Reduction of ELM energy loss by pellet injection for ELM pacing
Hayashi, Nobuhiko; Aiba, Nobuyuki; Takizuka, Tomonori*; Oyama, Naoyuki
The reduction of ELM energy loss by pellet injection for ELM pacing has been studied by an integrated code TOPICS-IB. It is found that the energy loss can be siginificantly reduced by a small pellet injected from the low-field-side (LFS) with a speed fast enough to approach the pedestal top, which penetrates deeply into pedestal and triggers high-n ballooning modes with localized eigenfunctions near pedestal top, where n is the toroidal mode number. On the other hand, a small pellet injected from high-field-side triggers lower-n modes with wide eigenfunctions before the pellet penetrates deeply into pedestal, resulting in a large energy loss. The dependence of energy loss on pellet injection timing in one cycle of natural ELMs is studied. Early injection reduces energy loss because high magnetic shear prevents onset of lower-n modes, but leads to reduction of target pedestal pressure and enlargement of pellet size to trigger ELM. On the other hand, late injection induces a large energy loss comparable to a natural ELM. Therefore, LFS pellet injection to pedestal plasma equivalent to that at middle timing in a natural ELM cycle is found to be suitable for ELM pacing. With these suitable conditions, ELM pacing with reduced energy loss is successfully demonstrated in simulations, in which core density increase due to additional particle fueling by pacing pellet can be compensated by reducing gas puff and enhancing divertor pumping.