23P064-P
Edge Ion Saturation Current Fluctuation Measurement in PANTAPANTAにおける周辺プラズマイオン飽和電流揺動計測	 
Noriko Ohyama1, Shigeru Inagaki2,3, Yoshihiko Nagashima2,3, Stella Oldenbürger3,Tatsuya Kobayashi1, Katsuyuki Kawashima1, Yuuki Tobimatsu1, Tetsuo Yamashita1,
Soutaro Yamada1, Hiroyuki Arakawa4, Takuma Yamada3,5, Masatoshi Yagi2,3,6, Naohiro Kasuya3,7,Makoto Sasaki2,3, Akihide Fujisawa2,3, Sanae-I. Itoh2,3 and Kimitaka Itoh3,7
大山乃里子1,	 稲垣滋2,3,	 永島芳彦2,3,	 Stella	 Oldenbürger3,	 小林達哉1,	 川島勝之1,	 	 飛松祐紀1,	 山下哲生1,	 山田宗太郎1,	 荒川	 弘之4,	 山田琢磨3,5,	 矢木雅敏2,3,6,	 糟谷直宏3,7,	 
佐々木真2,3,	 藤澤彰英2,3,	 伊藤早苗2,3,	 伊藤公孝3,71Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,
6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan1
九州大学総合理工学府 〒816-8580 福岡県春日市春日公園6-1
2
Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan2
九州大学応用力学研究所 〒816-8580 福岡県春日市春日公園6-1
3
Itoh Research Center for Plasma Turbulence, Kyushu University,
6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
3
九州大学伊藤極限プラズマ研究連携センター 〒816-8580 福岡県春日市春日公園6-1
4
Japan Atomic Energy Agency, 801-1 Mukoyama, Naka, Ibaraki 311-0193, Japan4
日本原子力研究開発機構 〒311-0193 茨城県那珂市向山801-1
5Graduate School of Frontier Sciences, The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Tokyo 277-8561, Japan5
東京大学新領域創成科学研究科 〒277- 8561 千葉県柏市柏の葉5-1-5
6Japan Atomic Energy Agency, 2-166, Oaza-obuchi-aza-omotedate, Rokkasho, Kamikita, Aomori 039-3212, Japan6日本原子力研究開発機構 〒039-3212 青森県上北郡六ヶ所村大字尾駮字表舘2-166
7
National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
7核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
Fluctuations of ion saturation current at distant positions are simultaneously measured with a 5-pins radial
probe array and a 5-pins azimuthal probe array in the linear magnetized plasma device PANTA. In theedge region, large-amplitude positive bursts are observed and the probability density function (PDF) ispositively skewed. The propagation velocities of bursts are estimated both in radial and azimuthal
directions by the time delay estimation method.

1. Introduction
The understanding of plasma turbulence isstrongly required to realize fusion reactors since
turbulence-driven transport has a significant effecton plasma confinement. Intermittent coherent
structures (e.g. plasma blobs) are observed in the
scrape-off layer of magnetic fusion devices [1] andlinear devices [2]. The nonlinear nature inherent tointermittent fluctuation signals appear as
non-Gaussianity of their probability density
function (PDF). Intermittent heat pulses to the wallshould be avoided in the future fusion plasma
device such as ITER and the understanding ofintermittent phenomena is a great challenge inplasma turbulence physics.
In this study, we will report ion saturation current
bursts observed in the edge region of PANTA. The

spatio-temporal structures are measured with
Langmuir probes. The radial structure of statisticalproperties of this phenomenon is presented alongwith propagation velocities of burst signals in radial
and azimuthal directions.
2. Experimental Setup
A cylindrical plasma with a diameter ofapproximately 0.1 m and axial length 4 m isgenerated by helicon wave (7 MHz, 3 kW) in thelinear device PANTA. The operational conditions
are a magnetic field of 0.09 T and neutral argon gas
pressure of 0.8 mTorr. Ion saturation current (Iis)and floating potential (Vf) are measured with a
movable Langmuir probe array. Two probe arrays,
each of which have five tungsten tips, are installedat the axial position z = 1.675 m to estimate

Iis [mA]64
2200

Power [a.u.]r = 8 cm
(a)

202
204
time [ms]r = 4 cm

10-910-1010-11

10-12206

Power [a.u.]

