23P139-B
Two-dimensional fluctuation measurement of the edge turbulence in the TST-2
spherical tokamak
TST-2における周辺乱流の空間２次元計測	 
Masateru Sonehara, Yoshihiko Nagashima1, Hidetoshi Kakuda, Yuichi Takase2, Akira Ejiri2,
Junichi Hiratsuka, Takuya Oosako2, Osamu Watanabe2, Takashi Yamaguchi2, Takuya Sakamoto2,Takuma Wakatsuki, Takanori Ambo2, Ryouta Shino, Hirokazu Furui2, Takahiro Hashimoto,
Kunihiko Kato2 and Takahiro Shinya2
1
曽根原正晃, 永島芳彦 , 角田英俊, 高瀬雄一2, 江尻晶2, 大迫琢也2, 渡邉理2, 平塚淳一,
山口隆史2, 坂本拓也2, 若月琢馬, 安保貴憲2, 篠遼太, 古井宏和2, 橋本貴博,加藤邦彦2, 新屋貴浩2
Graduate School of Science, the University of Tokyo
5-1-5,Kashiwanoha, Kashiwa, 277-8561
1
Research Institute for Applied Mechanics, Kyushu University
6-1, Kasugakouen, Kasuga, 816-8580
2Graduate School of Frontier Sciences, the University of Tokyo
5-1-5,Kashiwanoha, Kashiwa, 277-8561
東京大学理学系研究科〒277-8561千葉県柏市柏の葉5-1-51
九州大学応用力学研究所 〒816-8580 福岡県春日市春日公園6-12
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
In order to investigate the poloidal structure of edge turbulence of ohmically heated plasma in Tokyo
Spherical Tokamak-2 (TST-2), the experiments were conducted using two-dimensionally movable
Langmuir probes. With monitoring floating potential and ion saturation current, the radial and poloidal
profiles of edge turbulence are observed. High coherence of fluctuations at f ~10 to 100kHz measured withtwo probes 90 toroidally distant from each other is also observed. In order to investigate the nonlinear
structure of the turbulence, bispectral analysis was used, and the observed auto-bicoherence patternsuggests the presence of the coupling and energy transfer between different modes.

1. Introduction
In fusion plasma research, study on anomaloustransport by turbulence is important to control thetransport and plasma operation. Recent researchesof turbulence have paid special attention on
nonlinear,
non-local,andnon-stationary
properties of multi-scale turbulence. In manymagnetic confinement toroidal plasma devices,
poloidal asymmetry of fluctuations on the same
flux surface has been observed [1,2]. Furthermore,
recent research progress highlighted nonlinear
couplings among multi-scale fluctuations as wellas their spatial variation and non-locality [3].
Therefor, measurement of poloidal profiles ofmulti-scale fluctuations on the same flux surface
is important to understand linear/nonlinear
processes of fluctuations behind the saturated
turbulence fluctuations and transport. In theposter, we introduce the progress of edge
turbulence measurement across the broad range ofthe low-field side edge plasmas in the TST-2
spherical tokamak.

2. Experimental device
Typical design parameters of TST-2 are major
radius R ≦ 0.36m,minor radius a ≦ 0.23 m,
aspect ratio A≧1.6, elongation κ ≦1.8, toroidal
field Bt≦0.4 T, plasma current Ip≦0.2 MA, anddischarge duration τ ≦40 ms. Inductive start-up
discharges are initiated by EC wave (2.45 GHz/5kW) in this study. The target plasma parameters
are: Ip ~ 100 kA, electron density !e ~ 1-10
1018 m -3, Bt ~ 0.2 T, and Deuterium gas was used.
The analysis target plasmas is the stationaryedge turbulence (several ms durations) without
significant reconnection events. Edge turbulence
was measured with a number of Langmuir probes(LP) installed at the low-field side: a midplane LPand upper/lower LPs. Each LP has several
electrodes, and can measure the floating potential
Vf and ion saturation current Iis simultaneously.
Electron temperature (Te) is also measured bysweeping the bias voltage every 1 ms.

Fig. 1. (Color online) Enlarged view of the upper
Langmuir probe. It has 4 electrodes and one electrodemeasures Iis and others Vf.

3. Results
First, we will give the information about theamplitude of fluctuations. The magnitude of Vf/Te
is roughly similar to that of !is /!is at R=630 mm,but Vf/Te>!is /!is inside the plasma boundary. This
result is similar to other tokamak experiments [4].
The typical auto-power spectra of the edge
floating potential fluctuations show two spectral
peaks at ~10 kHz and at ~100 kHz. In particular,
the second peak is especially prominent for Vf.

Fig.2 (Color online). Power spectra of Vf/Te (black) and!is /!is (orange). Coherent peak observed at 10kHz is
considered to be MHD-type fluctuation. (R=560 mm), aspectral peak is observed at ~100 kHz.

The both spectral peaks are prominent at innerarea, and the magnitudes of these peaks decay asprobe tips move away from the plasma boundary.
The 10 kHz peak shows strong coherence between
Vf and magnetic perturbation, thus the 10 kHz
fluctuation is considered to be an MHD type mode.
The second peak at several 10s kHz shows theincrease as the probe tips go inside the boundary.
In studying the nature of the transport, thecoherence and phase between density andpotential fluctuations are important. When we
assume that the electron temperature fluctuationis considerd to be negligible, Vf and Iis reflectpotential and density fluctuations respectively.
In the radial and poloidal profiles of coherence

and phase angle between Vf and Iis, high coherence
was observed only in the radially limited edge
region (R~570-600 mm), and phase shift varies
from -0.2π to -0.7π according to the location.
In order to explore the nonlinear structure of theedge turbulence, bispectral analysis was applied toVf. The total bicoherence has distinct peaks at f ~ 10kHz and several 10s kHz, where corresponding
spectral peaks of power spectrum are prominent.
These patterns indicate significant nonlinear
couplings between the second spectral peaks andthe turbulent fluctuations with the frequency
higher than the second spectral peak, suggestingthat the turbulence cascading of the fluctuation
with the second peak occurs.
The high coherence of several kHz to 100 kHz
measured with two probes 90 toroidally and ~75°	 
poloidally distant from each other are also observed.
This suggests the two probes were connected
through the magnetic field line, and the correlation
length of turbulence along the line is very long.
4. DiscussionThe coherence and phase between Vf and Iis varies
significantly around the edge region, suggesting thedifference in transport in those regions. The
bispectral analysis shows the presence of nonlinear
coupling between the MHD mode at 10 kHz, thecoherent mode at 70 kHz, and backgroundmicroscopic fluctuations. It should be noted A.
Ishizawa, and N. Nakajima pointed out that amacro-MHD mode is excited as a result of micro
turbulence and zonal flow [5]. However, when theturbulence intensity has radial profile, oscillation ofthe magnetic flux surface may modulate turbulence
intensity, and this may cause nonlinear coupling
between MHD mode and turbulence. Further study
is necessary to confirm nonlinear energy transferfrom turbulence to MHD mode on the basis ofequation of motion.
Acknowledgement
This work was partly supported by Grants-in-Aidfor Scientific Research (23360409, 21226021,21246137) of JSPS, Japan.
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020706.
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040702.