J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 19'1-200

Thomson Scattering Measurement in Heliotron J
OKADA Hiroyuki. TSURU Hirokir. IKEDA Yoichir. KONDO Katsumir.
SANO Fumimichi, MIZUUCHI Tohru and OBIKI TokuhiroInstitute of Advanced Energy, Kyoto University, Gokasho, Uji 611-0011, Japan1
Graduate School of Energy Science, Kyoto (Jniversity, Gokasho, Uji 6tl-0011, Japan(Received: 11 December 2001 / Accepted: 19 June 2002)

Abstract
Thomson scattering system has been developed for Heliotron J. Two-dimensional measurement isrequired at least since plasmas of Heliotron J are fully three-dimensional. For this purpose, a movable
collecting mirror system and a movable bending mirror system have been developed. Using this system,
the electron temperature and density in the range of 0.3 m in the z (vertical)-direction and 0.2 m in the R(major radius)-direction can be measured. The spectrally and spatially resolved light scattered byelectrons is injected into a image-intensifier tube and recorded by a CCD camera. An image of about 0.1
m in z-direction and 80 nm in the wavelength is obtained in one discharse.

Keywords:
Thomson scattering, electron temperature, Helioron J, two-dimensional measurement, camera system

1. Introduction

accuracy for the reduction of the structure material. The

Heliotron J is a helical-axis heliotron device withlocal quasi-ornnigeneous field []. The Heliotron J
device was designed to produce a wide variety of the
field configurations to study the optimization of the

optical image fiber bundle connects the collecting
optical system to the spectrometer followed by thedetector system in order to make it easy to align thesystem components.

helical-axis heliotron field configuration. Along thispu{pose, Thomson scattering device has been developed.

2. Experimental Setup

The spatial profile data of plasma temperature anddensity with sufficient spatial resolution are important
for the local transport analysis. Two-dimensional

The overview of the Thomson scattering system for
Heliotron J is shown in Fig. l This system is composedof a ruby laser injector with an energy of 4-5 J, theinjection optical system, the collecting optical system (aspherical mirror and an image fiber bundle), a spectrometer, a framing camera and CCD camera system. A
fast framing camera with an S-25 photocathode surface
is used as the front end of the detector since thebackground plasma light can be also obtained withseveral hundred-nanosecond intervals. The readoutdevice is a cooled CCD, which has 1000 x 1000 pixels
each of which corresponds to 0.1 mm (spatial length)
and 0.2 nm (wavelength). For practical purposes, themeasurement is required at least in Heliotron J since theplasma is fully three-dimensional.

The injection port and the scattered-light detection
port for the Thomson scattering system can be arrangedin the same poloidal plane in Helioron J since Heliotron
J has a L = I helical coil instead ofL =2 helical coils.

Therefore, the sampling point can be changed bymoving the injection optical system and the detection
optical system in the same direction. These two opticalsystems are separately installed and designed to change
the positions by the same distance within the sufficient

Corre sponding author' s e -mail: okada@ center.iae.lqoto-u.ac j p

r97@2002 by The Japan Society of Plasma
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ground frame.

spatial resolution is restricted by the diameter of thelaser beam (about 2 mm) and the scattered light
CCD camera, the left side of which was used for theThomson scattered light detection, and the right side ofwhich was used for the background light detection. Theframing camera attached in front of the CCD camera
made the change of the exposure side. The channel
number from 20 to 480 on the each side corresponded tointensity. The reason that the spatial resolution dependson the intensity is that a lot of signals must be summed
along the sampling volume to increase S/N ratio when
the signal is weak.
The beam injection point and the detection point
can be moved horizontally in the major radius direction
to any positions in the range of 0.2 m with an setting
error of 0.2 mm so that the various configurations ofHeliotron-J plasma can be measured. The detectablearea is shown in Fig. l. About 0.1-m sampling volume
along the laser beam can be measured by 1000 detector
elements in one shot. The spherical mirror of 0.5-m indiameter is used for the collecting optics. The solid

the wavelength from 790 nm to 710 nm. The
dependence of the signal on the wavelength was caused

mainly by reason that the tungsten radiation had thedependence on the wavelength and the central region of
the camera tube had better sensitivity.

