Speaker
Mr
Graham Wright
(Massachusetts Institute of Technology)
Description
The conditions for the growth of tungsten (W) or molybdenum (Mo) nano-tendrils have been well defined in linear plasma devices (helium plasma, Tsurface > 1000 K, E_He+> 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/m^2 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/m^2 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.
This work is supported by US DOE award DE-SC00-02060 and US DOE contract DE-FC02-99ER54512.
[1] M.J. Baldwin, R.P. Doerner, Nucl. Fusion 48 (2008) 035001.
Country or International Organization of Primary Author
United States
Primary author
Mr
Graham Wright
(Massachusetts Institute of Technology)
Co-authors
Brian Labombard
(Massachusetts Institute of Technology)
Bruce Lipschultz
(Massachusetts Institute of Technology)
Daniel Brunner
(Massachusetts Institute of Technology)
Dennis Whyte
(Massachusetts Institute of Technology)
Jim Terry
(Massachusetts Institute of Technology)
Matthew Baldwin
(University of California-San Diego)
Russ Doerner
(University of California-San Diego)
