE-VSC groups came in, developing a new WOW (“Wide Open Waveguide”) crab cavity that is compatible with the sputtering technique, as well as a new technique for depositing the coating, namely High-Power Impulse Magnetron Sputtering (HiPIMS), a sputtering method using voltage modulation that makes it possible to reach fairly high power levels in order to ionise a significant fraction of the sputtered niobium atoms. The potential of the niobium target is periodically reversed in order to repel the positive niobium ions, thereby increasing the speed of the scattered particles. They are thus projected more efficiently onto the cavity walls and the coating becomes denser and more homogenous.
Following three years of R&D, the first test on a cavity will take place this winter, having been postponed due to CERN moving to safe-mode. Fabio Avino of the VSC group is raring to go: “I witnessed the very beginnings of the project, three years ago, and since then, I’ve been delving into the principles of physics and engineering, and I’ve come up with a satisfactory result. The team and I hope that our work will one day be useful for an accelerator like the FCC.” Beyond high-energy physics, the depositing technique studied at CERN also has applications in the automobile, aerospace and medical industries, which use HiPIMS to coat objects with complex shapes and to obtain layers with challenging properties.