UC San Diego

Understanding the erosion and redeposition behavior of tokamak plasma facing materials is important to ensure component lifetime. The goal of this thesis was to characterize the erosion and redeposition of aluminum (Al) when exposed to a tokamak divertor plasma. Al was chosen for its similarities to beryllium (Be), which is intended for use as the first wall material in ITER but is toxic and restricted in DIII-D and most other tokamaks. The Divertor Material Evaluation Station (DiMES) was used to expose a set of Al-coated samples to low-density L-mode plasma discharges in the DIII D divertor. Different plasma conditions were used for each sample (including He and D plasmas), and samples with both ideal (smooth) and practical (rough) surfaces were exposed. Measurements of quantitative emission spectroscopy and film thickness change were compared to simulations of sputtering, ionization, and redeposition of Al to determine the gross erosion rate, redeposition fraction, and spectroscopic emission efficiency.

We present the first quantitative spectroscopic measurements of neutral emission anisotropy due to sputtering erosion in a tokamak divertor plasma. We present an ionization-emission model that reproduced the anisotropy by assuming full angular sputtering yield distributions predicted by grazing angle sputtering simulations and ion beam sputtering measurements. Grazing angle ions were expected due to the disappearance of the classical Debye sheath in favor of a thicker magnetic pre-sheath (MPS) at small magnetic field surface inclination angles. The direction of presumed sputtering anisotropy and E×B drift of ions within the MPS was consistent with the direction of deposition patterns found on the samples and within individual pores of the rough surfaces. A model of the erosion-redeposition cycle including re-erosion and material mixing reproduced observed film thickness change measurements. The characteristic Al redeposition length was on the order of its ionization length, 1-3mm at the 1×10^13cm^-3 plasma densities analyzed. Total redeposition fractions ranged from 30% to 76%, increasing with higher plasma temperature due to the higher sheath electric field strength. On rough surfaces on the order of 50% of redepositing material was retained in hidden or shadowed regions, while on smooth surfaces this effect was negligible.