Johannes Bernardi

Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools/reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. The content and structure of this report are guided by the scope and priorities defined in the mandate of the FCC Feasibility Study. It is therefore not intended to serve as an exhaustive review of the full physics potential of FCC. Several topics, already covered in earlier reports such as the FCC CDR, are not reiterated here or are addressed only briefly, in alignment with the study’s focus. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme.

Transition metal dichalcogenide (TMD) coatings have attracted enormous scientific and industrial interest due to their outstanding tribological behavior. The paradigmatic example is MoS₂, even though selenides and tellurides have demonstrated superior tribological properties. Here, an innovative in operando conversion of Se nanopowders into lubricious 2D selenides, by sprinkling them onto sliding metallic surfaces coated with Mo and W thin films, is described. Advanced material characterization confirms the tribochemical formation of a thin tribofilm containing selenides, reducing the coefficient of friction down to below 0.1 in ambient air, levels typically reached using fully formulated oils. Ab initio molecular dynamics simulations under tribological conditions reveal the atomistic mechanisms that result in the shear‐induced synthesis of selenide monolayers from nanopowders. The use of Se nanopowder provides thermal stability and prevents outgassing in vacuum environments. Additionally, the high reactivity of the Se nanopowder with the transition metal coating in the conditions prevailing in the contact interface yields highly reproducible results, making it particularly suitable for the replenishment of sliding components with solid lubricants, avoiding the long‐lasting problem of TMD‐lubricity degradation caused by environmental molecules. The suggested straightforward approach demonstrates an unconventional and smart way to synthesize TMDs in operando and exploit their friction‐ and wear‐reducing impact.

We demonstrate the combined use of scanning electron microscopy (SEM) and scanning Hall probe microscopy (SHPM) to analyse inhomogeneities in Nb₃Sn wires. Inhomogeneities of the A15 phase in Nb₃Sn sub-elements of a Ti-alloyed Restacked Rod Process wire and a Ta-alloyed Powder-In-Tube wire are investigated. Microstructural features are examined by SEM, elemental concentration gradients by energy dispersive x-ray spectroscopy (EDX) and the superconducting properties by SHPM. Correlations between the results are analysed to gain information about the impact of inhomogeneities in the microstructure on the superconducting properties. We find considerable differences in geometry and performance between sub-elements, as well as compositional and geometric inhomogeneities of the A15 phase inside single sub-elements. Additionally, simulations of the influence of Sn concentration gradients on the critical current density J _c are performed. We also demonstrate the viability of SHPM and EDX for determining the dependence of the critical temperature T _c on the Sn concentration and discuss possible performance gains by a reduction of inhomogeneities in Nb₃Sn wires.

REBCO based coated conductors (CCs) are a viable alternative to conventional superconductors for many applications, therefore the optimization of their current carrying capacity is an ongoing process. A promising route for the increase in performance is the introduction of artificial pinning centers such as BaHfO₃ (BHO) nanoparticles. However, granularity still imposes a substantial performance limitation, especially in REBCO CCs deposited on RABiTS based templates, as the critical current density is severely reduced by moderate misalignment angles of adjacent grains. A combined study of scanning Hall probe microscopy and electron microscopy of undoped and BHO-doped YBa₂Cu₃O_7-δ (YBCO) films on technical templates shows that BHO-doping leads to a denser microstructure of the superconducting layer and higher global and local critical current densities. The statistical evaluation of local current maps allows for a quantification of the magnetic granularity where a reduction of granularity with increasing film thickness, doping and increasing temperature is found. In particular, the dependence of granularity on the film thickness and enhanced film growth through BHO-doping shows the potential for further optimization of YBCO films on RABiTS based templates.

Iron-based superconductors are a popular candidate in the search for affordable and simple superconductors for high-field applications. In particular, the relaxed texture requirements fuel hope that films deposited on RABiTS with simple buffer layer architectures could enable cheap coated conductors. We find that a single Yttrium oxide buffer layer can act as a suitable diffusion barrier and epitaxial Fe(Se,Te) thin films were successfully grown by pulsed laser deposition. An analysis of the local current distribution by means of scanning Hall probe microscopy reveals current densities exceeding 1 MA cm⁻², however, granularity still seems to be an issue. Transmission electron microscopy images and analysis by transmission Kikuchi diffraction show that the out-of-plane orientation of underlying Ni-W grains in the substrate has a severe impact on the growth of Fe(Se,Te) films.

The resistive-type of superconducting fault current limiters (R-SFCL) using the second generation of high-temperature superconductors (2G HTS) are well adapted to power grid protection. Almost electrically invisible in normal operation, it quenches and becomes highly resistive in the event of a fault. The tape operation at the transition from the superconducting state to the resistive state is the basis of a R-SFCL. In this scope, we studied the onset of a quench occurring in 2G HTS tapes using high-speed imaging to record the bubble generation in liquid nitrogen. In the first milliseconds of operation, the bubble generation appears highly inhomogeneous. Dissipation is initiated on multiple spots over the tape surface, which first expands in the direction of the width of the tape and then along its length. A detailed analysis of the REBCO layer is performed by means of scanning Hall probe microscopy (SHPM) after the optical study of the tapes is completed. Analysis of the local current density distribution demonstrates that the bubble generation occurs at positions of local inhomogeneities in the REBCO layer. These inhomogeneities, well recognized through SHPM, are not reliably described by the critical current as a function of the position derived magnetically (e.g., by traditional TapeStar measurements). Similar results were obtained on samples from Superpower and SuperOx.