CERN Accelerating science

 Collaboration to develop HTS Tl-based coatings for FCC beam screens
 by Sergio Callatroni (CERN) 

Critical field of several HTS. Continuous lines indicate the maximum value as a function of temperature, where dashed lines indicate the maximum value at which these materials can be used as practical current carriers, such as in accelerator magnets. Image Credit: CNR-SPIN. 

A recently formed collaboration aims at developing beam screens for the FCC study based on the little-known HTS thallium cuprate in order to drastically reduce the beam impedance.

The research on HTS (High-Temperature Superconductors) has mainly been focused on two different families of materials, YBCO and BSCCO, which differ in terms of composition and crystal structure. BSCCO is used for example for the current leads of the LHC superconducting magnets, while YBCO-coated ribbons are a potential candidate for winding magnets of accelerator-grade quality reaching 20 T, as needed for the future circular collider designs examined under the FCC study.

In addition, the FCC study demonstrated on theoretical grounds that HTS could also be used to minimize the electromagnetic interaction of the beam with the surrounding vacuum pipe.

The proton beams circulating in the accelerator will produce several tens of watts per meter of synchrotron radiation due to their 50 TeV energy. To prevent this power from impinging on the dipoles, which are cooled to 1.9 K, they have to be protected, like in the LHC, by a beam screen.

Other considerations related to vacuum stability dictate that the beam screen has to be kept at a temperature between 40 and 60 K. At these temperatures, common electrical conductors such as copper may have not low enough beam impedance. HTS materials offer the only viable solution to overcome this obstacle. However, a study of the required performance has shown that BSCCO would be inadequate for the FCC requirements. YBCO could cope with the requirements, but is currentlyavailable only in the form of thin coated ribbons. Scaling the YBCO fabrication procedure to the beam screen shape remains a tough challenge, and would require a thorough rethinking of the coating technology.

Example of a beam screen for the LHC. Image Credit: CERN

To address this problem, a new collaboration between CNR-SPIN (Superconductors, oxides and other innovative materials and devices), TU Wien (formerly Vienna University of Technology) and CERN has recently been formed, with the goal of developing HTS coatings based on the little-known superconductor thallium cuprate (Tl-1223 / Tl-1212).

Tl-cuprates bear the potential for an exceptional performance. Their Tc (critical temperature) and Hc2 (upper critical magnetic field) are among the highest found so far, and their thin film deposition properties should be scalable to large dimensions and complex geometries. This seemingly superior HTS is not widely used because the required texture could not be established in previous studies leading to an unsatisfactory in-field performance – and the toxicity of thallium did certainly not contribute to its popularity.

CNR-SPIN in Genova possesses comprehensive knowledge and in-house expertise regarding the preparation of this compound, and will explore new production routes aimed at the fabrication of Tl-based HTS with a significantly improved microstructure in order to overcome the previously found limitations. The feedback required for optimizing the production process will be supplied by TU Wien, where both the microstructure of the samples and the superconducting properties will be analysed. It is the possibility of using advanced characterization techniques to correlate the local superconducting properties (flux pinning, grain coupling, current percolation) with microstructural features, which fuels the hope of developing a Tl-based HTS coating with satisfactory performance.

Collaboration agreements have been signed mid-November, and a Kick-off meeting at CERN has successfully attracted all collaborators, helping finalize the work plan. Other institutes from Spain have also expressed their interest in collaborating with CERN on HTS coatings.

This R&D project and its direct application are at the crossroads of several fields: accelerator science and technology, RF and beam dynamics, vacuum and cryogenics, materials and surface science, and of course superconductor science and technology. Both CNR-SPIN and TU Wien are leaders in the superconductor R&D field and the ideal partners to carry out this work together with CERN, who will guarantee the final validation of the material for the accelerator environment. As a welcome side effect, this research project might open up the possibility of introducing Tl-based HTS into the mainstream domain of conductor cables for the fabrication of high power devices, such as magnets or motors.

Read more about this project