26th IAEA Fusion Energy Conference - IAEA CN-234

Helium Ion Energy Threshold for Helium Retention and Nano-bubble Formation in Tungsten

21 Oct 2016, 14:00

4h 45m

Kyoto International Conference Center

Poster EXD - Magnetic Confinement Experiments: Plasma–material interactions; divertors; limiters; scrape-off layer (SOL) Poster 8

Speaker

Dr Cormac Corr (Australian National University)

Description

Tungsten has been chosen as the material for the ITER divertor and is a contender for the wall of DEMO, which will experience temperatures in excess of 1000 K. The interaction between high-flux helium plasmas with tungsten can lead to plasma-induced surface modifications. In particular helium retention in tungsten is problematic, since helium is known to form nano-scale bubbles beneath the surface, and are thought to be responsible for the formation of nano-fuzz and surface pitting [1,2]. Results from Grazing Incidence Small Angle X-Ray Scattering (GISAXS) measurements [3] performed at the Australian Synchrotron show that in tungsten exposed to pure helium plasmas in linear plasma devices, such as the MAGnetised Plasma Interaction Experiment (MAGPIE) [4], nano-sized bubbles of between 1.5 – 2.5 nm diameter are formed in near-surface layers of approximately 30 nm thickness. The findings are in excellent agreement with a direct observation by transmission electron microscopy. Depth distributions were estimated by taking successive measurements across a range of x-ray incidence angles. As an example, for tungsten at 700°C, the bubble layer is observed to be 31 +/- 4 nm deep. A helium ion energy threshold of approximately 9eV has been identified, above which helium nano-bubbles are formed and is strongly correlated with an increase in helium retention. The effects of surface temperature and plasma fluence on nano-bubble formation will also be presented. [1] S. Kajita, N. Yoshida, R. Yoshihara, et al., J. Nucl. Mater. 418, 152-158 (2011) [2] D. Nishijima, M. Y. Ye, N. Ohno, et al., J. Nucl. Mater. 313-316, 97 (2003) [3] M. Thompson, P. Kluth, R. Doerner, N. Kirby, C. Corr, Nuclear Fusion, 55, 042001 (2015) [4] B.D. Blackwell, J.F. Caneses, C.M. Samuell, J. Wach, J. Howard and C.S. Corr, Plasma Sources Sci. Technol. 21, 055033 (2012)