Growth of tungsten nano-tendrils in the Alcator C-Mod lower divertor

The conditions for the growth of tungsten (W) nano-tendrils have been well defined in linear plasma devices (T$_{surface}>$900 K, $\Gamma _{He+}>$10$^{22}$ m$^{-2}$s$^{-1}$, 10 eV$<$E$_{He+}<$150 eV) but, until now, there has been no documented nano-tendril growth in a tokamak environment. We have exploited the high power density and all-metal wall in Alcator C-Mod to successfully grow W nano-tendrils on a Langmuir probe (ramped $\sim $10$^{o}$ into the parallel plasma flux) in the lower divertor during a single run day. Scanning electron microscopy shows fully formed nano-tendrils over the surface of the probe after an estimated 15-30 seconds of growth time. Having shown that these nano-tendrils can form in a tokamak divertor and given that the growth conditions are met in an all-W ITER divertor during the DT phase, this provides strong evidence that these types of ``fuzzy'' surfaces will be present in the ITER divertor. The effects of this extreme surface morphology on plasma-surface interactions remains unclear, but possible implications for ITER operation will be discussed.