7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Study of asymmetry in heat and particle loads on divertor tiles in LHD

T. Sugiyama1, S. Masuzaki1,2, M. Shoji1,2, Y. Hayashi1,2, G. Motojima1,2, G. Kawamura1,2, H.
Ohtani1,2
1
Department of Fusion Science, The Graduate University for Advanced Studies
2
National Institute for Fusion Science, National Institutes of Natural Sciencese-mail (speaker): sugiyama.tsukasa@nifs.ac.jp

In the boundary plasma of the Large Helical Device(LHD), an asymmetry in heat and particle loads on
divertor tiles installed at positions of symmetricmagnetic field structure has been observed.[1-3] Theasymmetric heat and particle loads can cause asymmetricerosion/deposition of plasma-facing materials, which canaffect the lifetime of components and fuel accumulationin the vacuum vessel in fusion reactors. Hence,
understanding the physical mechanism of the asymmetry
should be considered for designing next-step fusion
devices. In LHD, the asymmetry has been investigatedin the standard magnetic configuration (a major radius of
the magnetic axis Rax of 3.6 m).[1, 3] However, we have
not yet completely understood the physical mechanism
causing the asymmetry observed in experiments. The
purpose of this study is to reveal the physical mechanismof the asymmetry.
In order to understand the mechanism causing theasymmetric particle flux, we analyze the discharges inseveral experimental conditions including the standard
magnetic configuration. There are two divertor tiles with
Langmuir probe arrays in 7 of 10 toroidal sections. These
are labeled as “R-array” and “L-array”, as shown inFigure 1. The Langmuir probe array consists of 20electrodes. The degree of the asymmetry (DOA) isdefined as the ratio of the total ion saturation current at
the L-array to that at the R-array. Figure 2 shows theFigure 1. Poloidal cross-section at thetoroidal positions where the Langmuirprobe arrays are installed.

dependence of DOA on the electron temperature at thelast closed flux surface (LCFS) in the cases where thetoroidal field direction is clockwise (CW) andcounterclockwise (CCW). The asymmetry is reversed bychanging the toroidal field direction including the effectof particle drifts. Figure 2 also shows the DOA is
influenced by the difference in the magnetic
configurations in which the magnetic field lines structure
is different.
The data shown in Figure 2 are scattered, andplasma parameters changing the DOA have not yet been
fully clarified. In order to find the plasma parameterschanging the DOA, we adopt a Neural Network model.[4]As an example, Permutation Importance is used to findplasma parameters changing the DOA, which shows thatthe toroidal field direction is the primary factor. Plasma
current, stored energy, NBI power, Rax, and magnetic
field strength are the secondary factors.
References[1] S. Masuzaki et al., Fusion Sci. Tech. 50 (2006) 361.
[2] H. Tanaka et al., Nucl. Mater. Energy 12 (2017) 241.
[3] S. Masuzaki et al., Nucl. Mater. Energy 18 (2019)281.
[4] I. Yamada et al., Plasma Fusion Res. 17 (2022)
2402061.

Figure 2. Degree of the asymmetry as a function of electron
temperature at the LCFS in three different magnetic configurations(the major radius of the magnetic axis 3.6, 3.75, and 3.9 m).