High-density experiments with hydrogen ice pellet injection and analysis of pellet penetration depth in Heliotron J. (3rd June 2019)

• Record Type: Journal Article
• Authors: Motojima, G; Okada, H; Okazaki, H; Kobayashi, S; Nagasaki, K; Sakamoto, R; Yamada, H; Kado, S; Ohshima, S; Minami, T; Kenmochi, N; Ohtani, Y; Nozaki, Y; Yonemura, Y; Nakamura, Y; Konoshima, S; Yamamoto, S.; Mizuuchi, T; Watanabe, K Y
• Is Part Of: Plasma physics and controlled fusion. Volume 61:Number 7(2019)
• Publication Date: 2019-06-03
• Subjects: pellet injection -- Heliotron J -- fast electron; Plasma (Ionized gases) -- Periodicals; Controlled fusion -- Periodicals; 530.44
• DOI: 10.1088/1361-6587/ab1d40
• Languages: English
• ISSNs: 0741-3335

Abstract

Abstract: A hydrogen cryogenic pellet injection system was fabricated and applied to high-density experiments in a Heliotron J device. The fueling through pellet injection was conducted in Heliotron J by using an injection barrel with a tapered structure for a small pellet with low speed. A single pellet with a speed of 260 ± 30 m s −1 and a typical size of 1.1–1.2 mm was injected into NBI + ECH plasmas and NBI-only plasmas. In both cases, a significant increase in electron density was observed upon pellet injection. The increase in line-averaged electron density due to pellet injection was 4.0 ± 0.7 × 10 19 m −3 in both cases. Pellet fueling leads to high stored energy in the high-electron-density regime. A peaked electron density profile with high core density is obtained in NBI-only plasmas after pellet injection, while the edge density is kept at a lower level. An innovative H α array measurement shows that the pellet has penetrated into the magnetic axis in NBI + ECH plasmas, while the penetration depth in NBI-only plasmas is beyond the magnetic axis, which is deeper than that in NBI + ECH plasmas. The difference in penetration depth cannot be explained by the simulation results of the ablation code, in which the effect of fast electrons/ions on pellet ablation is not considered. These results suggest that the effect of fast electrons/ions should be taken into account in the ablation code in order to explain the present experimental results.