ELM mitigation with high-frequency lithium granules gravitational injection in EAST *

Large edge-localized modes (ELMs) were mitigated by gravitational injection of lithium (Li) granules (0.7mm nominal diameter) into the upper X-point region of the EAST device, which has tungsten plasma-facing components (PFCs) [1]. The maximum ELM size was reduced by ~ 70% in Type-1 ELMy H-mode plasmas. Large ELMs were stabilized for up to ~ 40 energy confinement times. Constant core radiated power held constant, and there was no evidence of impurity accumulation. The Li granule injection reduced the edge plasma pedestal electron density and temperature, and also their gradients, due to increased edge radiation and reduced recycling. ELITE code calculations indicate that the stabilization of large ELMs correlates with improved stability of intermediate-n peeling-ballooning modes, due to reduced edge current resulting from the relaxation of edge profile gradients. The pedestal pressure reduction was partially offset by a core density increase, with the net result of a modest ~ 7% drop in core stored energy and normalized energy confinement. We surmise that the remnant small ELMs are triggered by the penetration of multiple Li granules just past the separatrix, similar to small ELMs triggered by deuterium.

[1] Z. Sun et al., 2021 Nucl. Fusion 61 066022