J. Plasma Fusion Res. SERIES, Vol.5 (2002) 437-441

CCD Camera Array System for Electron Density and Temperature Measurement in the LHD Plasma Periphery
SHOJI Mamoru, YAMAZAKI Kozo, ICHIMOTO Shinji and LHD Experimental Group
National Institute for Fasion Science, Toki 509-5292, Japan(Received: 1 1 December 2OOl I Accepted: 5 July 2002)

Abstract
An image data analysis system using four CCD cameras has been developed for measuring electron
density and temperature profiles on divertor legs. The image data are analyzed by image processing
software after every plasma discharge. The system was installed in a tangential port, and applied tomeasurement after the calibration of the CCD sensitivity of each camera. While the electron density on
the divt:rtor leg increased with the main plasma density, the decrease of the electron temperature on thedivertor leg was observed. These dependencies on the main plasma density agree with the experimental
data by a fast moving Langmuir probe and probes embedded in divertor tiles, indicating the availabilityof the CCD camera array system for plasma parameter measurements on the divertor leg.

Keywords:
CCD c:rmera, HeI intensity ratios, divertor leg, image processing, Large Helical Device

1. Introduction
Large Helical Device (LHD) consists of two
helically twisted magnetic coils and three pairs ofprocessing software.

The calculations by tracing the magnetic field lines

circular poloidal coils. Three dimensionally complicatedperipheral plasmas (ergodic layer and divertor legs) arewith a random walk process from the ergodic layer
predict that the distribution of the particle deposition
pattern on divertor plates is three-dimensionallyformed by the magnetic coils. The measurement of thecomplicated. Thus, the CCD camera array system can beplasma parameters on the divertor legs with visible CCD
cameras is useful for complicated shaped plasmas sucha useful device for measuring three dimensionallycomplicated profiles of the plasma parameters on theas LHD plasmas because of the high spatial resolution,

divertor leg which can not be measured by Langmuir

wide viewing areas and simple instrumental
configurations. It has been found that the electronprobes (0-dimensional measurement).

density and temperature in the plasma periphery canquantitatively be measured by HeI line intensity ratios[]. Conventionally, the HeI line intensity ratios havebeen measured by spectroscopes. Many spectroscopes,thus, are necessary to measure the plasma parameter
profiles. For observing simply the plasma parameter
profiles, we have developed CCD camera array system
in order to measure the images of the line intensity
ratios using an image capture board and image

2. CCD Camera Array System for Measuring
Plasma Parameter Profiles
In order to measure the profile of the HeI line
intensity ratios, three interference filters (center transmit

wavelength lo = 668.8 nm, 707.7 nm and 728.4 nm,respectively) are mounted at the front of the cameras[2]. We used the cameras (SONY: DXC-LSl) with aseparated small CCD head (Ql2 x 60 mm). By@2O02 by The Japan Society of Plasma

C o rre sp ondin g autho r' s e -mail : s hoj i @ LH D. ntfs. a c.j p

Science and Nuclear Fusion Research437

Shoji M. et al., CCD Camera Array System for Electron Density and Temperature Measurement in the LHD Plasma Periphery

mounting the CCD heads side by side at a tangential
port (7-T), we can actually neglect the difference of theposition of the cameras, because the distance between
each camera (- 14 mm) is much smaller than thefiber. The data are captured and saved in memories

distance between the cameras and the peripheral plasma

3. Calibration Experiment for the GCD

mounted on an image capture board (Microtechnica:MTPIC-CD).

(- 2.8 m). The gain and shutter speed can remotely be

CamerasBefore applying the camera system to plasma

controlled from the LHD control room. The HeI line
intensity (L = 728.1 nm) is much weaker than the othertwo HeI line intensity (L = 667.8 and 706.5 nm) [3].
Thus, we have observed the background intensity byanother camera with the interference filter which central
transmit wavelength is 72O.6 nm. We regard the exact
HeI intensity ()" = 728.1nm) as that which is reduced by
the background intensity. For applying interference
filters to cameras, it should be noted that the central
transmit wavelength of the filter is shifted to the lower
wavelength side when the incident angle of light to thefilters is not 0 degree, which is expressed as thefollowing formula [4]:1o= homeasurement, we had determined the relative sensitivityof the cameras by using a tungsten lump. The relative
sensitivity of the camera S is defined by the following
formula:(2)
where, Z (W.cm-3.str-r; is the emissivity of light fromthe tungsten lump at the center transmit wavelength ofeach interference filter, and T-u* (7o) is the maximum
transmittance of the interference filter. We determined
the parameter A by measuring the dependence of the intensity of the tungsten filament on the accumulated
light. Figure 1 gives the dependence of the average intensity on the image of the filament. We controlled the(1)accumulation of light by using neutral density (ND) filters (Z = 50,32, l0 Vo) and by changing the exposure
time of the cameras (// = 500, 250, 125, 100 ps). The
parameters T and At mean the transmittance of the ND
filters mounted in front of the camera, and the exposure
time, respectively. It is generally known that the intensity from cameras saturates when a measured object iswhere .1.6 is the center transmitted wavelength at normal
incidence, N" is refractive index of the extemal medium(air: N" = 1.0), lf is that of the interference filter (1.5), and a is the incident angle of light. In our camera
system, the maximum incident angle of light from theperipheral plasma is less than 5 degrees, thus, the ma.xi-mum shift of wave length due to the finite incident
angle is estimated to be about 1.0 nm. Therefore, weused relatively wide band pass filters (2r,r - 5.0 nm) so100

