Evaluation of hydrogen retention behavior in tungsten exposed to hydrogen plasma in QUEST

• Main Authors: Ayaka Koike, Moeko Nakata, Shota Yamazaki, Takuro Wada, Fei Sun, Mingzhong Zhao, Naoaki Yoshida, Kazuaki Hanada, Yasuhisa Oya
• Format: Article
• Language: English
• Published: Elsevier 2021-03-01
• Series: Nuclear Materials and Energy
• Subjects: Hydrogen isotope retention; Tungsten; Deposition layer; Plasma exposure; QUEST
• Online Access: http://www.sciencedirect.com/science/article/pii/S235217912030123X

The W (tungsten) samples were placed at Top, Equator and Bottom plasma facing walls of QUEST (Q-shu University Experiment with Steady-State-Spherical Tokamak) device and exposed to 754 shots of hydrogen plasma during 2017A/W (Autumn/Winter) campaign. Thereafter, their surface morphologies and chemical states were evaluated by TEM (Transmission Electron Microscope) and XPS (X-ray photoelectron spectroscopy). The XPS results showed that a thick carbon layer about 3–18 nm has formed throughout the wall surface. Among them, the Bottom wall had the deposition layer with the thickness of 2 nm, which was thinner than the top wall, namely erosion-dominated. On the other hand, a thick C layer about 18 nm was deposited on the Equator wall. The additional 1 keV D2+ was implanted into these samples and the D (deuterium) retention enhancement was estimated. The D2 TDS (Thermal Desorption Spectroscopy) spectra for all the samples had two major desorption stages at 400 K and 650 K, namely the desorption of D trapped by irradiation damages and deposition layer. The erosion/deposition profile would be caused by wall position and plasma condition, like a current start-up experiment. The desorption temperature of H2 (hydrogen) was shifted toward higher temperature side compared to that exposed to previous plasma campaign (2016 A/W), suggesting that H was mainly accumulated in the deposition layer with forming C–H bonds.