Active tungsten expulsion in ELM-absent H-mode plasmas via on-demand ELM triggering with lithium granule injection

Lithium granules gravitationally injected into the upper X-point region demonstrated on-demand edge-localized modes (ELM) triggering in otherwise ELM-suppressed H-mode plasmas on the Experimental Advanced Superconducting Tokamak. Sub-mm lithium granules dropped into enhanced D-alpha H-mode plasmas achieved a high triggering efficiency, while enabling ELM frequencies from several to hundreds of hertz. Core radiation from heavy impurities, dominated by W, was reduced by up to 60%, and the normalized energy confinement increased by up to 30%. At low injection frequencies, ELMs of substantially reduced size compared to spontaneous type-I ELMs were observed. At high injection frequencies, a transition to a mixed ELM phase occurred, characterized by intermittent larger ELMs and suppression of the quasi-coherent mode, achieving the most significant W reduction and energy confinement improvement. These results highlight a promising pathway for active W control via controlled, small ELMs in long-pulse, high-performance scenarios.. Authors: Sun, Z. [Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)] (ORCID:0000000272243592); Maingi, R. [Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)] (ORCID:0000000312388121); Cheng, Y. X. [Chinese Academy of Sciences (CAS), Hefei (China). Institute of Plasma Physics (IPP)] (ORCID:0000000291952757); Qian, Y. Z. [Hefei Comprehensive National Science Center (China). Institute of Energy] (ORCID:0009000198588898); Xu, W. [Hefei Comprehensive National Science Center (China). Institute of Energy] (ORCID:0000000186196953). DOE Contract: AC02-09CH11466. Subjects: EDA H-mode; ELM; lithium; pellet injection; tungsten