Replication of fusion tokamak plasma disruptions on Eurofer 97 steel using Magnum-PSI

Plasma disruptions will impose high magnitude, short duration radiative loads on the plasma-facing first wall of a commercial-scale tokamak, constituting a transient thermal ageing mechanism that affects the performance and lifetime of underlying structural and heatsink materials. In this work, the Magnum-PSI linear plasma device and an ancillary Nd:YAG laser were used to expose 9 samples of the structural steel Eurofer 97 to 1, 100, and 1000 reactor-representative thermal transients of 0.5, 1 and 3 ms duration. The resultant microstructural evolution was characterised via scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM), and microhardness mapping. A duration-dependent evolution of grain diameters, precipitate populations, and hardness were observed. Notably, after 1000 transients of 3 ms, evidence of dynamic recrystallisation, followed by grain coarsening was observed, accompanied by the dissolution of MX and M23C6 precipitates. For the 1000 transient set, hardness reductions of 13%, 6.3%, and 11.2% for the 0.5, 1, and 3 ms respectively suggest a reduction of Hall-Petch strengthening and precipitate hardening contributions. A damage depth of 110–120 µm is estimated from analysis of precipitate depletion. This is supported by carbon diffusion length calculations and hardness mapping.