Development of an in operando uniaxial fatigue testing regime for the linear plasma device Magnum-PSI

Tokamak divertors may be exposed to cyclic thermomechanical loads during service, giving rise to the fatigue cracking of plasma-facing monoblocks. Fatigue is a surface-sensitive dislocation-mediated degradation phenomenon, and monoblock fatigue life may be affected by plasma-material interactions such as D/T/He implantation, blistering, fuzz, and ELMs that modify the morphology, microstructure and mechanical response of the plasma-facing surface. To investigate the synergistic effects of plasma-material interactions on fatigue cracking, a novel experimental method has been developed for the Magnum-PSI linear plasma device that combines cyclic thermomechanical loading with simultaneous plasma exposure. A DEMO strikepoint sweeping scenario (45 MW m−2 at 1 Hz over 100 mm span) is translated into experimental parameters via time-dependent thermal–mechanical finite element modelling of a monoblock, which calculated the uniaxial strain imposed at the centre of the plasma-facing surface to be 0.648%. The cyclic thermal loading of ITER-grade W targets is achieved in Magnum-PSI via sinusoidal modulation of the plasma source, with real-time monitoring via synchronous Thomson scattering, optical emission spectroscopy, pyrometry, and infrared thermography. Initial results are presented which demonstrate a proof-of-concept in operando fatigue testing method, qualitatively explore surface roughness/treatment effects, and indicate that strikepoint sweeping may significantly contribute to plasma-facing surface cracking.