Effects of helium plasmas on tungsten surfaces characterized by helium ion microscopy, ion channeling, and in-situ spectroscopic ellipsometry*

We have used helium ion microscopy and in-situ spectroscopic ellipsometry to characterize how tungsten surfaces are modified by low-energy helium plasmas. The RF plasmas were generated in a small linear plasma device equipped with a Lisitano coil (exposure conditions: Γ_ion = 8.5×10²⁰ m^-2 s^-1; F = 7.9×10²³ - 3.6×10²⁵ m^-2, T_sample = 450 - 930 °C.) Helium ion microscopy was used to image the initial surface morphology changes and nano-tendril growth. We then correlated the microscopy data with the surface optical properties measured by an in-situ, real-time spectroscopic ellipsometry system. We observed a continuous decrease in both the extinction coefficient and index of refraction as a function of fluence between wavelengths of 280-1000 nm. In addition to these results, we present preliminary low and medium energy surface channeling experiments. We considered the W(110) model system, using ion beams to assess the crystallinity of the near-surface.