Comparison of tungsten nano-tendrils grown in Alcator C- Mod and linear plasma devices

Growth of tungsten nano-tendrils (“fuzz”) has been observed for the first time in the divertor region of a high-power density tokamak experiment. After 14 consecutive helium L-mode discharges in Alcator C-Mod, the tip of a tungsten Langmuir probe at the outer strike point was fully covered with a layer of nano-tendrils. The depth of the W fuzz layer (600 ± 150 nm) is consistent with an empirical growth formula from the PISCES experiment. Re-creating the C-Mod exposures as closely as possible in Pilot-PSI experiment can produce nearly-identical nano-tendril morphology and layer thickness at surface temperatures that agree with uncertainties with the C-Mod W probe temperature data. Helium concentrations in W fuzz layers are measured at 1-4 at.%, which is lower than expected for the observed sub-surface voids to be filled with several GPa of helium pressure. This possibly indicates that the void formation is not pressure driven. PACS: 52.40.Hf, 81.07-b, 81.05.Je, 52.55.Fa PSI-20 keywords: Alcator C-Mod, Helium, Bubbles & Blisters, Divertor, W fuzz Corresponding author address: MIT Plasma Science and Fusion Center, 77 Massachusetts Ave. NW17-121, Cambridge, MA, USA, 02139 Corresponding author e-mail: [email protected] Presenting author: G.M. Wright