23P044-P
Study of Intermittent Edge Plasma Transport in LHD
LHD周辺領域で発生する間欠的プラズマ輸送現象の研究
Hirohiko Tanaka1, Suguru Masuzaki1, Noriyasu Ohno2, Tomohiro Morisaki1, Masahiro Kobayashi1,
Yoshiyuki Tsuji2, Yuki Hayashi3, Naomichi Ezumi3 and LHD Experimental Group1
田中 宏彦1, 増崎 貴1，大野哲靖2，森崎友宏1，小林政弘1，
辻 義之2，林 祐貴3，江角直道3，LHD実験グループ1
1

National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan3
Nagano National College of Technology, 716 Tokuma, Nagano 381-8550, Japan1
核融合科学研究所 〒509-5292 土岐市下石町322-6
2
名古屋大学大学院工学研究科 〒464-8603 名古屋市千種区不老町3
長野高等専門学校 〒381-8550 長野市徳間716

2

We have investigated electrostatic fluctuations measured by using fast scanning probes and a divertor
probe array in LHD. Main topic in this study is to evaluate edge transport characteristics of coherent
plasma structures, such as blobs, in the helical configuration. By the correlation analysis, local velocity ofthe propagation was estimated quantitatively on the assumption following the theory of the blobby plasmatransport.

1. Introduction
In the edge region of tokamak devices, blobby
plasma transport is a well-observed phenomenon.
Propagation of plasma blobs are mainly detected byusing a fast imaging camera and/or Langmuirprobes. The propagation generally points to
low-field side (LFS) direction and the velocity
amplitudes of the blobs range from a few hundred
meters to several kilometers per second at thedifferent radii [1].
In the recent attempt, edge turbulence database
composed of electrostatic fluctuations measured intokamaks and several stellarators is being created. Itindicates that there are common features such as
positive skewness of ion saturation current, I sat ,outside of the last closed flux surface [2], implying
an existence of the plasma blobs.
In the Large Helical Device (LHD), positive
skewness was clearly observed at the LFS region ofa divertor leg by using a fast scanning probe (FSP)[3]. The LFS direction from the divertor leg points
to a private region. Thus, this result suggests thatplasma blobs might broaden heat and particle fluxes
on the divertor plate. However, propagation
direction and typical velocity are not yet
experimentally estimated in that analysis.
In this study, we have measured I sat signals byusing FSPs and a divertor probe array with hightemporal resolution in LHD. By applying statisticalanalysis techniques and KMAG code [4], we
investigated characteristics of the intermittent
cross-field transport.

2. Experimental Setup
Figure 1(a) shows a Poincaré plot of magnetic
field lines for Rax = 3.6 m. Insertion trajectory ofone of the FSPs is also depicted. Insertion speed
was approximately 1-2 m/s. In the past, the FSP had
two electrodes at interval of 5 mm horizontally, asshown in Fig. 1(b). In the 15th experimental
campaign of this year, these electrodes have aninterval of 5 mm horizontally and 5 mm vertically,as shown in Fig. 1(c). Sampling frequency was 1
MHz.

Fig.1. (a) Poincaré plot of magnetic field lines and(b)(c) photographs of probe heads.

3. Statistical Analysis of Fluctuations
Figure 2(a) shows I sat signals measured withtwo electrodes of the FSP shown in Fig. 1(b)around the LFS region of a divertor leg. Figure 2(b)shows distributions of skewness, which is definedbyI sat  I sat 3

I sat  I sat 2

32. Because of

the interval between the two electrodes (ch 1 and ch2), there is a gap of approximately 6 mm alongz-axis between these distributions. Positive
skewness is confirmed in wide z range at the LFS of
the divertor leg.
Figure 2(c) shows cross-correlation coefficient
between high-passed I sat ( f  10 kHz) measured
with the two electrodes, C12   . Positive
correlation is observed at  ~ 0 around z = 1.19 m
where the skewness of two electrodes are positive
and negative. At z > 1.2 m, positive correlation is
distributed at around  ~ 4 s. Positive peak at  > 0means that I sat fluctuation of ch 2 is delayedcompared to that of ch 1. Therefore, blobs would
propagate from the electrode of ch 1 to that of ch 2.

that the electrode of ch 2 enters the divertor leg
earlier than that of ch 1 during an insertion. Inaddition, streamlines along the gradient of themagnetic field, B , are depicted. At z > 1.2 m,
both the electrodes are distant from the divertor leg,and the LFS direction corresponds to the vector
passing through the electrodes from ch 1 to ch 2.
If perpendicular length of the plasma blobs wasmuch larger than that of interval between theelectrodes, typical speed of the blobs propagating
toward the LFS direction, vb , could be estimated.
Effective distance between the electrodes alongB axis is approximately 4.6 mm. Therefore, on
the above assumption, vb ~ 4.6 mm 4 s  1.15 km/s .
It is comparable value to that of tokamaks.

Fig.3. Magnetic field configuration and insertion
trajectory of the two electrodes of the FSP.

Fig.2. (a) Distributions of Isat, (b) skewness, and (c) C12.

4. Analysis of Magnetic Field Configuration
In order to interpret the analysis results, weinvestigated three-dimensional magnetic fieldconfiguration around the LFS of the divertor leg.
Figure 3 shows the magnetic field configuration
inside a cube 100 mm on a side. One axis indicatesa depth from the R-z plane shown in Fig. 1. Roughposition of the divertor leg is displayed as
isosurfaces where the connection length, Lc , isequal to 20 m. From this figure, it can be confirmed

5. Summary and Discussion
We have estimated the typical velocity of theblob propagation on the assumption.
At z ~ 1.196 m where both the skewness are
positive, C12   peak is located at  ~ -1 s, as
shown in Figs. 2(b)(c), meaning that positive
spikes propagated from the electrode of ch 2 to ch
1. This position is located near the divertor leg
which would be generation region of the plasma
blobs; thus, this result might reflect the time delay
related not to the propagation but the generation.
Acknowledgments
This research was supported by Grant-in-Aid for
Research Activity Start-up (23860064).
References[1] J.A. Boedo et al: Phys. Plasmas 8 (2001) 4826.
[2] B. Nold et al: Proc. 37th EPS Conf. Plasma Phys,
Dublin, 2010, P1.1073.
[3] S. Masuzaki et al: Proc. ITC/ISHW2007, Toki, 2007.
[4] Y. Nakamura et al: J. Plasma Fusion Res. 69 (1993)
41.