Reduction of ELM energy loss by pellet injection for ELM pacing

ELMペーシングのためのペレット入射によるELMエネルギー損失の低減

林 伸彦; 相羽 信行; 滝塚 知典*; 大山 直幸

Hayashi, Nobuhiko; Aiba, Nobuyuki; Takizuka, Tomonori*; Oyama, Naoyuki

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