23P33-P
Edge plasma fluctuation measured with multiple Langmuir probes
in Heliotron J
ヘリオトロンＪ装置における静電プローブによる周辺プラズマ揺動計測とその構造の解明
Shinsuke OHSHIMA1, Kohei HASHIMOTO2, Satoshi YAMAMOTO2, Kazunobu NAGASAKI2,Tohru MIZUUCHI2, Hiroyuki OKADA2, Takashi MINAMI2, Shinji KOBAYASHI2, Kiyoshi
HANATANI2, Shigeru KONOSHIMA2, Masaki TAKEUCHI2,3 and Fumimichi SANO2
大島慎介1，橋本紘平2, 山本聡2，長崎百伸2，水内亨2，岡田浩之2，南貴司2，小林進二2，花谷清2，木島滋2，竹内正樹2,3，佐野史道2
1

Kyoto University Pioneering Research Unit for Next Generation, Gokasho, Uji 611-0011, Japan
京都大学 学際融合教育研究推進センター次世代開拓研究ユニット, 611-0011 京都府宇治市五ケ庄
2Institute of Advanced Energy, Kyoto University, Gokasho, Uji 611-0011, Japan2
京都大学 エネルギー理工学研究所, 611-0011 京都府宇治市五ケ庄
3Japan Atomic Energy Agency, 311-0193, Mukoyama 801-1, Naka-shi, Ibaraki-ken, Japan
3
原子力開発研究機構, 311-0193 茨城県那珂市向山801-1

1

Edge fluctuation characteristics and its structures are investigated using multiple probes separated in
toroidal/poloidal directions in Heliotron J. Two kinds of characteristic fluctuations, an engergetic ion
driven instability with higher harmonics and a low frequency mode, are observed in neutral beam heatingplasma. Though these modes have quite different characteristics, both modes have nonlinear phaserelationhiop with broadband fluctuations, implying the modes have influence on the broadband turbulence.

1. Introduction
Fluctuations localized at edge region have asubstantial role in determining transport andperformance of plasmas. Although there arevarious interesting phenomena relating to edge
fluctuations, one of the recent issues is generationof meso-scale structure such as zonal flow or
geodesic acoustic mode, which are radiallylocalized,symmetric
shear
flowsinpoloidal/toroidal directions[1,2]. These modes aregenerated by turbulence itself, and can regulate
and suppress the turbulence and the transport
through shearing process due to the flow orenergy dissipation from turbulence to the flows.
These modes are studied intensively in theframeworks of both theory and experiment [1-6].
Here, from the perspectives of configuration
optimization, geometry-dependent behavior of
turbulence on magnetic configurations should beinvestigated and clarified. Heliotron J, having awidely controllable magnetic configuration, isone of good testbeds to investigate such a
relationship among edge turbulence, meso-scalestructures and magnetic configurations.
In this presentation, we will show and discuss
the details of the characteristics and its structuresof the turbulence measured with Langmuir probes
in Heliotron J.

helitoron with an L/M =1/4 helical coil, Heliotron
J [7,8]. The averaged major and minor plasma
radii are R = 1.2 m and a = 0.17 m and themagnetic field strength of B < 1.5 T.
Figure 1 shows the probe locations in the topview of Heliotron J. The device equips with four
sets of probes at different toroidal/poloidal
sections. In this study, #8.5 and #11.5 probes,
separated about 70 degrees in the toroidal
direction each other, were used [9]. The probehead structures at #11.5 and #8.5 sections are

II. Experimental set up
The experiment was conducted in a helical-axis

Fig.1 (a)Probe locations in the top view ofHeliotron J. (b)Probe heads at #11.5 section and (c)probe head at #8.5 section.

(a)

(b)
(c)

ne[x1019 m‐3]1(a)#41381

0.50ECH

NBI(b)

Fig.2 (a)Time development of line averaged density
measured with interferometer. (b)Spectrogram of
floating potential fluctuation at 4mm inside LCFS at
11.5 section.

shown in the photographs of Fig. 1(b) and (c),respectively. The probe heads mainly consists ofcarbon and boron nitride to be tolerant of hightemperature and thermal shock.
III. Experimental Results and Discussions
Edge fluctuations at 4mm inside last closed
flux surface (LCFS) were measured using theLangmuir probes in the neutral beam injection(NBI) heating plasma with low density less than 1x 1019m-3, as shown in Fig. 2(a). Two kinds ofcoherent modes were observed in the probe
signals, as shown in the spectrogram of Fig. 2(b).
Thesemodes
arecategorizedasanenergetic-particle-driven mode at 90 kHz
accompanying higher harmonics (~180, ~270….
kHz), and a lower frequency mode (LF mode) at~20 kHz.
The energetic particle driven modes, which arerelating to the Alfven eigenmode (AE), are often
observed in NBI plasma of Heliotron J. Energeticparticle mode (EPM) or global Alfven eigenmode
are the candidates in the case of Heliotron J. Themodes shown in Fig. 2 (b) are likely to be EPM
since the density dependence of the frequencychanges was not clear although further analysis
will be needed.
The LF mode has long range correlation intoroidal direction. The correlation and phasedifference between two signals measured with theprobes at different location were investigated.
Here one probe position was fixed at a point andthe other probe position was scanned radially on
shot-by-shot bases. As a result, the LF mode wasfound to be localized in narrow radial range
inside LCFS. Additionally, since magnetic probes
cannot detect the mode, the magnetic fluctuation
components of the LF mode were considered not
to be so strong. These characteristics of the LF
mode are not inconsistent with fluctuations withmeso-scale structures observed in other devices[4,5,6].
Furthermore, nonlinear characteristics of theseFig.3 Result of bicoherence analysis for ion saturation
current fluctuation measured with #11.5 probe at 4mm
inside LCFS.

modes were investigated by applying bicoherenceanalysis to the probe signals. In Fig.3, nonlinear
couplings were clearly observed between the LF
mode at ~ 20 kHz and broad-band fluctuation in
the frequency range of ~ 100-500 kHz, and also
between the energetic ion driven MHD at ~90
kHz and broad-band fluctuation in the similarfrequency range. This implies that these modes
would affect the broadband turbulence.
IV. Conclusion
Two kinds of characteristic modes of edge
plasma fluctuations were observed in NBI plasmaof Heliotron J. One is a kind of Alfven
eigenmodes and the other is a low frequency
mode having the characteristics similar tomeso-scale structures. These modes have quite
different
characteristics
andstructures,nevertheless, both modes were found to couple
with broad-band fluctuation and it implies thatthese modes would affect the broadband
turbulence.
Acknowledgments

This work is performed with the support and under theauspices of the Collaboration Program of the Laboratory forComplex Energy Processes, Institute of Advanced Energy,
Kyoto University and the National Institute for Fusion
Science (NIFS) Collaborative Research Program ofNIFS10KUHL030
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