Surface Stress Evolution of Tungsten-Based Thin Films Under Low-Energy Deuterium Ion Irradiation

Plasma-facing materials (PFMs) undergo constant erosion and redeposition that lead to film growth in the form of redeposited or reconstituted films. These films can be μ m to mm thick, are often porous, and are comprised of a mix of impurities and the first wall material (e.g., W, B, C, O, and N). The properties of these films (i.e., sputtering and retention) can differ significantly from their pure polycrystalline precursors, which could have important implications for the PFM lifetime. Multi-beam optical stress sensor (MOSS) is an optical technique that can measure the surface stress of a material in situ in real time. Surface stress is affected by a variety of factors, including material deposition, substrate temperature, or defects in the substrate. In this study, this technique is used for the first time to study fusion-relevant thin films. Thin (∼ 500 nm) tungsten films were deposited on the surface of polycrystalline tungsten substrates to mimic redeposited films. These artificial films contained ∼ 30 % oxygen and were irradiated using low-energy (∼ 165 eV) deuterium ions for a total fluence of 1.9 × 10²⁰ m^-2. MOSS data acquired during the ion irradiations indicates that thin tungsten films underwent an irreversible change in surface stress as a result of the deuterium ion irradiation. This result suggests that deuterium retention could be responsible for the change observed, which can open the door to using MOSS as a technique to monitor PFMs in real time.