Strongly Reduced Penetration of Atomic Deuterium in Radiation-Damaged Tungsten

Radiation-damaged tungsten is exposed to high-flux, low-energy deuterium plasmas at self-bias conditions. We observe that the fraction of deuterium that penetrates is only ${10}^{-5}–{10}^{-7}$ of the plasma flux and strongly dependent on the local surface temperature. We propose that deuterium does not directly penetrate bulk tungsten but that it thermalizes at the surface, where it forms a protective chemisorbed layer. We find an energy barrier of 1–2 eV between the surface and bulk, causing the influx of deuterium to be low as compared to the number of defects and leading to slow filling of the damaged layer.

Figure 1

The influx ${\Phi }_{\mathrm{in}}$ is only a small fraction $f$ of the incoming plasma flux ${\Phi }_{\mathrm{plasma}}$.

Figure 2

Deuterium depth distributions in damaged (0.45 dpa) tungsten targets exposed to high-flux deuterium plasmas at self-bias conditions (a)–(c) and biased at $-40\mathrm{V}$ (d). The exposure times are as indicated. The deuterium depth profiles were measured at three positions on the target (center at 560 K, squares; 3 mm off-center at 530 K, circles; 6 mm off-center at 500 K, triangles).

Figure 3

The integrated, total amounts of deuterium in the top $6\mu \mathrm{m}$ for the self-bias experiments. The lines show the results of the TMAP7 simulations.

Figure 4

The simulated influx as a function of $1000/T$ with $T$ the local target temperature.