Stainless steel performance under ITER-relevant mitigated disruption photonic heat loads

ITER grade stainless steel (SS) 316L(N)-IG was tested in QSPA-T (photons) and JUDITH (electrons) under heat loads relevant to those expected from photon radiation on the ITER diagnostic first wall (DFW) during disruptions mitigated by massive gas injection. Repeated pulses slightly above the melting threshold eventually lead to a regular, “corrugated” SS surface, with hills and valleys spaced by 1-2 mm. The negligible mass loss observed after the heat pulses indicates that hill growth (growth rate of ∼1-2 μm per pulse) and SS plate thinning in the valleys is a result of melt-layer redistribution. A Similar behavior is observed on SS samples exposed in QSPA-T (pulse length 0.5 ms) and JUDITH (pulse length ⩽ 3.0 ms) at the same heat flux factor. The combined data suggests, for the total lifetime, a surface roughening of ⩽1.5 mm on parts of the ITER SS DFW exposed to the highest transient photon loads, after 1200 mitigated disruptions at high stored energy. The results also indicate that the surface roughness increase may be significantly reduced by variation of the SS impurity composition. This experimental observation is supported by a proposed theoretical mechanism for the surface roughness formation based on the growth of capillary waves in the melt layer.