Plasma and Fusion Research: Rapid Communications

Volume 17, 1202093 (2022)

First Observation of Pellet Ablation Clouds Using
Two-Directional Simultaneous Photography
in GAMMA 10/PDX
Masayuki YOSHIKAWA, Yousuke NAKASHIMA, Junko KOHAGURA, Yoriko SHIMA,Hiroyuki NAKANISHI, Youhei TAKEDA, Shinji KOBAYASHI1) , Ryutaro MINAMI,
Naomichi EZUMI and Mizuki SAKAMOTO
Plasma Research Center, University of Tsukuba, Tsukuba 305-8577, Japan1)
Institute of Advanced Energy, Kyoto University, Uji 611-0011, Japan(Received 12 May 2022 / Accepted 22 June 2022)
The study of pellet ablation mechanisms is an important subject for plasma fueling in fusion plasmas. InGAMMA 10/PDX, sub-millimeter hydrogen pellet injection experiments are conducted in the higher electrondensity plasma for detached plasma experiments. We observed the pellet ablation cloud by two-directional simultaneous photography during the GAMMA 10/PDX pellet injection experiments. The three-dimensional pelletablation cloud and its trajectory were clearly obtained for the first time.
c 2022 The Japan Society of Plasma Science and Nuclear Fusion Research⃝Keywords: pellet ablation cloud, two-directional simultaneous photography, high-speed camera, GAMMA
10/PDX
DOI: 10.1585/pfr.17.1202093
Pellet injection is one of the most useful techniques
for particle fueling in fusion plasma devices [1–7]. To understand ablation cloud formation around a pellet on interaction with the background plasma, temporal and spatially resolved imaging observation is useful. In many fusion plasma devices, radiation intensity profiles of ablation clouds were observed using a high-speed camera system and Hα line emission measuring array systems. However, there have been few observations of the pellet ablation cloud by the two-directional simultaneous photography using two-to-one optical fiber and a high-speed camera system. In Ref. [3], three-dimensional observations forpellet ablation traveling in plasma were shown with using
the stereo observation technique. However, using the stereogram method, it is impossible to obtain the information
about the back of the pellet. Simultaneous measurement inboth vertical and horizontal directions can obtain not only
the back-side information but also the three-dimensional
profile of pellet ablation clouds.
GAMMA 10/PDX is a tandem mirror device with themain confining region of central cell, two axisymmetricminimum-B anchor cells, and two plug and barrier cells[8, 9]. In the west end region, the divertor simulation experimental module (D-module) was installed for divertor
simulation experiments using the end-loss plasma from thecore plasma. The x and y axes are defined perpendicular
to the magnetic field in the vertical and horizontal directions, respectively, and the z axis is parallel to the magneticfield. The mid-plane is z = 0 m. Typical electron den-sities and temperatures are about 2 × 1018 m−3 and 25 eV
in the central cell and 1 × 1017 m−3 and 25 eV in the end
cell without additional gas injection into D-module. We
could successfully simulate the detached plasma formation with the injection of an additional neutral gas in Dmodule. The plasma density of the core plasma was low
compared to the SOL plasma of normal fusion devises. To
study the mechanisms of the detached plasma formationin the higher plasma density region, we perform the pellet
injection into the central cell plasma to increase the coreplasma density. The pellet ablation study is also importantfor understanding the pellet fueling mechanisms. In addition, it is useful for studying pellet ablation mechanisms
to observe the pellet ablation cloud and the pellet trajectory in the plasma directly. In this paper, we describe thetwo-directional simultaneous photography system and thefirst result of the pellet ablation measurement in the tandem
mirror GAMMA 10/PDX.
Figure 1 shows the schematic of the two-directional
simultaneous photography (TDSP) system [10] and a submillimeter hydrogen pellet injection system [6, 7] installedin the central cell. The TDSP system consists of wideangle lenses, dual branch optical fiber bundles, TV lens,and a high-speed camera (MEMRECAM GX-1, NACInc.). The x- (vertical) and y-direction (horizontal) images are observed through the horizontal side port at z =0 m and on the upper side port at z = −0.10 m, respectively. The frame rate of the high-speed camera, imaging
size, and bit depth are 10,000 fps, 320 × 240 px, and 8 bit,respectively. The vertical and horizontal direction images

author’s e-mail: yosikawa@prc.tsukuba.ac.jp

1202093-1

c 2022 The Japan Society of Plasma⃝Science and Nuclear Fusion Research

Plasma and Fusion Research: Rapid Communications

Volume 17, 1202093 (2022)

Fig. 2 Pellet ablation clouds observed using the TDSP system. (a), (b), and (c) show t = 350.6 ms, 351.7 ms, and 352.7 ms, respectively.
The left and right side images show the vertical and horizontal images, respectively.

