23P063-P
Measurement of electric field fluctuations in PANTAPANTAにおける電場揺動計測	 
Yuuki Tobimatsu1, Shigeru Inagaki2,3, Yoshihiko Nagashima2,3, Stella Oldenbürger3,
Tatsuya Kobayashi1, Katsuyuki Kawashima1, Noriko Ohyama1, Tetsuo Yamashita1,
Soutaro Yamada1, Hiroyuki Arakawa4, Makoto Sasaki2,3, Takuma Yamada3,5, Masatoshi Yagi2,3,4,
Naohiro Kasuya3,6, Akihide Fujisawa2,3, Sanae-I. Itoh2,3, Kimitaka Itoh3,6
飛松祐紀1,	 稲垣滋2,3,	 永島芳彦2,3,	 Stella	 Oldenbürger3,	 小林達哉1,	 川島勝之1,	 	 
大山乃里子1,	 山下哲生1,	 山田宗太郎1,	 荒川弘之4,	 佐々木真2,3,	 山田琢磨3,5,	 	 矢木雅敏2,3,6,	 糟谷直宏3,7,	 藤澤彰英2,3,	 伊藤早苗2,3,	 伊藤公孝3,7	 
1Interdisciplinary 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, Japan3
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, Japan7
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
In Plasma Assembly for Nonlinear Turbulence Analysis (PANTA), radial electric field fluctuations aremeasured by using a multi-channel probe array. The initial results in the spatial structures of electric field
and its fluctuations are reported.

1. Introduction
A good confinement is essential to realize an
economic fusion reactor with a magnetic andlaser confinement devices of plasma. Many
studies have demonstrated that electric field hasstrong impact on plasma turbulence transport inmagnetically confined plasmas [1,2], e.g. shearedflows and zonal flows [3]. Thus, to clarify themechanisms of plasma structural formation, it isimportant to observe electric field. In this study,
we report the initial results of the measurementsof floating potential, radial electric field and theirfluctuations, using a newly made probe array, in a
linear magnetized plasma of PANTA device.
2. Experimental Setup
PANTA is a device to produce a cylindricallinear plasma (diameter 0.1m, length 4m) with ahelicon wave (7MHz, 3kW) by using a double-loop

antenna around a quartz tube. In this experimenttypical experimental parameters are 0.09T magnetic
field and 3mTorr neutral gas pressure (argon). A
64-channel Langmuir probe array is used to observe	 
the azimuthal structure of potential fluctuation. To
evaluate electric field fluctuations, a 3-channel
radial Langmuir probe array is used. The distance
between adjacent pins is 2mm. The cut-off
frequency of this probe system is 400 kHz, which
was improved from that of previous value, 10kHz,
by adding a new circuit to compensate the phasedelay due to stray capacitance of probe. Figure 1
shows the comparison between the spectra offloating potential fluctuations measured with thenew probe and a reference probe (one of the64-channel Langmuir probe array) at r = 4cm. The
spectrum measured with the new probe suggests
existence of fluctuations in higher frequency range
than 30kHz, while the fluctuation power in thefrequency range is evaluated to be smaller with thereference probe.

By using cross phase and two-time two-point
correlation between two probes whose positions
differ on the same magnetic line, it is suggested thatthe fluctuation at f = 2.5kHz propagates bothinward and outward from the radial point at r=3.8
cm. The inward and outward velocities are almost
the same, that is, approximately v = 200m/s.

Fig.1
Auto-power spectra of floating potential
fluctuations measured with the new probe and areference probe with less sensitivity to higher frequency.

3. Experiment results
Figure 2 (a) shows the radial profile of mean
floating potential measured by scanning of radial
probe array in a set of discharges showing a good
reproducibility with an identical condition. Theprofiles measured with different pins on the probearray are consistent with each other within error
bars. Figure 2 (b) shows the radial profile of meanelectric field. The black one is evaluated by two
floating potentials, V f 2 and V f 3 , which aremeasured at a pair of adjacent pins, denoted as thesecond and third pins, simultaneously. The pair isselected because it has similar property. The electric!
! asfield is evaluatedE r = "(V f 2 " V f 3 ) /d ,where, d is distance between the second and thirdpin (0.002m). The red one is evaluated by thegradient of floating potential of the third pin. The! electric field is found to be negatively strong
radialin the region of r = 3 - 4.5cm.

Fig.2 Radial profile of (a) mean floating potential and(b) mean electric field.

Figure 3 (a) shows radial profile of electric field
spectra. The spectrum peaks of floating potentialand electric field are observed at the harmonicfrequencies (1.2, 2.4, 3.6kHz). Figure 3 (b) shows
2D spectra of the floating potential fluctuations
measured by the 64-channel Langmuir probe arrayat r = 4cm. The comparison of Fig.3 (a) with Fig.3(b) indicates that f = 1.2kHz, 2.4kHz and 3.6kHz
should correspond to m = 1, 2 and 3, respectively.
Here m is azimuthal mode number. The azimuthal
phase velocity is evaluated as approximately
380m/s in the electron diamagnetic direction fromthe dispersion relation in Fig. 3(b).

Fig.3 (a) Radial profile of auto-power spectra ofelectric field. (b) 2D spectra of the floating potentialfluctuations.

4. Summary
Using a new probe array we have performed themeasurement of the radial profiles of floating
potential, electric field and the power spectrum ofelectric field fluctuations in a linear cylindricalplasma of PANTA device. In the discharges withmagnetic field at 0.09T and gas pressure at 3mTorr,
the rather coherent fluctuations characterized withharmonic frequencies and wavenumbers are found,which propagate azimuthally in the electron
diamagnetic direction, and radially both inward andoutward from the source position of r = 3.8 cm inthe gradient regime.
In future, utilizing the new probe, it should beinteresting to investigate the effects of harmonic
structural oscillation on high frequency fluctuations
and couplings between them. The effects of thebackground flows on the harmonic oscillation andhigher frequency fluctuations will be possible in acontrolled manner after the biasing electrode isinstalled on PANTA.
Acknowledgment
This work is partly supported by a grant-in-aid forscientific research from JSPS (21224014, 23360414) andbythecollaboration
programsofNIFS(NIFS10KOAP023) and of the RIAM of Kyushu
University.
References[1] S.-I. Itoh and K. Itoh, Phys. Rev. Lett. 60, 2276
(1988).
[2] H. Biglari, P.H. Diamond and P.W. Terry, Phys.
Fluids B 2, 1 (1990).
[3] K. Itoh and S.-I. Itoh, Phys. Plasmas 13, 055502(2006)