Neutron reflectometry for in-situ surface characterization of plasma-facing components

Measuring the evolution of plasma-facing-components (PFCs) in a tokamak or stellarator environment is extremely challenging, in part due to the difficulty in establishing in-situ techniques to diagnose materials during irradiation under a controlled setting and measure the evolution of the plasma-material interface (PMI). To address this challenge, we study diagnostics which rely on neutrons to characterize fusion-relevant PFC materials during plasma exposure. Proof-of-concept modeling for neutron reflectometry (NR) indicates that changes in near-surface material composition near-surface due to deuterium implanted in pure W (19.35 g/cm3) at 500–1000 eV can be detected by NR at fluences as low as 10¹⁶ cm-². Fluences of 10¹⁷, 10¹⁸, and 10¹⁹ cm⁻² are distinguishable from each other, and differences for larger fluences must be detected at higher q values. NR was also modeled for D implantation at 1000 eV for various dispersion-strengthened W materials. At each fluence, the reflectivity curves are qualitatively similar regardless of the composition. Differences between the curves as a function of dispersoid concentration are due to the differences in the scattering lengths of the dispersoids. We conclude that the range of incident angles needed to capture relevant features depends on sample density. This result will be important for designing future experiments, to ensure that the relevant features are captured. Given the observed, large changes in NR spectra as a function of fluence, it is expected that NR will be an effective technique for the in-situ diagnosis of PFCs during plasma irradiation. A multi-diagnosis system, which will be implemented and tested at the NIST Center for Neutron Research, has been developed to perform in-situ NR experiments.