Fig. 1 Schematic of the TDSP system, sub-millimeter hydrogenpellet injection system, and multi-channel microwave interferometer system in the central cell.

are obtained in the same frame of the camera system. The
pixel resolution is 6.5 × 10−3 m/pix. The pellets are injected from the lower side port at z = −0.10 m throughthe Teflon guide tube from the pellet injector, which is apipe-gun type pneumatic pellet injector to produce the submillimeter pellet. The diameter and length of the pellet arecontrolled by the barrel size and by adjusting the feed gas
flow rate. We used the pellet of approximately 0.66 mm
in diameter and 1.9 mm in length in this experiment. Theinjected fueling pellet speed is approximately 200 m/s.
The pellet ablation clouds were observed by the TDSP
system for the first time (Fig. 2). The vertical and horizontal wide-angle images were obtained in the same frames.
Figure 2 (a) shows the images of both the vertical (left) andhorizontal (right) directions just after the pellet incident in
the plasma at t = 350.6 ms. The shape of the pellet ablation clouds is elliptical in both the vertical and horizontal
directions. As seen in images Fig. 2 (b), the pellet ablation
cloud is located at the plasma center in the vertical (left)
direction and is slightly moved from the plasma center at y∼ 0 m to the position at y ∼ 0.05 m in the horizontal (right)

Fig. 3 Temporal behavior of the radial profile of line density
observed by the multi-channel microwave interferometersystem. The times of ablation cloud images shown in
Fig. 2 are indicated. Peak line density temporal positionis also presented in arrows.

direction at t = 351.7 ms. In addition, the outline shape of
the pellet ablation cloud is a teardrop shape, which extends
to the direction along the magnetic field. Since it has almost the same shape in both the vertical and horizontal directions, it is considered to be a teardrop shape in three dimensions. This suggests that there is some non-uniformity
in the plasma particle flow into the pellet ablation cloud. InFig. 2 (c), the pellet ablation cloud in the vertical direction(left) moves to the upper part of the plasma, and that in thehorizontal direction (right) also moves significantly fromthe center to the plasma edge at t = 352.7 ms. Using the algebraic reconstruction technique method [11] for each x–yplane at every z position, we can study the tomography
image of the pellet ablation cloud and its motion. Detailsof the tomography technique will be presented elsewhere.
The pellet crossed diagonally in the plasma and the pellet
ablation cloud had a teardrop shape in the plasma centerregions. This observation would not have been possible if
only one-directional photography measurement was used.
This is also confirmed by the behavior of the electron line
density radial profiles obtained using multi-channel microwave interferometer system [12] in Fig. 3. The electron

1202093-2

Plasma and Fusion Research: Rapid Communications

Volume 17, 1202093 (2022)

line density increases from t = 350.6 ms and the FWHM
of the increased electron line density radial profile remains
almost constant, about 0.18 m, before t = 351.7 ms. The
electron line density peak moves to the outer region after t= 351.7 ms, which is the same motion as observed for thepellet ablation cloud in the horizontal direction. The line
density profile is strongly correlated by the pellet ablation
cloud motion.
We performed observations of the pellet ablation
cloud and its trajectory in plasma using the TDSP systemin the GAMMA 10/PDX pellet injection experiment for thefirst time. This system can observe both the pellet ablation
cloud and pellet trajectory in three dimensions. Moreover,it shows the pellet ablation cloud formation by backgroundplasma particle flow and the correlation between the pellet
ablation cloud motion for particle fueling and the electron
density radial profile. We continue to develop the analyzing method for the TDSP system.
This study was supported by the bilateral collaboration research program in the University ofTsukuba and Kyoto University (NIFS20KUGM148,
NIFS20KUGM159,
NIFS21KUGM165,andNIFS21KUHL101). The author sincerely thanks themembers of the GAMMA 10 group for their collaborationon the experiments.
[1] S.K. Combs et al., Rev. Sci. Instrum. 56, 1173 (1985).
[2] R. Sakamoto et al., Nucl. Fusion 44, 624 (2004).
[3] R. Sakamoto et al., Rev. Sci. Instrum. 76, 103502 (2005).
[4] J. Baldzuhn et al., Plasma Phys. Control. Fusion 61,095012 (2019).
[5] S.H. Park et al., Fusion Eng. Des. 154, 111535 (2020).
[6] E. Kawamori et al., J. Plasma Fusion Res. SERIES 3, 473(2000).
[7] Y. Kubota et al., Plasma Fusion Res. 2, S1057 (2007).
[8] Y. Nakashima et al., J. Nucl. Mater. 463, 537 (2015).
[9] M. Yoshikawa et al., AIP Advances 11, 125231 (2021).
[10] M.M. Islam et al., Plasma Fusion Res. 11, 2402053 (2016).
[11] H. Guan and R. Gordon, Phys. Med. Biol. 41, 1727 (1996).
[12] Y. Hasegawa et al., Trans. Fusion Sci. Technol. 63, 337(2013).

1202093-3