STUDY OF THE IRRADIATION EFFECTS ON SUPERCONDUCTING FILMS OF IRON BASED SUPERCONDUCTORS

The magnetic fields needed for applications such as nuclear fusion plants are in the range between 8 T and 20 T. In such conditions, superconducting materials are needed in order to build magnets for plasma confinement. Magnets developed and manufactured for nuclear fusion up to now are in Nb-Ti and Nb3Sn, but there are materials which can be better candidates for these applications such as the High temperature superconductors like REBCO, which show better performances in field, but are very complicated to be manufactured as conductors in long lengths. One of the main difficulties is due to the fact that REBCO needs to be grown epitaxial in order not to suffer from weak link grain boundaries which limit the current flow. Therefore, REBCO wires must be fabricated as coated conductors, i.e. highly textured, biaxially aligned films deposited on a metal tape substrates covered with suitable buffer layers. Iron Based Superconductors (IBS) since their discovery in 2008 have grown to become a new class of high magnetic field superconductors. At low temperature, their upper critical fields are high, proving that they are very promising for fusion magnets. Although not yet considered "technological" conductors, the investment in terms of research worldwide on IBS is large. IBS are semi-metallic materials with transition temperatures up to 55K. The combination of extremely high upper critical fields, moderate anisotropies and high critical field, makes this class of superconductors particularly appealing for high-field applications. Moreover, they are less sensitive to grain boundaries and therefore they can be grown as coated conductors on less textured substrates and with architectures and buffer layers much simpler than those required by REBCO. There are different IBS families, each deriving from a common parent material. FeSe (so-called 11 family) is very interesting because it is the only not containing arsenic, and it has the simplest structure. FeSe has a critical field up to 50 T and a critical temperature of 9K, but an enhancement of the TC was observed with the substitution of Te for Se, for which the TC increased up to 75%, while 21 K can be reached upon strain induced by substrates. The aim of my project is to improve the growth of FeSeTe thin films, to be able to grow this phase on coated conductors with superconducting properties appealing for conductors for fusion applications. Inside a fusion plant, our material would be subjected to many particles radiation, so an important stage of this study is to understand how irradiation affects the phase. As we could imagine, a too high dose of radiation will ruin its superconducting properties, but, actually, radiation could bring some positive effects too, creating defects that can increase the upper critical field and pin the flux lines, that allows to reach higher critical current density JC. Although many irradiation experiments on IBS single crystal have been reported, only few studies report on the effects of irradiations in thin films: a complete understanding of the effects of irradiation with different particles and energies is foreseen. This thesis is inserted in the Project 4 of the JRC Fusion “High Tc superconductors for magnetic confinement fusion: development of materials and production processes” within the Joint Research Agreement between CNR and Eni. The partners of this Project are Eni itself, and the two CNR Institutes SPIN (Genova and Salerno) and IMM (Catania). Moreover, a significant part of my project has been carried out in collaboration with the PRIN project HIBISCUS.