J. Plasma Fusion Res. SERIES, Vol. 3 (2000) 415418

Development of CO2 Laser Polarimetryfor Long-Pulse Operations of LHD
TSUJI-IIO Shunji*, AKIYAMA Tsuyoshi, SATO Eiji#, MURAYAMA Hirohider,
NOZAWA Takeo, SHIMADA Ryuichi, NAKAYAMA Kazuya2, OKAJIMA Shigeki2
and KAWAHATA Kazuoj
Tolqo Institute of Technology, Meguro-ku, Tolcyo 152-8550, Japan
lHitachi Works, Hitachi Ltd., Hitachi-shi 317-8511, Japan
2Chubu Llniversity, Kasugai- shi 487 -850 I, Japan
3National Institute for Fusion Science. Toki-shi 509-5292, Janan(Received: 18 January 2000 / Accepted:23 May 2000)

Abstract
A CO2 laser densitometer measuring the Faraday rotation was developed for long-pulse operationsof the Large Helical Device (LHD). The frequency-shift heterodyne techniques with the use of acoustooptic modulators (AOM) were adopted to measure the Faraday rotation angle with a high resolution. A
two-channel polarimetry system was installed on LHD. The first results reveal that a rotation angle
resolution of about 0.02 degrees was achieved with a time resolution of 16 ms.

Keywords:
polarimeter, frequency-shift heterodyne, Faraday rotation, CO2 laser, LHDAn accurate measure of the rotation angle would

1. lntroduction
Tangential polarimetry in the midplane has been
proposed to monitor the electron density in large
tokamaks []. This technique has several advantages

enable us to monitor the electron density of stellarator/

over interferometry. The polarimetric measurement does
not depend on the past history of the discharge since theFaraday rotation angle can be designed to be less than
Ztt with a proper choice of the probe beam wavelength.

infrared polarimeter using acousto-optic modulators has
been developed [3] to back up the interferometer systemon the Large Helical Device (LHD). The Faraday
rotation angle is less than a degree except high density

Hence the polarimetry is free from the miscount offringes from which interferometers often suffer so that it
is suited for long-pulse operations of fusion devices. It
has another advantage of being insensitive to vibrationof optical components, by which the system becomes
simpler. The feasibility of the tangential polarimetry has
been demonstrated by a CO2 laser polarimeter using acouple of photoelastic modulators on the JT-60Udischarges even along the tangential chords of LHD

tokamak [2].

asainst common-mode refractive effects.

*C orre s pondin g author' s e -mail : s iio @ nr. tite ch. ac

heliotron plasmas, where the confining magnetic field
can be computed at least for low plasma pressure. An

plasmas when we use CO2 laser beam to avoid
refraction effects. Thus sophisticated techniques arerequired to measure the angle with a resolution of lessthan 0.1 degrees. The frequency-shift heterodyne
techniques [4,5] were adopted to measure the angle withhigh resolution. The method is insensitive to beam
ellipticity and laser power fluctuations and robust

ip'Present address: Kawasaki Heavy Industries, LTD., Noda-shi 278-8585, Japan415@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchTsuji-Iio S. et aL, Development of CO, Laser Polarimetry for Long-Pulse Operations of LHD

The polarimeter was installed on LHD and its first
results are presented.

signals is four times the Faraday rotation angle owing tothe double beam path in the plasma.

The total beam path length from the CO2 laser(MPB GN-802-MES) to detectors was designed to be

2. Optical Arrangement
Figure I shows the optical setup of the frequency-

about 30 m. The beam path in the vacuum vessel to andshift heterodyne polarimeter. The laser beam passes

from the retro-reflector is about 20 m. To focus thethrough an acousto-optic modulator (AOM), operating at
about 40 MHz. The Doppler shifted beam is thenrecombined with another diffracted beam such that thetwo slightly frequency offset components are polarized
perpendicularly to each other. These are then passed

beam diameter at the retro-reflector to about 10 mm, abeam expander with a beam expansion ratio of three
was placed at a path length of 2.5 m from the laser. The
beam diameter at the ZnSe window with an aperture of
88 mm is calculated to be less than l8 mm.

through a quarter wave plate to generate counterrotating circularly polarized beam components andlaunched into the plasma along a few chords using

3. Probe Chords in LHD
The Faraday rotation of the CO2 laser beampolarization insensitive beamsplitters.

through LHD plasmas was numerically evaluated as afunction of the tangent radius to optimize the probe
chords. We used the MHD equilibrium database of LHD
computed with the VMEC code. Figure 2 plots thedistribution of the Faraday rotation angle divided by theIine electron density with a tangent radius of 3.8 m in
the two cases of volume averaged plasma betas of
0.32Vo and 1.767o. We assumed the electron density
profile as ne = z,u^ (l-S)r, where S stands for theRetro-reflectors with a clear aperture of 70 mm areused to displace the returning beams by 40 mm
vertically so that the detector optics are separated fromthe probe beam optics. The retro-reflector wasmanufactured from aluminum alloy (46063) and its
mirror surface was coated with gold. It weighs only 540
g. Although mirror blocks were screwed on a base plate
to avoid the use of resin or adhesives, an accuracy ofless than 0.5 mrad of parallelism was achieved.
The beam emerging from the plasma is passed
through a polarizer and detected to give an oscillating

toroidal magnetic flux normalized to that at theoutermost flux surface. Flat and parabolic profiles areexpressed by,l = 0 and 1, respectively.

signal at the beat frequency that suffers a phase

When we choose a probe chord with a tangent

retardation proportional to the Faraday angle. Note thatthe phase difference between the reference and probe

radius of around 3.8 m, the dependence of thenormalized Faraday rotation on the electron density@ioal bilc 1

t,a--4'aaa--I*-\@tiut w;i
dctector ;-]dltcctorxs iIl/2 wsvc platc

polarizer

H:lrto+610. beampolarizcr x 3

arb: arbt-&rbn

V:trlo+o)6'1 oxpandar

cor$inlr l/4 wavs platoTTi [,s'rna',,iI Rdro+60,\--Yi!,t-r*-9lr-l.31-"0"*'Fig. 1 Schematic of the frequency-shift heterodyne polarimeter system on LHD.

