Assessment of Tungsten Nano-tendril Growth in the Alcator C-Mod Divertor

Full text: The conditions for the growth of tungsten (W) or molybdenum (Mo) nano-tendrils have been well defined in linear plasma devices (helium plasma, Ts > 1000K, EHe+ > 20 eV). We have exploited the high power density in Alcator C-Mod to successfully grow W nano-tendrils on a Langmuir probe (ramped approximately 11 degrees into the parallel plasma flux) in the lower divertor during a single run day, demonstrating for the first time that these nano-tendril structures can be grown in a tokamak divertor. Scanning electron microscopy and focused ion beam cross-sectioning shows a 600 ± 150 nm thick nano-tendril layer on the surface of the W Langmuir probe after approximately 15 seconds of accumulated growth time. This layer thickness is in agreement with calculations using a preliminary empirical growth formula proposed by Baldwin et al.[1]. The W nano-tendrils show no sign of melting despite receiving surface heat fluxes of approximately 35 MW/m2 and three full current (900 kA) plasma disruptions during the growth sequence. There is also no indication of unipolar arcing from the nano-tendrils. Sputtering calculations show that sputtering is playing a minor role in nano-tendril growth on the W surfaces. However, strong sputtering is likely inhibiting nano-tendril growth on nearby Mo surfaces that received heat fluxes of 10 MW/m2 and achieved surface temperatures > 1000 K but showed no indications of nano-tendril growth. Having shown that these nano-tendrils can form in a tokamak divertor and given that the key growth conditions are met in an all-W ITER divertor during operation in He or the DT phase, there is a strong need to understand how other plasma conditions not re-created in this work, such as ELMs and impurity seeding, can affect growth and how these nano-tendril layers can impact plasma-material interactions and tokamak operations. (author)