Irradiation tolerance of high temperature superconducting tapes, coils, joints and associated magnet components

Irradiation studies to date have focussed exclusively on criticalcurrent properties of high temperature superconducting (HTS) tapes . This has been of prime concern as it is a device design driver, influencing shielding thicknesses and thus power plant size and economics. However, irradiation is also expected to affect many other factors within the magnet system that are so far unexplored. This joint project with Tokamak Energy, a private fusion company based in Oxfordshire, will address this lack of knowledge, by investigating a range of other key issues, including the mechanical integrity of the tapes and joint resistance. These need to be explored because engineering mitigations and/or smart materials selection choices may be required to prevent magnet degradation over their operational lifetime in a fusion device.

HTS tapes have a complicated, multilayer structure consisting of a flexible metal substrate coated with a series of thin oxide layers, a 1-2 micron thick REBa₂Cu₃O₇ (REBCO) layer and a protective silver overlayer, usually encased in a copper stabilising layer. There are multiple commercial manufacturers of REBCO tapes, using different materials combinations, deposition techniques and layer thicknesses, which may play an important role in their overall radiation tolerance. During magnet manufacture and in operation, the HTS tapes will experience considerable mechanical stresses that can lead to damage if improperly managed, such as delamination of the layered structure. In this project, the effects of irradiation on the mechanical integrity of HTS tapes will be investigated. In particular, damage to the buffer layers and the buffer-REBCO interface will be assessed to determine the impact of irradiation on the delamination strength of HTS tapes.

Another important factor that needs to be considered is the effect of radiation on the performance of non-superconducting joints within the magnet. This involves investigating the REBCO-metallic cladding interfacial resistivity within the HTS tapes themselves, which is a key contributor to joint resistances in HTS magnets, and thus the viability of thermally efficient and maintainable power plant concepts. Since contact resistance is known to be influenced by oxygen at the interface, this strand of the project will correlate changes in interface chemistry and morphology on irradiation. In addition, the joints between HTS magnet components, including the bulk properties of jointing solders and metals, and on the intermetallic compound (IMC) layers contribute to dissipation in the magnets. Proposed demountable magnet jointing schemes commonly employ low melting metals and alloys to enable low resistance joints to be achieved under practical limitations posed during magnet construction and maintenance activities. These low melting alloys and their interactions with substrates such as copper, have received only limited investigation, and their radiation tolerance has not been widely researched. In addition, the effect of radiation on the integrity of other magnet components such as partial insulation, resins, insulators, and other materials, may be explored.

The project will assess the most relevant and pragmatic radiation sources to simulate fusion environments in this study, employing cryogenic irradiation with in-situ testing wherever possible.