UC San Diego

For magnetic confinement fusion reactors, physical sputtering of the plasma-facing materials (PFMs) can influence the lifetime of PFMs and the migration of impurities. During plasma exposure, the surface morphology of a PFM can be modified, resulting in the formation of complex submicron-scale structures. This dissertation demonstrates how such modifications can influence physical sputtering of PFM, the subsequent transport and migration of the physically sputtered impurities. In Chapter 3, the microscopic cone structures that can spontaneously form on a Cr surface physically sputtered by He plasma are studied in the PISCES-A linear plasma device with hyperspectral imaging measurements and in simulations couple SDTrimSP and GITR codes. Due to line-of-sight redeposition of sputtered Cr atoms onto cone structures, the effective sputtering yield is reduced, and the angular distributions at smaller angles with respect to the macroscopic surface normal is enhanced. In Chapter 4, the influence of fuzz structures on physical sputtering of Mo by He plasma is studied using the similar methods. While sputtering yield reduction due to the line-of-sight redeposition in the fuzz layer is confirmed, fuzz structures do not influence the angular distributions of sputtered Mo significantly. According to the depth profile of the differential sputtering yield from the simulations, a simple analytical model that characterizes the fuzz layer by a single collision mean free path is proposed, which successfully explains the effect of fuzz on physical sputtering. In Chapter 5, how angular distributions of physically sputtered W influence migration of W in the ITER divertor region is conceptually studied using ERO 2.0 simulations. When using simple idealized unidirectional distributions, the line-of-sight redeposition locations of W atoms clearly vary with the direction of the initial velocity. However, despite the differences between the angular distributions of W sputtered on a flat surface and on a fuzzy surface, the corresponding W deposition profiles look similar, since the suppression of neutral W atom population by ionization dominates the profiles, enhancing atomic deposition at locations closer to the strike points. From the results of this dissertation, the influence of surface morphology should be considered when studying physically sputtered impurities.