Fast-ion induced ablation of Li granules in DIII-D[1]

In DIII-D, sub-millimeter Li spheres are injected at ${\textless120}$\hspace{2.5 mm}m/s to pace edge localized modes[2]. Typically, granule ablation, monitored by fast visible imaging, begins at the last closed flux surface (LCFS), with strong field-aligned emission from Li$^{1+}$. During counter $I_p$ neutral beam (NB) injection, non-field-aligned Li emission was observed from the vacuum region between the LCFS and wall, suggestive of a neutral cloud evaporating from the granules. This is ascribed to a relatively high density of fast-ions arising from 80 kV counter-$I_p$ NB injection. Simulations with the full-orbit Monte-Carlo code SPIRAL[3] find fast-ion densities up to 1x10$^{16}$ m$^{-3}$ with expected heat flux $\sim$50-100 W/cm$^2$ at the granule surface, sufficient to induce melting. The non-isotropic fast-ion pressure may cause droplets to leave the melted layer accelerating along characteristic trajectories, a phenomenon observed during injections at $\textless$50$\hspace{1.1 mm}$m/s.\par \vskip9pt \noindent [1] Supported by the U.S. Department of Energy (DE-AC02-09CH11466, DE-AC05-00OR22725, DE-FC02-04ER54698, DE-FG02-07ER54917).\par \noindent [2] A. Bortolon, NF 2016, 056008\par \noindent [3] G.J. Kramer, PPCF 2013, 025013