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

Abstract

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 × 1022 m−2 s−1) at temperatures ranging between 103 and 554 °C. Retention of deuterium trapped in the bulk, assessed through thermal desorption spectrometry, reached a maximum at 230 °C and diminished rapidly thereafter for T > 300 °C. Post-mortem examination of the surfaces revealed non-uniform growth of bubbles ranging in diameter between 1 and 10 μm 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 μm 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.

Deuterium trapping and surface modification of polycrystalline tungsten exposed to a high-flux plasma at high fluences

“…Despite more than one order of magnitude variation of the incident fluence, the appearance of blisters and protrusions, namely their number density and average size, barely changed with increasing fluence. Furthermore, grains without any blister and protrusion were also present, which is consistent with previously reported results [19,29]. It should be noted, however, that the method of surface preparation could potentially influence the appearance of the observed surface modifications [8].…”

“…As a result, fluences in excess of 10 30 ions m −2 will be reached by the end of ITER operation. However, in experiments performed so far fluences did not exceed 10 27 ions m −2 [3,[14][15][16][17][18][19]. Only in recent experiments at PISCES-B W targets were exposed to a deuterium (D) plasma to fluences up to 2 × 10 28 D m −2 .…”

Combined effects of crystallography, heat treatment and surface polishing on blistering in tungsten exposed to high-flux deuterium plasma

“…A major difference between the experiments described here and those previously reported [3][4][5]) is in the surface treatment of the samples. Here electrochemically polished samples were used, leading to a deformation-free surface.…”

“…Exposure of tungsten to plasma with a high ion flux of hydrogen isotopes under conditions comparable to those at the plasma-wall boundary in a fusion reactor leads to the formation of macroscopic sub-surface cavities, associated with surface blistering [1,2]. An oftenreported feature of the blistering process is its non-uniformity-micrographs of the plasma-exposed surfaces reveal regions of intense blistering directly neighbouring blister-free regions [3,4]. Orientation imaging using electron backscatter diffraction (EBSD) indicates that these regions of different blistering characteristics correspond to different grains separated by high-angle grain boundaries [5].…”

Hydrogen bubble nucleation by self-clustering: density functional theory and statistical model studies using tungsten as a model system

“…Though it is normally difficult for the hydrogen concentration to reach C * H due to the extremely low hydrogen solubility in tungsten, it can be possible when tungsten is exposed to low-energy high flux hydrogen plasma when the hydrogen concentration can be significantly enhanced. Generally, the maximum concentration of hydrogen at the material surface during ion implantation, C Ion H , can be evaluated by [61]:…”