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Previous work utilizing deuterium (D) found that while displacement damage increases D retention in tungsten (W) due to generated D trapping sites [1-3], helium (He) pre-treatment or seeding reduces D retention in pristine W for low-energy D since He nanobubbles act as a diffusion barrier to D [2,4]. He pre-treatment or seeding also reduces D retention in pre-damaged W such that under certain conditions when both effects are considered, retention is the same as for pristine W exposed to pure D [2,3]. This work investigates how He bubbles and D retention are affected by displacement damage that occurs simultaneously to plasma exposure.
Bulk W samples were exposed to (i) pure D plasma, (ii) mixed D-10%He plasma, or (iii) D-10%He plasma during simultaneous W$^{6+}$ damaging. For all, D ion flux was kept at 4.2$\times$10$^{22}$ m$^{-2}$, D/He ion energy at 75 eV, damage rate at 6.3$\times$10$^{-5}$ dpa/s, W$^{6+}$ energy at 20 MeV, and duration at ~3600 s, while the sample temperature $T_{\mathrm{s}}$ was varied from 550-750 K. An additional sample was exposed to 1 MeV instead of 20 MeV W$^{6+}$ to reduce the peak damage depth from 1.35 μm to 35 nm, and hence to focus the damage to the near-surface He bubble layer while minimizing the damage in the underlying bulk. Post-exposure characterization included scanning electron microscopy, transmission electron microscopy, nuclear reaction analysis, and will include thermal desorption spectrometry.
Up to 300 nm diameter blisters were observed at $T_{\mathrm{s}}$ = 550 and 650 K for both pure D and D-He samples (in contrast to [4]), while simultaneous damage suppressed blister formation (in agreement with [5] for pre-damaged W). At $T_{\mathrm{s}}$ = 650 K, D retention was increased by two orders of magnitude with 20 MeV damage versus without, while retention peaked at shallower sample depths for 1 MeV damage. In view of all, results point to simultaneous displacement damage as having a measurable impact on He-induced diffusion barriers and damage-induced trapping, which in turn, has important implications for predicting hydrogen-isotope retention in tungsten under fusion-relevant conditions.
[1] W.R. Wampler and R.P. Doerner, Nucl. Fusion 49 (2009) 115023
[2] V.Kh. Alimov et al., J. Nucl. Mater. 420 (2012) 370-373
[3] Q. Bai et al., Nucl. Fusion 59 (2019) 066040
[4] M. Miyamoto et al., Nucl. Fusion 49 (2009) 065035
[5] S. Wang et al., J. Nucl. Mater. 508 (2018) 395-402
Work is supported by US-DOE-FES (DE-SC0022528) and NIFS Collaboration Research program (NIFS25KIET012).