Iis [mA]r = 4 cm8(c)0510frequency [kHz]

15

10200

202
204
time [ms]

(d)

10-11
10PDF

PDF-202~
Iis / m0510
frequency [kHz]

15(f)

0.1

04-4-202~
Iis / m4

Iis [mA]
skewness

Fig.1. (a), (b) Typical time evolution, (c), (d) power
spectrum, (e), (f) PDF of Iis
864
202(a)(b)

10kurtosis

-1(c)

63002

positively skewed with S=1.5 and K=4.6 whereas
S=0.4 and K=2.4 at r = 4 cm. Radial profiles ofas well as S and K ofare plotted in Fig. 2.
Skewness gradually increases with the observed
radii, from r = 2 cm to 10 cm. Kurtosis stays around
K=3 inside r = 6 cm and increases with the radius to
K=8.2 at r = 10 cm.

-12r = 8 cm(e)

-4

206r = 8 cm

10-10r = 4 cm

0.05

0(b)

24

6810r [cm]Fig.2. Radial profiles of (a) average, (b) skewness and(c) kurtosis of Iis

propagation velocity of signals. One has aseparation of 3 mm between tips in the azimuthal
direction and another a separation of 10 mm in theradial direction. The data sampling frequency is 1MHz.
3. Statistical Analysis and Fluctuation Properties
Typical time evolutions of Iis at r = 4 cm and r = 8cm measured with a 5-pins radial probe array are
shown in Fig. 1 (a) and (b), respectively. Radial
gradient of() is maximal at r = 4 cm andtime-averaged fluctuation level () reaches its
maximum at r = 8 cm. The power spectra shown inFig. 1 (c) and (d) have peaks at the same frequency
f = 1.7 kHz and the coherence is high (~0.9) at thatfrequency. By using a 64-channel azimuthal probearray, it was found that f = 1.7 kHz has azimuthalmode number m = -1 (propagating in ion
diamagnetic direction) at r = 4 cm.
A degree of deviation of PDFs from the Gaussian
distribution is indicated by skewness (S) andkurtosis (K). The values are S=0 and K=3 for a
Gaussian distribution. PDFs at the two positions areshown in Fig. 1 (e) and (f). At r = 8 cm, PDF is

4. Velocity of Burst Signals
The simultaneous multi-point measurement ofIis allows us to estimate propagation velocities.
Velocities of burst signals () are estimatedby the formula v=d/Δt	  where d is the distance
between two probe tips and Δt is the time delay
obtained by using cross correlation between twosignals. At r = 8 cm, estimated azimuthal velocity vθ
is ~1000 m/s and propagation is in ion diamagnetic
direction. Radial velocity vr varies widely but is~1400 m/s (outward) on average.
5. Discussion and Summary
The radial structure of statistical properties of ion
saturation current fluctuations is investigated at aneutral pressure of 0.8 mTorr and a magnetic fieldof 0.09 T. Skewness and kurtosis increase with theradial position in peripheral region (r > 6 cm). This
indicates that the fluctuations become moreintermittent as going away from the density steep
region. Fluctuations are characterized by the power
spectra and the PDFs in the plasma core (r = 4 cm)
and edge (r = 8 cm). The spectra have a common
peak at f = 1.7 kHz with high coherence. Azimuthaland radial velocities of burst signals are found to be~1000 m/s and ~1400 m/s at r = 8 cm, respectively.
These results indicate there can be some coherent
structures propagating in the edge region whose
formation is strongly related to the mode (m, f) =(-1, 1.7kHz) at r = 4 cm. We will investigate therelationship between this intermittent phenomenon
and the existence (or strength) of the mode m = -1
in various operational conditions. Also we have thefuture plan to investigate the effects of shear flows
induced by end-plate biasing on the statisticalproperties of ion saturation current fluctuations.
Acknowledgement
This work is partly supported by a grant-in-aid forscientific research from JSPS (21224014,23360414) and by the collaboration programs ofNIFS (NIFS10KOAP023) and of the RIAM ofKyushu University.
References[1] J. A. Boedo et al.: Phys. Plasmas 8 (2001) 4826
[2] T.A. Carter: Phys. Plasmas 13 (2006) 010701