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

Fast-framing camera observations of the trajectory and the ablation position ofdust particles injected by the multi-species impurity powder dropper in theLarge Helical Device
M. Shoji1,3, S. Masuzaki1,3, N. Ashikawa2,3, F. Nespoli4, R. Lunsford4, E. P. Gilson4, D. A. Gates4,
and the LHD User Group1,3
1
Transports in Plasma Multi-phase Matter System Unit, National Institute for Fusion Science,2
Applied Superconductivity and Cryogenics Unit, National Institute for Fusion Science,
3Fusion Science Program, The Graduate University for Advanced Studies (SOKENDAI),
4
Princeton Plasma Physics Laboratory, Princeton University
e-mail (speaker): shohji.mamoru@nifs.ac.jp

Impurity powder injection has been expected to be
an attractive optional technique for impurity seeding,wall conditioning, and dust transport studies in magnetic
plasma confinement devices [1]. A multi-species
impurity powder dropper (IPD) [2] was installed at anupper port in the Large Helical Device (LHD). The
control of the trajectory of the dust particles injected bythe IPD in the peripheral plasma is a critical issue forrealizing high-performance plasma discharges [3]. The
ablation positions of dust particles were routinely
observed with a fast-framing camera and a CCD camera
for monitoring the plasma discharges from upper ports.
Figure 1 shows snapshots of the ablation imagesof injected boron dust particles (150 m in diameter) in
the LHD peripheral plasma observed with the fastframing camera installed at the position close to the IPD.
They were taken in an initial phase of the start of the dust
particle injection for three different plasma densities in a
typical magnetic configuration (the radial position of themagnetic axis Rax=3.60 m). The dust particles dropped
from the IPD pass through an upper divertor leg before
reaching the peripheral plasma (the ergodic layer). A few
moving small luminescent spots were detected by thecamera, showing the ablation of the dust particles causedby the interactions between the dust and the peripheral
plasma. The images clearly indicate the change in theablation positions toward the outboard side of the torus
for higher plasma densities.
Analyses using a three-dimensional edge plasmasimulation code (EMC3-EIRENE) [4] coupled with adust particle transport simulation code (DUSTT) [5]revealed that the trajectory of impurity dust particles isdeflected toward the outboard side by the effect of theion drag force due to the plasma flow in an upper
divertor leg [6]. The simulation also predicted that theablation positions of the dust particles further movetoward the outboard side with the increase in the plasma
density by the larger ion drag force on the dust particles
in higher plasma densities. The simulations are
consistent with the observations.
At this conference, the results of the moresophisticated analyses of the trajectories and ablation
positions of the dust particles are presented by including
the effect of the impurity ion flow caused by the impurity
powder injection into the simulation code.
References[1] A. Bortolon, V. Rohde, R. Maingi, et al., Nucl. Mater.
Energy 19, 384 (2019).
[2] A. Nagy, A. Bortolon, D. Mauzey, et al., Rev. Sci.
Instrum. 89, 10K121 (2018).
[3] F. Nespoli, N. Ashikawa, E. Gilson, et al., Nucl.
Mater. Energy 25, 100842 (2020).
[4] Y. Feng, H. Frerichs, M. Kobayashi, et al., Contrib.
Plasma Phys. 54, 426 (2014).
[5] A. Pigarov, R. Smirnov, S. Krasheninnikov, et al., J.
Nucl. Mater. 363-365, 216 (2011).
[6] M. Shoji, G. Kawamura, R. Smirnov, et al., Contrib.
Plasma Phys. 60, e201900101 (2020).

Figure 1: The snapshot of the ablation images of boron dust particles injected by the IPD observed with a fast-framing
camera installed in an upper port in an initial phase of the start of the dust particle injection for three different plasma
densities in a typical magnetic configuration (Rax=3.60 m).