Investigation of gamma-induced changes to screening currents and AC losses in mono- versus multi-filamentary REBCO coated conductors using DC and AC magnetometry
REBCO coated conductor tapes are a highly attractive option for magnet materials in future tokamak fusion power plants. However, the threat of intense neutron and gamma radiation, plus AC loss power dissipation during magnet coil ramping, raises concerns around REBCO magnet coil lifetimes. Irradiation-induced flux creep, or AC loss, has been identified as the major performance limiting factor in REBCO tapes at low temperatures and high fields post-irradiation with gamma rays. Knowing that multi-filamentary tapes are under consideration for tokamaks as an AC loss mitigation strategy during coil ramping, magnetic measurements and gamma irradiation experiments were conducted on a YBCO striated tape versus a control YBCO mono-filamentary tape to investigate the differences in post-irradiation screening currents and AC losses. Reduction in AC losses improved magnetisation-Jc maintenance after 1 MGy in the multi- relative to the mono-filamentary samples. After the 5 MGy dose, striations then made the multi-filamentary tape more susceptible to Jc degradation because of the thinner individual filament width. STEM analysis on a previously studied analogous GdYBCO tape revealed that gamma radiation does not introduce microstructural damage. It was postulated that gamma-induced ionisation of core electrons disrupts screening current capability only; nonetheless, in applications such as fusion where there is a transport current and an external field acting on the tapes, this is vital information. Thus, this work provides critical information regarding changes to screening capability in REBCO tapes caused by gamma radiation and concludes that gamma effects should be considered in tokamak tape qualification studies.