22P108-P
Development of Thomson scattering diagnostics in the QUEST sphericaltokamak device
QUEST球状トカマクにおけるトムソン散乱計測の開発
Takashi Yamaguchi1 , Akira Ejiri1 , Junichi Hiratsuka1 , Makoto Hasegawa2 , Yoshihiko Nagashima2 ,Kazumichi Narihara3 , Yuichi Takase1 , Hideki Zushi2 , and the QUEST group
山口 隆史1 , 江尻晶1 , 平塚淳一1 , 長谷川真2 , 永島芳彦2 ,成原一途3 , 高瀬雄一1 , 図子秀樹2 ,QUESTチーム
The University of Tokyo 1 5-1-5, Kashiwanoha, Kashiwa 277-8561, Japan
Kyushu University2 6-1, Kasuga-koen, Kasuga 816-8580, Japan
National Institute for Fusion Science3 322-6, Oroshi-cho, Toki 509-5292, Japan東京大学 1 〒277-8561 柏市柏の葉5-1-5
九州大学 2 〒816-8580 春日市春日公園6-1
核融合科学研究所3 〒509-5292 土岐市下石町322-6
The goal of QUEST spherical tokmak research is steady state operation by RF current drive and controlled
PWI. Tho mson scattering diagnostics is important to analysis the RF current driven plasma. We used aNd:YA G laser with an energy of 1.65 J , a repetition rate of 10 Hz, a spherical mirror for collection of thescattering light, and polychromators for spectroscope. Since the Thomson scattering signal is weak at low
density, many pulse signals from steady state plasmas are accumu lated. We obtained preliminary re sult andthe electron temperature is about 90 eV for the plasma with a plas ma current of 10 kA.

1. Introduction
A steady state operation by RF current drive andcontrolled PWI is goal in QUEST spherical tokmak(ST) research. The QUEST is an ST with a major
radius R = 0.68 m and a minor radius a = 0.4 m [1].
RF system with a frequency of 8.2GHz is used forthe current drive, and excitation of electron Berstein
wave is expected. The typical electron density is 2×1017 m-3 by interferometer.
Thomson scattering (TS) measurement is ahighly reliable measurement of electron
temperature, and many devices are equipped with aTS system. The temperature and density is usefulfor equilibrium analysis and also useful forunderstanding the RF current driven plasmas.
The scattering signal is weak and diagnostic is
expected to be difficult for the low density current
driven plasma. However, the period of steady stateis over a second and laser of repetition rate is 10Hz.
Therefore, accumulation of many pulse signals ispossible.
2. Diagnostic system
The TS system is similar to TST-2 [2] using thesame power of Q-switch Nd:YAG laser, the same
polychromator and the same optical fiber. The
system consists of the incident optics, lightcollection optics, and spectroscopic system. The
Nd:YAG laser have a wavelength of 1064 nm, apulse width of about 7 ns, a repetition rate of 10 Hz,

and an output beam diameter of less than 9 mm.
For collection of scattering light, we use aspherical mirror. The spherical mirror has a radiusof 0.5 m, a curvature radius of 1 m and a coating ofgold. The measurement is backward scattering of165 - 171 degree. A diagram of laser injection, lightscattering and light focusing is shown in Fig.1.
There are six measurement points in the system.
Table I shows the scattering angle, the scattering
length and the solid angle at each measurement
point. We used a large-N.A. (= 0.37) optical fiber[2] to collect the reflected light from the sphericalmirror.

Nd:YA G laser

Spherical mirror

Fig.1. A diagram of QUEST device and collect ion
optics.

A fast response polychromator unit wasdeveloped for QUEST and TST-2 [4]. We areplanning to install a multiple path configuration,

which is very attractive because of the simultaneous
measurement of the forward and the backward
scattering. The polychromator has avalanche
photodiodes and interference filters.
Table I. scattering angle, length and solid angle at eachmeasurement points

Radius
[mm]
340
488
636
784
932
1080

Scattering
angle[degree]
171
169
168
167
166
165

Scattering
length
[mm]
52.9
39.2
32.4
26.7
21.9
17.7

Solid angle[sr]0.02820.03650.04300.0506
0.05970.0711Time [ns]Fig.4. Accumu lated signals fro m 21-40th laser
injections.

3. Measuring result
We measured the plasmas with gas puff during
discharge. The plasma current is 10 kA andduration time is 2s (Fig.2.). We measured at pointof major radius = 784.

Fig.5. Fitting to a Maxwell distribution.
Time [sec]Fig.2. The p lasma current evolution of Shot No.15069.

The signal to noise (S/N) ratio of one pulse signal
is low. We integrate a pulse signal and plot in Fig.3.
The recording system is not synchronized to theQUEST operation at present. 21-40th pulses seems
to be the scattering signals from and the plasma,
because the period of large (integrated) signal issimilar to the plasma current duration. Thesesignals were accumulated and shown in Fig.4, andsubtitle shows central wavelength and FWHM ofeach filter in the polychromator. We integrate
accumulated signals and fit to a Maxwell
distribution (Fig.5). The calculated electron
temperature is 93±9 eV.
Integratedsignal[nVs]
Laser pulseFig.3. A Integrated signal of a detected signal.

4. Summery and future works
We built a Thomson scattering system for
QUEST, and then Thomson scattering signals wereobtained for the first time. The preliminary electron
temperature was about 90 eV for a QUEST steadystate plasma with a plasma current of 10 kA. We
are planning the following works. A trigger system
synchronized to the QUEST device operation isneeded. For absolute calibration of the density,
Ramman scattering calibration is in progress.
Recording system for simultaneous multi-channnel
measuring is also needed.
Acknowledgments
This work was supported by Japan Society for
the Promotion of Science (JSPS) Grant-in-Aid for
Scientific Research No. 21246137, 21226021 andNational Institute for Fusion Science (NIFS)
Collaborative
Research
Program
No.
NIFS09KUTR039.
References[1] K. Hanada et al. Plasma Fusion Res. 5, S1007(2010)[2] T. Yamaguchi et al. Plasma Fusion Res. 5, S2092(2010)[3] A. Ejiri et al. Plasma Fusion Res. 5, S2082 (2010)