Erosion of Ta₂O₅ and HfO₂ coated silicon nitride radio frequency window material

Linear plasma devices using a helicon plasma source experience impurity generation^{[1, 2]} due to erosion of the plasma source window material by a radio-frequency (RF) sheath formed at its surface. Materials such as nitrides of silicon and aluminum, possessing a high thermal shock resistance and low coefficient of thermal expansion, are ideal to withstand the harsh plasma and heat fluxes at the MPEX plasma conditions. However, these materials have shown considerable erosion during Proto-MPEX operations due to their low-Z components. In response, we propose coating a non-conducting material containing high-Z components on a Si₃N₄ window. This approach is hypothesized to significantly reduce impurity generation due to the component’s low sputtering yield. Our work reports helium and deuterium plasma exposures of Ta- and Hf-oxide coatings on the Si₃N₄ base surface in RF-PIE^[3] (RF-Plasma Interaction Experiment) at 500V RF bias. Optical emission during plasma exposures of the Ta₂O₅ surface has shown increased Ta-line emission with increased RF bias. No preferential sputtering of Ta or oxygen was observed in both plasma conditions. Mass loss measurements were used to calculate the sputtering yield. A similar analysis of HfO₂ surfaces will be presented to compare both materials' durability to withstand MPEX plasma conditions.



References:



[1] Beers C, Lau C, Rapp J, et al. Physics of Plasmas 2021 28

[2] Dhamale G, Balwin M. J., Islam M. S., et al. Plasma Physics and Controlled Fusion (submitted)

[3] Caughman J, Butler K, Curreli D, et al. IEEE Transactions on Plasma Science 2024