The Rayleigh scattering measurement was carriedout to estimate the coefficient for converting themeasured signal of Thomson scattering to the photon

angle of the collecting optics is 0.09 Sr, which isnumber. The Rayleigh scattering with nitrogen gas wasselected since nitrogen is non-flammable. The measured
signals with the various nitrogen pressures by themanometer are shown in Fig. 3(a). The intersection of P
= 0 line and the data line corresponds to the stray lightstrength and the slope of the data line corresponds to thesensitivity of the detector system. The sensitivity of theincreased by three times larger than that of Heliotron E[2,3]. The light guide system from the collecting optics
to the spectrometer is also changed from the lens system
to the imaging fiber bundle. The collecting optics andthe detector could be aligned separately by using thefiber system.

3. Calibration of the Systemsystem was determined from the linear fit of the data up

The framing camera and the CCD camera had thewavelength sensitivity and the sensitivity profile on thedetector surface. Other optical components such as thespectrometer and the optical fiber bundle also had
sensitivity dependence on the wavelength and surface

to 7l Torr. It was found that the detector signal wasposition. The relative calibration of the whole system
was carried out with a tungsten lamp (Toshiba, 8V10cd)
whose radiation dependence on the wavelength was wellknown. The lamp was placed at the edge of the opticalfiber to calibrate the whole system. The image of thelamp is shown in Fig. 2. There were two frames in thethe summed signal within 10 nm in the wavelength and0.01 m in the real space. In this figure, the data counts
for S/N - I was 1000. This noise was caused by thestatistical error. We thought that S/N > 2 are needed forthe determination of the temperature. This means thatthe signal counts from the four-time larger number ofsaturated beyond 7l Ton.

From this sensitivity, the Thomson scattering
signals were estimated for the various plasma
parameters in Fig. 3(b). Each data point corresponded to198

Okada H. et dl., Thomson Scattering Measurement in Heliotron J

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710 7m 7& 740 750 750 770 7N 790 800
Wavelength (nm)Fig. 3 (a) Rayleigh scattering signals for various nitrogenpressures.

Fig. 4 (a) Raw signal of the shot number 5513. The signals are summed in the range of 0.05 m.

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Fig. 3 (b) Estimated Thomson scattering signals for various plasma parameters.

lLuLnpFig. a (b) Electron temperature determined by the cali-brated signal. The triangles are averaged values
for the S-nm range. Dashed line shows the fitting
for the triangles.

the pixels should be summed. Therefore, the spatial
resolution for the electron density of I x 10re m-3 wasestimated at about 0.05 m.

From the signal in Fig. 2,3(a) and Fig. 4(a), theelectron temperature is determined. The estimatedtemperature is 365 eV + 89 eV at the center of the4. Experimental Results
The raw data from a Heliotron-J plasma generated
53.2-GHz ECH (400 kW) is shown in Fig. 4(a). Theexposure time of the framing camera should be as short
as possible to avoid the noise of the background plasma
light usually. The fire timing of the laser, however, hada jitter about 100 ns against the start trigger. Therefore
the exposure time was about 400 ns typically to ensurethat the laser pulse of about 50 ns (FWHM) is contained
within the exposure time. The interval of the two signals
was 900 ns. In this wavelength region and the exposuretime, however, there was no significant plasma
background light. In Fig. a(a), the background signals
are subtracted only for the offset correction of the ADCof the CCD carnera.

plasma with the line-averaged density of 2 x l01e m-3 asshown in Fig. a@). The spatial resolution of this shot isabout 0.05 m determined by the statistical error.

However, the required spatial resolution of 0.01 m
under the Heliotron-J plasma condition could not be
attained. In the estimation from the Rayleigh scatteringexperiment, it was found that the detector sensitivity offive times larger than that of our detector is required.
The accuracy ofthe alignment ofthe optical system

for the two-dimensional measurement were examinedusing the stainless target in the vacuum chamber ofHeliotron J and a helium-neon laser of which the optic
axis was coincident with that of the rubv laser of ther99

Okada H. et al., Thomson Scattering Measurement in Heliotron J

Thomson scattering system.

also effective. A new camera with a GaAs photocathodehas been prepared in order to increase photon signals for5. Summary

the next campaign.

The TV Thomson scattering system was installed
and the electron temperature measurement was carriedout in Heliotron J. This system covers the detectablearea of 0.3 m x 0.2 m with 0.2 mm setting error. TheReferances

Ill T. Obiki et a/., Plasma Phys. Control Fusion 42,llsl (2000).

electron temperature was obtained by this system in the[2] S. Sudo et al.,Jpn. J. Appl. Phys.22,485 (1983).

medium electron density region. For lower density

t3l H. Okada et al., Proc. 1996 Int. Conf. on Plasma
Physics, Nagoya, 1434 (1997).

plasma or better spatial resolution, the statistical error
must be reduced. The signal piling up method should be200