ELM Triggering Dependence on Deuterium Pellet Size on DIII-D

The triggering of small ELMs by pellet injection has been demonstrated as a method to prevent large ELMs that can erode plasma facing components [1]. Small deuterium pellets $<$ 1 mm in size have been shown to reliably trigger ELMs on the DIII-D tokamak in the ITER like scenario plasmas. A variation in pellet size and speed was used to determine the minimum pellet size needed to trigger ELMs as a function of edge pedestal pressure. Pellets $<$ 0.8 mm in size were found to be insufficient to trigger ELMs. These results show smaller pellets than predicted by nonlinear MHD simulations can destabilize high-n ballooning modes from a local pressure perturbation well in excess of the pedestal pressure [2]. The implications of these results for pellet ELM mitigation and the design of the pellet injection system for ITER will be discussed.\par \vskip6pt \noindent [1] L.R. Baylor et al., Phys. Rev. Lett. 245001 (2013)\par \noindent[2] S. Futatani et al., Nucl. Fusion. {\bf54}, 073008 (2014).