Characterizing the effects of hydrogen and helium plasmas on tungsten surfaces with in-situ ellipsometry and low-energy ion beam analysis*

This study includes the use of in-situ techniques to characterize the structure and composition of plasma-exposed surfaces. For analysis of surface structure, we use spectroscopic ellipsometry to quantify the rate of tungsten nanostructure growth during high-flux He plasma exposure. Under appropriate conditions, we have also used ellipsometry to measure erosion of thin oxide layers. For surface composition measurements, we have also developed a high-fidelity in-situ surface technique based on ion scattering spectroscopy that is capable of operating at mTorr pressures. In this case, the surface is probed using a pulsed beam of 1 - 20 keV ions (typically Li+ or Na+ ), and energies of the scattered/recoiled particles are measured using differentially pumped time-of-flight (TOF) detectors. The atomic mass of each surface species can be related to the particle energies using classical kinematics. Both approaches provide insight into the mechanisms governing dynamic surface modification by plasmas.