Microstructural characterization of advanced superconducting materials for different components of the CERN hadron-hadron future circular collider

Abstract

How far can the curiosity push our capabilities? Is our thirst of knowledge stronger than the fear of our human limits? This is not the incipit of a philosophy manual, but the beginning of a magnificent idea. This idea led the great scientists at the European Organization for Nuclear Research (CERN) to start the huge project of the Future Circular Collider (FCC). We have been asking for ages about the evolution of our Universe and the nature of the matter, antimatter and particles describing it. The Large Hadron Collider (LHC) is the first great machine built at CERN with the aim of satisfying all the open questions related to the origin of the Universe. The discoveries it led to, together with the breakthrough represented by the observation of the Higgs boson, can be considered the starting point transforming the need of exploring new physics into a worldwide collaboration for the realization of a new extraordinary machine. In fact, the FCC aims at pushing the energy and intensity frontiers of particle colliders towards reaching collision energies of 100 TeV: the discovery of a new physics, possible when such energies are at stake, can lead to an extension of the known "Standard Model\ and a better understanding of the Higgs boson. Within this study, scenarios for three different types of particle collisions are examined: hadron (proton-proton and heavy ion) collisions, like inthe LHC, electron-positron collisions, as in the former LEP, and proton-electron collisions [1], [2], [3], [4]. This magnificent collider of the future will be the next large research facility after the LHC and its High-Luminosity upgrade (HL-LHC), whose realization is ongoing, once they approach the limits of their discovery potential [5].According to CERN recently published conceptual design study for a future hadron collider (FCC-hh), this extraordinary machine with its center-of-mass energy target of 100TeV would be located in a 100 km circumference ring close to Geneva (Switzerland).A key requirement for such a machine is the development of high-field superconducting accelerator magnets, capable of satisfying the requirements given by a non-copper critical current density (Jc) of at least 1500 A/mm2 at 16 T and 4.2 K [1]. Nb3Sn, a low temperature superconductor with a critical temperature Tc up to 18.3 K, is currently the best candidate for such magnets, since it is the only affordable material able to meet the afore mentioned requirements. To feed the FCC-hh magnets, new superconducting lines will be developed. Similarly to the LHC electrical layout, also for the FCC case the transfer of the current from the surface to the tunnel, where the magnets are located,would be possible via superconducting links containing tens of cables feeding different circuits [6]. The work related to the realization of such links focuses on the development of novel types of cables made out of Mg B2. Furthermore, since the FCC-hh is expected to produce unprecedented amounts of synchrotron radiation, a superconducting beamscreen is necessary in order to protect its sensitive components. Two suitable candidates for the beam screen coating are the high temperature superconductors YBCO and the technologically still unexploited thallium-based cuprate Tl-1223. Since YBCO is expensive and has a complex preparation on large scale, Tl-1223 could represent the properchoice for the material addressed to the beam screen coating. The introduced materials (Nb3Sn, MgB2 and Tl-1223), which represent the best candidates for some of the FCChh fundamental components, are the"main characters\ of my PhD thesis. In particular,I investigated their structure on a micro- and nanoscale level, and a greater emphasis was given to the study of Nb3Sn for the FCC-hh bending magnets.The microstructural investigation is an essential tool for understanding how the material superconducting properties can be enhanced, in order to exploit them for future applications. The electron microscopy, the science allowing the material microstructuralanalysis, plays a very important role in terms of studying the material intrinsic and extrinsic attributes for understanding its superconducting behavior. The manufacturing processes the superconducting materials come from are crucial in defining their characteristics: a key point in my work is to understand how the parameters involved in such processes influence the material microstructural features, strictly connected to its superconducting properties. In this way, it would be possible to give the manufactures an effective contribution in terms of producing wires and thin films with enhanced superconducting performance. For the material microstructural characterization electronmicroscopes were employed: these instruments use accelerated electrons under vacuumconditions in order to generate highly magnified images of specimens. In particular, both a scanning electron microscope (SEM) and a transmission electron microscope (TEM) were used. The first one provides information from the surface of a sample: in fact, electrons scattered by or emitted from its surface are used for image generation. TEM uses instead the electrons transmitted from a very thin specimen (approx. 100 nm) to build images. Since these microscopes are equipped with several attachments such as X-raydetectors (both SEM and TEM) or energy filters (TEM), they provide element-specific information from the sample, e.g. chemical composition, elemental distributions, grain size and shape, doping agents size and density.This work is part of the Marie Sklodowska-Curie Action EASITrain, funded by the European Union’s H2020 Framework Programme under grant agreement no. 764879.