A multi-technique analysis of deuterium trapping and near-surface precipitate growth in plasma-exposed tungsten

作者：Kolasinski R D^*; Shimada M; Oya Y; Buchenauer D A; Chikada T; Cowgill D F; Donovan D C; Friddle R W; Michibayashi K; Sato M

来源：Journal of Applied Physics, 2015, 118(7): 073301.

DOI：10.1063/1.4928184

In this work, we examine how deuterium becomes trapped in plasma-exposed tungsten and forms near-surface platelet-shaped precipitates. How these bubbles nucleate and grow, as well as the amount of deuterium trapped within, is crucial for interpreting the experimental database. Here, we use a combined experimental/theoretical approach to provide further insight into the underlying physics. With the Tritium Plasma Experiment, we exposed a series of ITER-grade tungsten samples to high flux D plasmas (up to 1.5 x 10(22) m(-2) s(-1)) at temperatures ranging between 103 and 554 degrees C. Retention of deuterium trapped in the bulk, assessed through thermal desorption spectrometry, reached a maximum at 230 degrees C and diminished rapidly thereafter for T>300 degrees C. Post-mortem examination of the surfaces revealed non-uniform growth of bubbles ranging in diameter between 1 and 10 mu m over the surface with a clear correlation with grain boundaries. Electron back-scattering diffraction maps over a large area of the surface confirmed this dependence; grains containing bubbles were aligned with a preferred slip vector along the < 111 > directions. Focused ion beam profiles suggest that these bubbles nucleated as platelets at depths of 200 nm-1 mu m beneath the surface and grew as a result of expansion of sub-surface cracks. To estimate the amount of deuterium trapped in these defects relative to other sites within the material, we applied a continuum-scale treatment of hydrogen isotope precipitation. In addition, we propose a straightforward model of near-surface platelet expansion that reproduces bubble sizes consistent with our measurements. For the tungsten microstructure considered here, we find that bubbles would only weakly affect migration of D into the material, perhaps explaining why deep trapping was observed in prior studies with plasma-exposed neutron-irradiated specimens. We foresee no insurmountable issues that would prevent the theoretical framework developed here from being extended to a broader range of systems where precipitation of insoluble gases in ion beam or plasma-exposed metals is of interest.

• 出版日期2015-8-21