22P106-P
Polarization based laser beam combining for improvement of accuracy on theLHD Thomson scattering system
偏光レーザービーム結合によるLHDトムソン散乱装置の高精度化
Ryo Yasuhara, Ichihiro Yamada, kazuto Narihara and Hiroshi Hasyashi
安原 亮, 山田一博、成原一途、舟場久芳、林浩
National Institute for Fusion science322-6 Oroshi-cho Toki-city, GIFU 509-5292
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
The polarization based beam combining from multiple lasers is developed for an improvement of time
resolution and an accuracy of the LHD Thomson scattering system. Two or multiple beams from thedifferent aperture of laser heads are combined to the coaxial beam line in time series by optical componentsof this beam combining scheme. We have installed this type of beam combining method for the LHD
Thomson scattering system from 15th experimental campaign. In the presentation, we will discuss thedetailed design and performance of polarization based laser combining.

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. TS system installed inLarge Helical Device (LHD) was put into theoperation from 1998 and has been obtained
numerous data of Te and ne data from LHD
experimental cycles [1]. The LHD TS systemmeasures Te and ne profiles of plasmas along themajor radius at a horizontally. It has several
Nd:YAG lasers which wavelength is 1064nm fora flexible multi-laser operation. Thomson
scattered light is collected with a large (1.5 mx1.8m) spherical mirror and analyzed bypolychromators that have five wavelength
channels. A partial high reflected coat mirror isadapted to make the bundle beam emitted frommulti-lasers. This mirror was developed in DIII-D
tokamak[2]. The laser beams are packed along acommon beam path by packing mirror and partiallyoverlap in the far field where they are focused to acommon point inside the LHD vessel. However,each laser beam passes in the LHD vessel is not
same. Electron density profiles measured by eachlasers have different errors due to misalignment ofeach laser beams and corrective optics. A
comparison of electron density profile in time line
is difficult in this situation.
To minimize the multiple-laser error requires a
special arrangement of the laser optical system. A
newly beam combining technique was created tosend pairs of the lasers co-axially into the plasma.
This scheme employs high performance polarizing
optics to combine each pair of orthogonallypolarized laser beams. The LHD Thomson
scattering system adapts an oblique backscattering
configuration. Polarization loss for the amount of

scattering light can minimize in this scheme.
2. Polarization based laser beam combining
methodA schematic diagram of newly beam combing
method of polarization based system is shown infigure 1. It is seen that both lasers enter withhorizontal polarization. Input 2 from laser2 isprepared for the polarization beam splitter (PL) byrotating the beam using a Half-wave plate (QR).
This then reflects at the PL. Input 1 from laser 1 on
the other hand passes though the PL and hence
lasers 1 and 2 are now made co-axial.

Beam combiner
PLInput 1sOutput
PQRs
Input 2Fig.1. A schematic diagram of polarization based beamcombining method. PL: polarizer, QR: Half-wave plate

3. Experimental result
To evaluate the effect of polarization forThomson scattering signal, Rayleigh scattering
signal was measured as a function of polarization
angle. A Thomson scattering signal has same actionof a Rayleigh scattering signal for the polarization.

We can assume the response of Thomson scattering
signal from LHD plasma by this measurement.
Figure 3 shows a Rayleigh signal as a function ofpolarization direction at the laser output. From thisfigure at the polarization direction of 90 degree aRayleigh scattering signal is decreased to 85% ofmaximum value at 0 degree. This loss can tolerate
for the benefit of coaxial beam line of two lasers.

4. Summary
The polarization based beam combining frommultiple lasers is developed for an improvement oftime resolution and an accuracy of the LHD
Thomson scattering system. Two or multiple beams
from the different aperture of laser heads are
combined to the coaxial beam line in time series by
optical components of this beam combining scheme.
We have installed this type of beam combining
method for the LHD Thomson scattering systemfrom 15th experimental campaign. In thepresentation, we will discuss the detailed designand performance of polarization based laser
combining.

Acknowledgments
This work was supported by a Grant-in-Aid forYoung Scientists (B), Grant No. 23760813 from
MEXT and is partially carried out under the LHD
projectfinancialsupport(GrantNo.
NIFS11ULHH005).
Fig.2. Sensitivity of polarization
Rayleigh scattering signals.

directionfor

In LHD 15th experimental campaign we install
this beam combining method into LHD Thomson
scattering system. Figure 3 shows a normalized ne
profiles measured by coaxial laser of laser1 andlaser2 and off axial laser of laser3 which uses apacked mirror for beam bundling. This figure show
the good agreement of ne profiles of laser1 andlaser2. In contrast ne profile measured by laser3 hasa difference from coaxial lasers. It shows thecoaxial beam combining is effective for reliabilityof ne profile measurement by use of Thomson
scattering.

Fig.3. Normalized ne profiles measured by coaxial
laser of laser1 and laser2 and off axial laser 3.

References[1] K. Narihara, I. Yamada, H. Hayashi, K. Yamauchi.
Rev. Sci. Instrum. 72 (2001) 1122.
[2] T. N. Carlstrom G. L. Campbell J. C. DeBoo R.
Evanko J. Evans, et al. Rev. Sci. Instrum. 63 (1992)
4901.