Description du sujet de thèse

Superconductivity is a state of matter that allows electricity to flow, and therefore magnetic fields to be generated, without dissipation. Over the past decade, the commercial advent of High Temperature Superconducting (HTS) tapes has opened up revolutionary prospects for certain breakthrough technologies requiring high magnetic fields: medical imaging (MRI), ultralight and compact turbines or electric motors (wind turbines, boats, airplanes, etc.), and magnetic confinement in fusion reactors and particle accelerators. HTS tapes can indeed generate much higher magnetic fields, and the fusion power density is proportional to B4, potentially paving the way for simple, compact and commercially viable fusion reactors maybe much more quickly than the ITER fusion project would let us hope, according to several start-ups in the running.
However, the electrical transport properties of HTS tapes in high magnetic fields (above 30 T) remain largely unexplored. These properties are driven by the physics concept of superconducting vortex pinning. In HTS, the application of even a relatively weak magnetic field leads to the penetration of magnetic field tubes carrying a quantum of flux ϕ0 and surrounded by swirling supercurrents: the vortices. When a current flows through the superconductor, if the vortices are not pinned then they move under the action of the Lorentz force, dissipating energy: there is no more electrical transport without dissipation. So, to maximize the critical current (and avoid destroying the tape), it is necessary to have both non-superconducting nanometric defects that energetically pin the vortices and a high crystalline quality around the defects to maximize the robustness of the superconducting state.
Over the last two decades, research and industry have made considerable progress in the production of reliable high critical temperature (HTS) superconducting tapes in long lengths (thousands of km/year) and with a nanostructure optimized to maximize the superconducting critical current. Synthesizing these tapes is a real technological feat, requiring control of the material on a nanometric scale. It requires cleanroom technologies to perfectly grow epitaxially a brittle quaternary (Rare Earth-Ba-Cu-O) superconducting ceramic on a flexible metal tape, and to include controlled nanometric defects in this superconducting matrix to pin the superconducting vortices.
One of the current lines of research is aimed at increasing the critical current under a magnetic field, which would make it possible to envisage these new cutting-edge 'electromechanical' machines. The question of the effective ultimate limit of vortex pinning at very high magnetic fields (and therefore very high vortex densities) remains open. Experimentally, there is a vast space to explore, as there are many natural and artificial vortex pinning centers: point defects, grain boundaries, stacking faults, nanocolumns, nanoparticles, etc. And theoretically, only intensive numerical methods can provide ab initio results that remain qualitative for the critical current, in the face of this problem of out-of-equilibrium statistical physics of 'elastic lines with quantum properties' (vortices) in a disordered medium.
Project objectives:
The 1st part of the thesis will consist of an experimental study of the link between nanostructure and electrical transport properties of HTS tapes at low temperature (77 K -1.5 K) up to 60 T at LNCMI-Toulouse. Tapes with different nanostructures will be sourced via our existing collaborations.
The 2nd part will involve integrating these results into the numerical tools for modelling superconductors already developed at the Néel Institute in Grenoble, by iterating between modelling and experiment. The 30-60T magnetic field range is essentially unexplored in this field because the measurements did not work in pulsed fields. The project will benefit from an innovative experimental setup that enable measurements in this range at the LNCMI, one of only 4 laboratories in the world where it is possible to regularly access fields in excess of 30 T.

This thesis project will be associated with all stages of research: sample preparation, measurement and analysis/understanding of data in relation to theory, integration of results into numerical models, presentation of results in articles and national/international conferences.

Contexte de travail

The Laboratoire National des Champs Magnétiques Intenses (LNCMI), located on 2 sites (Toulouse and Grenoble), is a CNRS laboratory (UPR3228) and a Très Grand Instrument de Recherche (TGIR). It is associated with INSA Toulouse, Université Grenoble Alpes (UGA), and Université Paul Sabatier (UPS, Toulouse).
The LNCMI enables researchers to carry out experiments in some of the world's most intense magnetic fields. Continuous fields up to 36 T are available at the Grenoble site.

This PhD project based at LNCMI Toulouse will take place as part of a nascent collaboration between the LNCMI (Toulouse an Grenoble) and the Néel Institute (Grenoble). Therefore, several stays of up to several months in Grenoble will be planned.
The research project relates to the electrical properties of high critical temperature (HTS) superconducting tapes in the presence of high magnetic fields, within the PEPR "Suprafusion.
The PhD thesis will take place in the quantum Nanostructures and topological matter group at the LNCMI Toulouse, which consist of 5 permanent scientists.
The project will benefit in particular from an innovative pulsed I-V system in Toulouse, which enable studies up to 60 T (pulsed), and in Grenoble from a 2000 A and 35 T (DC) system as well as the modelling tools already developed. Tapes with different nanostructures will be sourced via our existing collaborations. The LNCMI-EMFL is one of only 4 laboratories in the world where fields above 30 T can be routinely accessed.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

Several stays of up to several months in Grenoble will be planned. National and international travel of up to a few weeks per year will also be required to present results at conferences and get training at summer schools on most recent research topics.