Combined high fluence and high cycle number transient loading of ITER-like monoblocks in Magnum-PSI

It is highly desirable to understand the long term evolution of the divertor material under the extreme steady-state and transient heat and particle loads expected during ITER operation. Here the impact of ELM-like transient loading under combined high-flux plasma and transient ELM-like heat loading in Magnum-PSI was explored to determine how plasma affects the fatigue cracking threshold of tungsten due to ELMs. Mock-ups consisting of five ITER-like monoblocks in a chain were simultaneously exposed to high flux plasma and a high power pulsed laser which closely simulated the ELM impact in terms of heat flux and duration. Loading conditions were chosen to enable comparison to existing data from electron-beam loading, while the influence of surface base temperature (750 °C, 1150 °C or 1500 °C) and impurity seeding (addition of 6.5ion% He+ and/or 8ion% Ne+) were also investigated. The plasma loading leads to differences in surface morphology and indicates synergistic effects on the extent of the surface damage. Base temperatures at or above 1150 °C are found to lead to a significant reduction in the fatigue cracking threshold by a factor of two or more compared to at 750 °C. Cracked surfaces are found to be more than ten times rougher than the original microstructure, and additionally when seeding impurities are added surface roughness can be significantly increased by up closely factor of two compared to roughening using pure H plasma. Overall the results indicate that avoiding fatigue cracking in ITER will be very challenging, and that understanding the level to which this can therefore be tolerated is vital for anticipating divertor lifetime and reliability.