22P107-P
Development of the polarization controlled multi-pass Thomson scatteringsystem for tandem mirror GAMMA10
タンデムミラーGAMMA10のための偏光制御マルチパストムソン散乱システムの開発
Ryo Yasuhara1, Masayuki Yoshiwaka2, Ichihiro Yamada1, Kazuo Kawahata1, Hisamichi Funaba1,Tsuyoshi Imai2, Mizuki Sakamoto2, Yousuke Nakashima2, Masato Morimoto2, Takashi Minami3
安原 亮1、吉川正志2、山田一博1、川端一男1、舟場久芳1、今井剛2、坂本瑞樹2、中嶋洋輔2、森本真人2、南貴司312National Institute for Fusion science 322-6 Oroshi-cho Toki-city, GIFU 509-5292
Plasma Research Center, University of Tsukuba, 1-1-1 Tennodai, Tsukuba-city, Ibaraki 305-8577, Japan
3
Institute of Advanced Energy, Kyoto University, Gokasho, Uji-city, Kyoto 611-0011, Japan1
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
2
筑波大学プラズマ研究センター 〒305-8577 茨城県つくば市天王台1-1-1
3
京都大学エネルギー理工学研究所 〒611-0011 京都府宇治市五ケ庄

The novel configuration of the multi-pass Thomson scattering (TS) system is proposed for theimprovement of time resolution and accuracy of electron temperature by use of the polarization controltechnique. This configuration can realize a perfect coaxial multi-passing at each passes. Number of roundtrips is not limited by the optical configuration. Now, we are developing this new multi-pass TS system in
GAMMA10. As the first step, a scattering signal has increased to 1.6 times by the double pass system.

1. Introduction
Thomson scattering (TS) diagnostics is one of
the most reliable methods for measuring theelectron temperature (Te) and density (ne)profiles in fusion plasmas. However, due to thesmall Thomson cross-section σts=6.6510−21 m2,
Thomson scattering is challenging for lowdensities plasma such as the GAMMA 10 plasma.
The typical electron density, electron and ion
temperatures of GAMMA10 are about 2 1018 m-3,0.1 keV and 5 keV, respectively, during
application of P/B-ECH[1].
For increasing the scattering probability at low
densities plasma, a multi-pass Thomson
scattering scheme is effective. It allows the laser
pulse to be focused several times onto the scattering
volume, thus increasing the scattering photonnumber. Multi-pass Thomson scattering systems aredemonstrated at many institutes. TEXTOR group
has demonstrated the improvement of signal tonoise ratio by use of multi-pass TS system whichuses a pair of concave mirror for recycling
photon[2]. JT60 TS group has constructed a double
pass system by the use of phase conjugate mirror
for reflection[3]. Although those systems haveincreased the reliability of TS system, they have thelimit from the optical system. Each laser beam
passes are different in a concave mirror type TS
system (TEXTOR system). It should set thescattering volume near the focal point of a concave

mirror, or need to calibrate for each different beam
passes. A phase conjugate mirror requires the purityof laser bandwidth.
In this study, we propose a newly scheme ofmulti-pass TS system by the use of a polarization
optics. This scheme can be modified from the basic
single pass Thomson scattering system by adding
the high reflection mirror for cavity mirror, lenses
used for image relaying the laser beam andpolarization control devices. It has a collinear beamline in the multi-pass cavity. The system design andexperimental results of this multi-pass scheme havebeen reported at following sections.
2. Design of the polarization controlled
multi-pass system
A schematic diagram of the new multi-passmethod of a polarization based system is shown in
the figure 1. This system is based on theGAMMA10 Thomson scattering system which has
successfully observed a electron temperature of theGAMMA10 plasma in 2010 [1]. Horizontal
polarized laser light from the 2 J by 10 Hz Nd:YAGlaser (Continuum, Powerlite 9010) is focused into a
plasma by the first convex lens. After theinteraction with plasma, laser light emits from theport window which has the anti-reflection coating
and collimated by the second convex lens. A pair oflenses is a key component of this optical system. It
makes the image relaying optical system from IP1

to IP2 to maintain the laser beam quality during themulti-pass propagation. The laser light is reflectedby the mirror for the second pass and focused again
into the plasma. A Pockels cell is used for apolarization control device. It switches horizontal
polarization to vertical polarization during thedouble pass traveling of the laser light. The verticalpolarized laser light is reflected by polarizer andend mirror at the image point of IP1’. The
polarization of reflected laser light is changed again
by Pockels cell for the traveling of third pass. Thenlaser light is confined between the mirrors at theIP2 and IP1’ for the multi-pass.
MirrorFrom the result, the scattering signal height ofdouble pass configuration is 1.6 times larger than
the signal of single pass configuration. It shows us
the improvement of data quality of GAMMA10
Thomson scattering system and the feasibility of theproposed polarization based multi-pass system.

Polychrometor

IP1'IP2

IP1
Polarizer

Lens

Polarization control device

Lens

MirrorGAMMA10Fig.1. A schematic diagram of the new multi-passmethod of a polarization based system.

Fig.3. Double pass (solid line) and single pass (dottedline) Thomson scattering signals.

To design the optical system of the multi-passsystem, we have carried out the ABCD matrix
analysis and the polarization analysis by the use ofJones matrix. Figure 2 shows the multiplication of
the scattering light as a function of a pass number
from the result of the optical design. This result
indicated that scattering light signals are increased
to two times larger at the double pass configuration.
At the sixteenth pass configuration, scattering light
was about six times larger than the single passconfiguration.

4. Summary
The novel configuration of the multipass
Thomson scattering (TS) system is proposed for theimprovement of time resolution and accuracy ofelectron temperature by the use of the polarization
control technique. This configuration can realize aperfect coaxial multipassing at each passes. Numberof round trips is not limited by the opticalconfiguration but only by the optical loss. Now, weare developing this new multipass TS system in
GAMMA10. In this presentation we will report a
conceptual design, an estimation of TS signal atmultipassing and preliminary results of the double
pass experiment.

Acknowledgments
This work was supported by a Grant-in-Aid forYoung Scientists (B), Grant No. 23760813 fromMEXT and the NIFS Collaboration Research
program (NIFS-KOAH025).
Fig.2. Scattering light power as a function of pass
number

3. Experimental result
To evaluate the effect of the polarization based
configuration, a mulitpass system without apolarization control device is installed in theGAMMA 10 system. This system can demonstrate
the double pass scattering. Figure 3 shows double
pass and single pass Thomson scattering signals.

References[1] M. Yoshikawa, et al., Plasma Fusion Re. 6, 1202095(2010).
[2] M Yu Kantor, et al. Plasma Phys. Control. Fusion 51(2009) 055002.
[3] T. Hatae, O. Naito, M. Nakatsuka, and H. Yoshida,Rev. Sci. Instrum. 77, (2006) 10E508.