22P113-P
Double-pass Thomson scattering diagnosis
on the TST-2 spherical tokamak device
TST-2球状トカマクにおけるダブルパストムソン散乱計測
Junichi Hiratsuka1, Akira Ejiri1, Makoto Hasegawa2, Yoshihiko Nagashima2, Yuichi Takase 1,Hiroshi Tojo3, Takashi Yamaguchi1, Takahiro Ambo1, Hidetoshi Kakuda1, Takuya Sakamoto1,Ryota Shino1, Masateru Sonehara1, Takuma Wakatsuki1 and Osamu Watanabe1
平塚淳一1, 江尻晶1, 長谷川真2, 永島芳彦2, 高瀬雄一1, 東條寛3, 山口隆史1,安保貴憲1, 角田英俊1, 坂本拓也1, 篠遼太1, 曽根原正晃1, 若月琢馬1, 渡邉理1
1The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa 277-8561, Japan1
東京大学 〒277-8561 柏市柏の葉5-1-5
2
Kyushu University, 6-1, Kasuga-koen, Kasuga 816-8580, Japan2
九州大学 〒816-8580 春日市春日公園6-1
3
Japan Atomic Energy Agency, 801-1, Mukoyama, Naka 311-0193, Japan3
日本原子力研究開発機構 〒311-0193 那珂市向山801-1

A double-pass Thomson scattering system, in which a laser pulse makes a round trip through the plasma,has been constructed. Using a fast detection unit, backward and forward scattering pulses in the signal canbe resolved. Since these scatterings reflect electron temperature along different directions, electrontemperature anisotropy can be estimated by the system. As a result, electron temperature anisotropy can bemeasured within the error bar of 10%. In order to search off-axis temperature anisotropy, new collection
optics have been implemented. Using four polychromator units, electron temperature at four points inplasma can be measured simultaneously. In this conference, preliminary results by the double-passmulti-point system will be presented.

1. Introduction
Multi-pass Thomson scattering scheme isattractive for low-density plasma and electron
temperature anisotropy measurements. Especially,
direct measurement scheme of electron
temperature anisotropy has not been established,
while temperature anisotropy has important
effects on plasma equilibrium and transport.
Asignificant
improvement
inthesignal-to-noise ratio was achieved using a
multi-pass Thomson scattering scheme on
TEXTOR [1]. However, it is difficult to apply asimilar multi-pass system to other devices
because of the use of the intra-cavity
configuration. In contrast, the configuration using
a confocal spherical mirror is simple. The
theoretical performance of this system wasanalyzed and confirmed experimentally [2].
Bowden et al. measured temperature anisotropy
using a single-pass Thomson scattering forelectron-cyclotron-heated low density plasmas (Te~ 1 eV and ne ~ 1017 m−3) [3]. They switched thedirection of the incident laser and measured
reproducible plasmas. They detected slighttemperature anisotropy.
When the plasma is in a collisionless regime,Te⊥ can be higher than T e|| in the high field side,
and Te|| can be higher than T e⊥ in the low fieldside.
Therefore,
temperature
anisotropy
distribution is expected in this case. For
preliminary study of multi-pass configuration,and in order to search for the anisotropy
distribution, we have developed a double-passmulti-point Thomson scattering system.
2. Double-pass Thomson scattering system
On the Tokyo Spherical Tokamak 2 (TST-2)
device, a compact and bright Thomson scattering
diagnosticsystemusing
alargenumerical-aperture fiber and a high power
injection laser (Nd:YAG) was developed [4,5]. In
order to measure electron temperature anisotropy,a double-pass Thomson scattering configuration
has been implemented to the compact Thomsonsystem using a spherical mirror [6]. Contrary tomulti-passconfiguration,
double-passconfiguration is relatively simple and easy toconstruct, since only one mirror should be alignedon the double-pass configuration. Figure 1 showsa schematic diagram of the principle oftemperature
anisotropy
measurementbydouble-pass Thomson scattering scheme. Injected
laser travels through the plasma and reflectedback by a concave mirror, and scattering lightpulse at each laser transit is measured by onepolychromator unit. In order to distinguish thesescattering pulses from each laser transit, a fastand low-noise detection system was developed,
and the measured full width at half maximum(FWHM) of the laser pulse was about 10 ns [7].
Combining the double-pass configuration withthe fast detection system, the forward andbackward scattering pulses can be detected
almost simultaneously (i.e., within the time
separation of a few tens ns). Therefore, we canmeasure electron temperature anisotropy even if
plasma reproducibility is poor. This is importantwhen we study phenomena related to instabilities.
→→

When the angle between k s - k i and the toroidal
field is far from 45 ° , the double-passconfiguration provides the opportunity fortemperature anisotropy measurements.
As a pilot experiment for the temperature
anisotropy measurement, electron temperatures atthe center of ohmic plasma has been measured by
the double-pass system. In such region,
temperature anisotropy is not expected. Figure 2
shows a typical waveform by the double-passconfiguration. As a result, the degree oftemperature anisotropy isTe  Te||Te  3%  10%(1)for high density ohmic plasmas. This result showsthat electron temperature anisotropy can bemeasured within the error bar of 10%. Thus,
result (1) implies that the significant temperature
anisotropy was not found at the center of highdensity ohmic plasma.

Fig.1. Principle of temperature anisotropy
measurement by the double-pass system

Fig.2. Typical waveform of the double-passmeasurement

3. New collection optics for off-axis temperature
anisotropy measurement
In order to search for the off-axis temperature
anisotropy in collisionless regime for ohmic
plasmas, new collection optics has beenimplemented (Fig. 3). A larger collection mirror
and a new fiber bundle were installed. Using fourdetectors (polychromators) and four fibers,
measurement can be performed at four points inplasma simultaneously, in which electron
temperature can be measured around low fieldside, high field side and center of plasma.
Combining the double-pass and the newcollection optics (double-pass multi-point system),radial distribution measurement of electron
temperature anisotropy has been performed.

Fig.3. Photograph of the collection window and four
fibers

Acknowledgments
This work was supported by Japan Society for
the Promotion of Science (JSPS) Grant-in-Aid for
Scientific Research Nos. 21246137, 21226021 andNational Institute for Fusion Science Collaborative
Research Program No. NIFS09KUTR039.
References[1] M. Yu. Kantor. et al. Plasma Phys. Control. Fusion
51, 055002 (2009).
[2] J. Hiratsuka et al., Plasma Fusion Res. 5, 044(2010).
[3] M. D. Bowden. et al. J. Appl. Phys. 73 (6), 15(1993).
[4] S. Kainaga. et al. Plasma Fusion Res. 3, 027 (2008).
[5] T. Yamaguchi. et al. Plasma Fusion Res. 5, S2092(2010).
[6] J. Hiratsuka. et al. Plasma Fusion Res. 6, 1202133(2011).
[7] A. Ejiri. et al. Plasma Fusion Res. 5, S2082 (2010).