Predicting performance of high temperature superconductors under fusion conditions

Supervisors: Professor Susannah Speller

Pulsed magnetic field measurements on REBCO coated conductor taken at Los Alamos National Laboratory (NHMFL facility).

The economic viability of a tokamak power plant (TPP) is a function of its size, toroidal field (TF) strength and availability during its operating lifetime. This optimisation has led the designers of tokamaks to adopt both a compact design and the use of coated conductors (CC) made with rare-earth barium copper oxide (REBCO) high temperature superconductor as the current carriers for their magnets. However, designing TF magnets for TPPs has some unique difficulties, notably that the properties of the superconductor at the very high magnetic field of a TF magnet (20 tesla) can only be accessed at specialist international user facilities, and that radiation emitted by fusion reactions causes damage to REBCO CCs that affect their ability to carry current.

This collaborative project between the Universities of Oxford and Cambridge and the UK Atomic Energy Authority involves using a combination of phenomenological modelling and experiments. The aim is to develop scaling relationships that will enable the prediction of superconducting performance under fusion magnet operating conditions (20 T, 20 K) from more easily accessible measurements at lower field and/or higher temperature. This will initially require comprehensive microstructural and electromagnetic characterisation of a selection of typical coated conductors (e.g. with and without artificial pinning centres) over the full range of magnetic fields, temperatures and field angles using a combination of facilities at the partner organisations and international user facilities such as the Pulsed Field Facility at Los Alamos National Laboratory. This data will be used to determine where scaling of easily accessible high temperature, low field data can be safely performed. This will allow us to determine the optimum qualification test conditions for the coated conductor that balances the ability for testing large quantities of material quickly and cheaply, with a high degree of confidence in the extrapolation to fusion magnet operation conditions provided by the robust scaling relationships. Given that the performance of a REBCO CC is dependent on the defect structure within REBCO, the work will be extended by applying irradiation to the samples to change the defect landscape in the REBCO and assess how that changes performance. Understanding the effect of irradiation is of great importance for fusion magnets because the REBCO will be exposed to fast neutrons that seriously degrade the superconducting performance, limiting the lifetime of the magnet and determining the thickness of shielding required. Further work may include the extension of the dataset to include field angles not perpendicular to the direction of current flow and/or to include the effects of strain on REBCO performance and be combined with molecular dynamic simulations using machine learning potentials developed for magnet for REBCO materials from first-principles modelling. In addition to developing and validating scaling laws that are essential for fusion magnet design, the student will contribute to the understanding of vortex physics in HTS, which is a rich area of high scientific interest more broadly than fusion.

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