Radiation damage in superconducting materials for Fusion applications

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

Future designs of fusion power machines will all require large superconducting magnet windings, and these superconductors will, in service, be subjected to high fluxes of energetic particles, most seriously the 14 MeV neutrons generated in the deuterium/tritium fusion reaction. Modern designs of small tokamaks usually rely on high temperature superconducting tapes to generate the large magnetic fields required for plasma confinement, and it is an engineering priority to generate these fields in as small a volume as possible.

However, little is known about the effect on the superconducting properties of the lattice damage resulting from the impact of energetic particles, especially in 2nd generation high temperature superconducting tapes where the ceramic superconducting layer that carries currents of hundreds of amps is only 1 micron thick. Much of the damage will be in the form of clusters of point defects in irradiation cascades, or possibly the dissolution of nanoscale flux pinning particles, and so can only be studied by very high resolution techniques - many of which have been developed in previous work on radiation damage in fission materials.

The project will use advanced electron microscopy techniques to identify the key damage mechanisms in a range of different superconducting materials exposed to high energy particles, and to correlate these with direct superconducting property measurements. National ion beam damage facilities (at Surrey and Huddersfield Universities) will also be used to study the effect of proxy irradiation by heavy ions, and to correlate the ion damage mechanisms with those found in n-damaged materials provided in both bulk crystals and 2nd generation tapes by our partners in the University of Vienna. We will explore novel measurements of ion damage degradation with the superconductor below its transition temperature, and if possible carrying current, to be as close to the real operating conditions as possible. This will be the first time that this in-situ damage experiment will have been attempted.

The primary aim of the project will be to identify the maximum neutron dose that a magnet winding can be subjected to without severe and irreversible damage, as fundamental input data for the designers of next generation tokamak machines. The novelty of the approach is to take techniques well known to the fission community in steels and zirconium alloys and to apply them to the novel ceramic materials now being selected for fusion magnets. The project is aligned with the EPSRC theme of Energy. This studentship is a collaboration with a local small company, Tokamak Energy.

The Themes are:
Energy
Physical sciences

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1922085 Studentship EP/N509711/1 01/10/2017 31/03/2021 William Iliffe
 
Description As a result of a literature review, it has been established that the current carrying performance of second-generation (2G) high temperature superconducting (HTS) coated conductors, in the high magnetic fields present in magnetic fusion devices, are affected by impacts by energetic particles. This effect is first positive, improving their performance up to a relatively low damage level, and then negative, until superconductivity is extinct, at much higher levels of damage.

Establishing more details about the types of impacts and resulting damage due to energetic particle bombardment in 1) conceptual fusion devices and 2) experiments where 2G HTS have been irradiated in fission reactors has been a key area of my research. I have used the material simulation software "SPECTRA-PKA" to calculate the effects of various energetic particle environments on 2G HTS. Results have allowed me to gauge how neutron shielding in fusion devices effects which neutron-impacted ions lead to the highest proliferation of damage in 2G HTS. This has informed my choice of which energetic ions to use to experimentally simulate damage from fusion neutrons. It has also allowed me to gauge the differences in damage creation dynamics between fusion and fission spectrum neutrons.

Based on these findings, I have performed various room-temperature irradiation experiments on a variety of 2G HTS with 2MeV helium and 3.1MeV oxygen ions. This has allowed me to establish the base-line effects of energetic ion irradiation on superconducting properties. Coupled with literature data from similar experiments, these experiments have added to the knowledge base surrounding these materials and informed my choice of ion-sample combinations to use for in-situ irradiation experiments.

The most significant progress I have made since the beginning of my award is the development of a new experiment to allow the in-situ testing and ion beam irradiation of 2G HTS samples at temperatures below -240C. This experiment, built in collaboration with Surrey Ion Beam Centre, has now been completely assembled and several samples have been tested. As of the time of writing, most of the tested samples have been damaged during high current testing and therefore finessing of the samples and experimental procedure is required before ion beam irradiation experiments can begin.
Exploitation Route 5) The over-arching goal of this work is to further inform the designers of magnets going into magnetic confinement fusion devices on how the 2G HTS is affected by fusion neutrons. Current estimates of the performance of these superconductors is based on irradiation experiments performed at moderate temperature (60'C) in fission devices and on mono-energetic ion bombardment experiments performed at room temperature. My specific goal is therefore to establish whether irradiation at low temperature (-240'C) - akin to the actual operating temperature of these magnets - more or less significantly affects the properties of 2G HTS than room temperature irradiation. This will allow for a significant de-risking of these magnets, which tend to be complex to build and expensive to manufacture.
Sectors Energy