In-operando observation of helium-irradiation ion effects on surface deuterium retention through LiOH bonding in lithium films on tungsten substrates

The plasma surface interactions within a fusion device are one of the limiting factors to long pulse, power generating operation. A plasma facing component material will require effective heat tolerance, minimal erosion yield, and minimal fuel retention properties. Tungsten (W) has been selected as the divertor material for the International Thermonuclear Experimental Reactor (ITER) due to its high thermal conductivity and high sputter threshold. However, when W is exposed to high particle flux (>10 ²² ions/m ² s) at high surface temperatures (>600 °C), the surface will develop defects such as pits, blisters, and nano-structured tendrils, reducing the beneficial properties of W. To overcome this limitation, a more radiation tolerant thin film material could be used, such as lithium (Li). In addition, the lithium film can protect the plasma from high-Z W sputtered atoms. In multiple tokamak devices, a Li wall coating, has improved the plasma performance by reducing fuel recycling from the walls, stabilizing the edge plasma and decreasing the number of edge localized modes (ELMs). Since ELMs help eject impurities from the core plasma, the complete suppression of ELMs is detrimental. Methods to regulate the frequency of ELMs have been investigated using gas puffs. In this work we report a new method to control the ELM frequency by tuning fuel recycling via the intrinsic helium (He) ions produced as ash from the deuterium (D) – tritium (T) fusion reaction. In Li films, one mechanism to retain D is via a chemical interaction between Li, O (oxygen), and D. Previous work has shown that when He ions are introduced with D ions, in a dual beam irradiation of Li films on W, a reduction in the dynamic surface D retention is observed. To further investigate this phenomenon, 1–2 um films of Li on W were exposed to sequential irradiations of D and He. The He fluence was ≈5% of the D (3.3 × 10 ²⁰ ions/m ² ). The energies for the He and D ions were 1000 eV and 250 eV/amu, respectively and samples were exposed at room temperature. The surface chemistry was characterized with x-ray photoelectron spectroscopy (XPS) to determine changes in retention. The XPS scans were conducted in-situ and in-operando for the irradiations. Our results showed a decrease in the surface retention when He follows D ions and little change in the retention when D follows He. This indicates that He breaks the D retention mechanism in Li.