Confirmation of the Absence of Contact Between Edge Boundary Plasma and Inboard First Wall in LHD Discharges Based on Radial Profile Measurement of Hβ Line Emissions. (3rd October 2022)

• Record Type: Journal Article
• Authors: Kawamoto, Yasuko; Morita, Shigeru; Kawamura, Gakushi; Goto, Motoshi; Oishi, Tetsutarou; Kawate, Tomoko; Kobayashi, Masahiro; Shoji, Mamoru
• Is Part Of: Fusion science and technology. Volume 78:Number 7(2022)
• Page Start: 537
• Page End: 548
• Publication Date: 2022-10-03
• Subjects: Large Helical Device -- visible spectroscopy -- divertor discharge -- EMC3-EIRENE; Fusion reactors -- Periodicals; Nuclear fusion -- Periodicals; Fusion reactors; Nuclear fusion; Periodicals; 621.48405
• DOI: 10.1080/15361055.2022.2068897
• Languages: English
• ISSNs: 1536-1055

Abstract

Abstract: In the Large Helical Device (LHD), a high-performance plasma has been obtained at the inwardly shifted magnetic axis position of Rax = 3.60 m in which a spatial distance between the first wall on the vacuum vessel and the outermost edge boundary of the stochastic magnetic field layer existing outside the last closed flux surface takes a minimum value of ~12 mm at the inboard side. In order to investigate contact between the edge plasma boundary and the inboard first wall, a radial profile of Hβ line emissions at 4861 Å has been measured using a Czerny-Turner visible spectrometer and a 40-channel optical fiber array. All Hβ profiles measured at different magnetic axis positions of Rax = 3.60, 3.75, and 3.90 m showed a centrally peaked profile except for a few fiber channels observing the outboard edge plasma. The Hβ emission near the inboard first wall was negligibly weak, in particular, in the case of Rax = 3.60 m, suggesting no significant contact between the edge boundary plasma and the vacuum vessel first wall. The radial Hβ profile was then analyzed in detail using the EMC3-EIRENE edge plasma simulation code. The simulation well reproduced the measured profiles, including the extremely weak Hβ emission around the inboard first wall in the Rax = 3.60 m configuration. The centrally peaked profiles are found to originate in the Hβ emissions around X-points, while hydrogen neutrals are dominantly localized near the divertor plates. These results confirm the formation of a complete open divertor configuration in the LHD discharge without significant contact with the first wall. The presence of a region with extremely short magnetic field connection lengths (Lc < 5 m) between the inboard first wall and the outermost edge boundary is a key point to eliminate the strong plasma-wall interaction because sustainment of a significant edge plasma is entirely difficult in such a low Lc region.