Tungsten and CFC degradation under combined high cycle transient and steady state heat loads

Within the ITER divertor lifetime millions of transient events are expected during H-mode operation due to edge localized modes (type I ELMs). These will deposit their energy on plasma facing materials that are pre-heated to various surface temperatures, depending on the steady state heat load (SSHL) at the respective location, leading to synergistic effects. An electron beam facility was used to simulate ELM-like heat loads with ITER relevant power densities (≈0.5 GW/m2) and pulse duration (0.5 ms). At the same time additional SSHL was applied to obtain different base temperatures. Experiments were performed on actively cooled pure tungsten and the carbon fiber composite (CFC) NB41, applying 103–106 pulses of 0.5 ms duration with a power density of 0.14–0.55 GW/m2 and 0.55–0.68 GW/m2 on tungsten and CFC, respectively. Surface temperatures were about 200 °C, 400 °C and 700 °C for tungsten and about 450 °C for CFC. Crack formation in tungsten was preceded by roughening due to plastic deformation. In case of Tsurf ≈ 200 °C cracks propagated comparably fast (brittle material), while slow propagation and recrystallization around the crack edges indicated fatigue damage at higher temperatures. Compared to tungsten, CFC showed a higher damage threshold.

► Tungsten and CFC were subjected to transient high cycle heat loads.
► Tungsten showed fatigue damage (roughening, cracking, melting).
► Roughening occurred as precursor for cracking.
► Underlying mechanism depended on surface temperature.
► CFC showed 3× higher damage threshold and no long term fatigue mechanism.