CERN Accelerating science

A new sputtering technique for the coating of SRF cavities with 3D complex geometries

Cylindrical magnetron (center) with Cu samples (on the left) and mass and energy analyser head (on the right). (Image: CERN)

High power impulse magnetron sputtering (HiPIMS) is a growing technique promising high-quality film deposition for SRF/accelerator cavities. One of the basic requirements for Nb SRF coatings is the dense and void-free morphology. This is particularly challenging to be obtained in configurations with grazing angle of incidence of the impinging particles, which are therefore required to reach the substrate with higher energies to densify the growing film.

The highest power peak achieved during the pulsed plasma in HiPIMS allows ionizing the sputtered metal atoms. These ions can later on/then be accelerated towards the substrate by either biasing the substrate itself to a negative voltage, or by applying a positive voltage (positive pulse-PP) at the target during the afterglow. The PP approach provides some advantages, such as the simplified design of the magnetron, the use of a single power supply (or another way to say there is no need for a second one for biasing the substrate) and the possibility to keep the cavity at the ground voltage, which is critical for the coating of a SRF cavity that works also as vacuum chamber during the coating process. Ongoing R&D efforts are oriented towards the validation of the HiPIMS with PP technique in the framework of the FCC project, for the Wide-Open-Waveguide crab cavity. In a first step [1], the effective acceleration of the sputtered ions has been demonstrated for the first time by changing the duration of a 50V PP, observing a complete shift of the ion energy distribution function to values close to 30eV, as shown in Fig. 1.

Fig 1: Time-integrated EDFs of Nb+ ions for the case without PP, +50 V PP of 100 μs, and 250 μs duration.

In a recent paper [2], the densification of coated Nb films on copper samples at 90deg angle of incidence was explored for different techniques, from the standard direct current magnetron sputtering (DCMS), to the HiPIMS with PP (Fig. 2).

Fig 2: Nb films grown on copper substrate FIB cross sections of samples at 90∘ coated in: (a) DCMS with grounded substrate, (b) HiPIMS with grounded substrate, (c) HiPIMS with grounded substrate and +50 V PP, (d) HiPIMS with grounded substrate and +100 V PP.

A first step towards the validation of these coatings with respect to the SRF performances was also performed, by measuring the film critical temperature and the transition width in a dedicated test-stand at the CERN Central Cryogenic Laboratory, in the framework of the MSCA EASITrain project. An example of measured transition is depicted in Fig. 3.

Fig 3: Coil measurement of the superconducting to normal conducting transition of grounded sample coated at 90∘ in HiPIMS with +100 V PP. The data points of the voltage amplitude of the pickup coil are shown in black with their error bars as a function of the average temperature. The data fit is  displayed with a continuous blue curve. In red, the lines for the estimate of the transition width Δ.

A further validation on a quadrupole resonator sample has been performed providing residual resistance values comparable to those of bulk Nb. In the next months, the first coating of the full scale WOW cavity prototype will be performed in HiPIMS with PP.

Further reading:

[1] F Avino et al. 2019 Plasma Sources Sci. Technol. 28 01LT03.

[2] F. Avino et al. Thin Solid Films 706 (2020) 138058.

Rickard Ström (CERN)
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Ridding the accelerators of clouds is one of the major challenges of the vacuum teams. The clouds in question are electron clouds, which can disrupt the beams. Electrons are produced when the beams ionise the residual molecules in the vacuum chamber. They are then accelerated, hit the surface of the vacuum chamber and scrub off other electrons. An ‘avalanche’ phenomenon ensues, in which electrons multiply and form the dreaded electron clouds. At high beam intensities, these clouds can start to degrade seriously the proton beam quality. The problem therefore becomes critical when preparing the LHC for operation at high luminosity, which will require beams twice as intense.  

Since 2007, the Vacuum, Surfaces and Coatings group has been developing a solution, comprising a fine layer of amorphous carbon applied to the internal walls of the vacuum chambers. When struck by electrons, amorphous carbon emits a lot fewer secondary electrons than the metallic surface of the vacuum chambers. This amorphous carbon coating technique has proved to be effective, having been applied to sixteen SPS magnets during the first long shutdown, and then to more magnets during year-end technical stops.

As part of the LHC Injector Upgrade (LIU) project, this innovative solution is now being implemented on all the focusing quadrupoles and the short straight sections of the SPS, which comprise corrector magnets and beam instrumentation. “The priority magnets have been identified in order to achieve the best compromise between efficiency and the cost of the operation,” explains Wil Vollenberg, in charge of the carbon coating project in the SPS. In total, the vacuum chambers of 99 quadrupole magnets and as many short straight sections will receive an amorphous carbon coating.

A mobile treatment unit has been developed to apply the coating directly in the SPS tunnel and to reduce the handling of such heavy items as the magnets.  A cathode, a long graphite structure, is introduced into the vacuum chamber. The air in the chamber is pumped out and very pure argon is injected. A voltage of 900 volts is applied in order to ionise the argon atoms. The argon ions that are thus released bombard the graphite cathode, creating a shower of carbon atoms, which then stick to the walls of the vacuum chamber. The cathode is fitted onto an ingenious motorised system, which moves backwards and forwards to enable a uniform coating 400 nanometres thick to be applied. 

This mobile coating system was developed and tested for the first time in 2016, and has since been improved. From the beginning of the long technical shutdown, a team of three has been working its way along the SPS tunnel to treat half of the quadrupole magnets. However, accessing the vacuum chambers of these magnets requires removing the nearby components. This is why the short straight sections, adjacent to the quadrupoles but smaller in size, are removed and treated above ground. Furthermore, parts of the vacuum chambers are replaced by treated sections: this is the case for certain chambers such as those in the vicinity of the beam dump system that is currently being replaced.

From the handling teams who move the magnets, to the surveyors who reposition them to within a tenth of a millimetre, through to the electricians, surface analysis QA experts and specialists in vacuums and magnets, many teams are hard at work and have to carry out their tasks in a precise sequence. “It takes four days to treat each magnet, but many operations are necessary before and after, which results in complicated logistics,” says Wil Vollenberg, orchestrator of the operations.

Underground, the teams are already halfway there, working in collaboration with their colleagues on the surface. A new round of operations is envisaged during the third long shutdown, not only in the SPS but also in the LHC. The future will be less cloudy for the two large accelerators at CERN.

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