4t6

Tsuji-Iio S. et al., Development of CO, Laser Polarimetry for Long-Pulse Operations of LHDq)--uo

fo 0.6

EEoih'6--cco-o 0.4a)

Ea.9.9tso:=

G9Loto>P 0.2

EoSp
xol-N

U:Pc
Log-?

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3.25 3.5', 3.75

0.64fr".'.',"*;i6**4
ch3-6,N;-''""

Eoo:

Fig. 3 Comparison of the Faraday rotation angle fromdigital processing with the interferometer data.

nffi(1-s)-.-.-.- n*,(1-S)2So:oi:.N

coi*t------!

I\..'\.

o:=-o>+r 0.2oE) f------'-"--'"\time (s)=6 o.4.98*aljicEc'lEE}
E'6fE

ch2rG

Tangent radius (m)o-oshot number 13897

ou
G..\o.E

eo"-.-\E

g2 ='l..760/o05

rioEG43.25 3.5 3.75 42030time (s)Tangent radius (m)Fig. 4 Comparison of the Faraday rotation angle fromlock-in amplifiers with the interferometer data.

Fig. 2 Variation of the normalized Faraday rotation angle

as a function of the tangent radius with threeelectron density profiles at two volume averaged
betas.

complex demodulation [6] was employed to improve theaccuracy of phase evaluation. The trace ofinterferometer measurement along a vertical chord witha major radius of 3.67 m, which was close to themagnetic axis is shown by broken line for comparison.
The observation that the rotation angle of ch 3 (tangent

profile shape is so weak that the Faraday rotation angle
is a good measure of the line electron density. On theother hand, the normalized Faraday rotation along innerchords with a tangent radius of around 3.4 m is sensitive
to the electron density profile shape. Accordingly we

can draw information on the electron density profile

radius: 3.90 m) is almost the same as that of ch 2(tangent radius: 3.72 m) in the early phase of thefrom the polarimetric measurements along twodischarge is only explained by a hollow density profile.

tangential beam paths.

The results indicate that the resolution of theFaraday rotation is about 0.02 degrees, which isequivalent to a resolution of the line averaged electron
density of about I x 1018 m-3, with the use of digitalband-pass filtering from 99.99 kHz to 100.01 kHz.

Two retro-reflectors which correspond to tangent
radii of 3.72 m (ch 2) and 3.90 m (ch 3) were installedin the vessel in 1999. The installation of another retroreflector for inner chord measurement (ch 1) was

Figure 4 shows the polarimeter traces of a longpulse NB heated discharge at2.75 T. In this case, twolock-in amplifiers (EG & G Princeton Applied Research
Model 5302 for ch 2 and Stanford Research Systems SR
830 for ch 3) were used for the phase measurements.

postponed until the next vacuum vent.

4. Experimental Results
An example of the 2-channel polarimetermeasurement of an NB heated discharge at 2.75 T isshown in Fig. 3. The beat signals at 100 kHz weredigitized in 12 bits at a sampling rate of I MS/s. Digital

The rotation angle sometimes fluctuates with anamplitude up to 0.05 degrees in several seconds.
417

Tsuji-Iio S. et al., Development of CO, Laser Polarimetry for Long-Pulse Operations of LHD

Although no cause for the seemingly common modenoise is identified, an improvement in the long-term
stability of the phase measurement is required toWatanabe for providing the MHD equilibrium databasemonitor the line electron density of long-pulse

assistance and advice.

of LHD. They are grateful to Dr. K. Terai and Mr. M.
Takahashi at Toshiba Corporation for their technicaloperations.

References

tll F.C. Jobes and D.K. Mansfield, Rev. Sci. Instrum.

5. Gonclusions

A frequency-shift heterodyne polarimeter was63,5rs4 (1992).

developed to monitor the electron density of long-pulse
discharges in LHD. The first results of the two-channel

tzl Y.Kawano et al., Rev. Sci. Instrum. 70, l43O

polarimeter reveal that the resolution of the Faraday
rotation angle is about 0.02 degrees when digital band-t3l H. Murayama et al., Proc. of 1998 ICPP & 25th(1999).
EPS Conf. on Control. Fusion and Plasma Phvs..
Yol.22C, 1482 (1998).

pass filtering from 99.99 kHz to 100.01 kHz isemployed. The suppression of the low-frequency noise

t4l G. Dodel and W. Kunz, Infrared Phys. 18, 773up to 0.05 degrees is required for long-pulse operations( r978).

[5] J.H. Rommers and J. Howard, Plasma Phys.

of LHD.

Control. Fusion 38, 1805 (1978).

Acknowledgements

t6l D.W. Choi et a/., Rev. Sci. Instrum. 57, 1989
(1986).

This study has been carried out under the LHD

project collaboration. The authors thank Dr. K.

418