as to reduce the wavelength shift effect. Here, the pa-{-chl-r- flr2

rameter h1p means the full width of the half maximum(FWHM) wavelength of the transmittance. A demerit ofusing wide band pass filters is that some line emission
except the HeI lines can be mixed into the transmitted
light. We confirmed that no prominent line emission
near the wavelength of the three HeI lines exist by a visible spectroscope. Bremsstrahlung is another possible

t80-i60\40

obstacle for the measurement, experimental results withthe spectroscope show that the intensity by Bremsstrahlung is much smaller than the two HeI lines (,1. = 667.8,706.5 nm) in standard plasma discharges with He gas
fueling. The effect of Bremsstrahlung on another weak
HeI line (L = 728.1 nm) can be reduced by using thebackground light intensity (Lo = 72O.6 nm).
The images of the HeI line emission and6.4389x___,y:6.(r9:]1x-y:- -Y=4'0368x-----I::l'74x0510152A
T x Ar (A.U.)

background light taken by the CCD cameras are
combined into an image by a video multiplexer. The
image is transferred to the control room via an opticalFig. 1 Dependence of the average intensity of the tungsten filament on the accumulated light.
438

Shoji M. et ul., CCD Camera Array System for Electron Density and Temperature Measurement in the LHD Plasma Periphery

too bright. We controlled the exposure time so as not toexceed the intensity over about 100 (A.U.) in plasmameasurement. The parameter A was deduced from theleast mean square fitting of the intensity plots. Using theformula (2), v,e determined the relative sensitivity ofwhich indicates that the image of the divertor leg insidethe divertor region is emphasized by the line integration.

Figure 3 shows a raw HeI line intensity images in an
NBI heated plasma with He gas fueling in which theaverage main plasma density gradually increases. We
observed the images by controlling the exposure time ofeach camera (Chl: At = 17 ms, Ch2: 50 ms, Ch3: 250ms, Ch4: 250 ms) so as to maximize the measured
intensity on the divertor leg within the linear response
range of the intensity (see Fig. 1). Figure 4 and 5
illustrate the time evolution (1.1 s and 1.7 s) of theeach camera as follows:
Schr (668.8 nm): 1.0000, Sch2(707 .7 nm): 0.7368,ScB(728.4 nm): O.4796, Sc1'3(720.6 nm): 0.5490.

By dividing the measured intensity by the relative
sensitivity, we can obtain the relative intensity of theHeI lines and background light.
For measuring the plasma parameter profiles withthe cameras, the uniformity of the sensitivity of pixels
on CCD is important. We confirmed the uniformity inthe practical area on CCD by changing the position of
the tungsten lump and by measuring the intensity profile
with a light diffuser plate for flat-field measurement.

4. Analysis of lmage Data Taken by theCGD Cameras
The captured images are divided into four imagesby the software (Planetron: Image-Pro Plus) so as to get
the calibrated intensity profile of each HeI line and thebackground light. The images are transformed into astandard formatted image to cancel the difference of theviewing angle of each camera, enabling directcomparison of the intensity profiles. The electron
density and temperature are derived from the following
formula:".'fffi, r.i(3)Fig. 2 CAD image of the LHD plasma and the divertor
leg from a tangential port (7-T).

where, 1661,, expresses the calibrated intensity of the image taken by the camera (channel N (N = I - 4)) on apixel. The parrrmeters a and B mean calibration factors

for converting the HeI intensity ratios to the electron
density and temperature, respectively. We determinedthe calibration factors from the ref. [] using typicalplasma parameters in the LHD divertor plasma (theelectron densit;r and temperature are 5 x 1018 m-3 and 50
eV, respectively).

5. Measurement of the Plasma Parameterson the Divertor Leg
From vierv ports (except for tangential ports), thelight from the rlivertor legs is unclear, because the main
plasma disturbs optical measurement of the divertor

legs. We, thrrs, installed the camera system at atangential port. Figure 2 illustrates the CAD image

Fig. 3 Raw image taken by the CCD camera array system in an NBI heated plasma with He gas fueling.

showing typical LHD plasma from the tangential port,
439

Shoji M. et al., CCD Camera Array System for Electron Density and Temperature Measurement in the LHD Plasma Periphery

Te{ ev)
t4lFi
tn 4S^tr-ltEt
444{ilH a,JOtffi ffi

24.

m16

llt*I40Tf {eV)5r,d8.

u{036nx2&s16ts.

to4RFig. 5 Time evolution of the electron temperature profileon the divertor leg measured by the CCD camera
array system.

Fig. 4 Time evolution of the electron density profile onthe divertor leg measured by the CCD camera array system.

measurements of Langmuir probe embedded in divertorplates. The measurements show that the divertor plasma

electron density and temperature image after image
processing, respectively. The area enclosed by a grayand a black line in these figures corresponds to thedivertor region. Considering the CAD image shown in
Figure 2, we think that the upper region inside thedivertor region is appropriate to estimate the plasma
parameters on the divertor leg. This is because thelength of the line of sight being across the divertor leg

density rises with the main plasma density and theelectron temperature gradually decreases with the main

plasma density [5], indicating the availability of thecamera array system for the plasma parametermeasurement on the divertor legs.
Next, we discuss possible experimental errors inour camera system. It can be listed as follows:
1. the wavelength shift effect by the finite incident
angle of the emission,in the region is longer than that in the other region. The
electron density on the divertor leg rises from - 3 x l0r8
m-3 at 1.1 s to - 8 x 1018 m-3 at 1.7 s with the average
main plasma density (from - 2 x l}te m'3 at t = 1.1 s to- 4 x lDte m-3 at 1.7 s), while the electron temperatureon the divertor leg decreases from - 45 eV at 1.1 s to 24 eY at 1.7 s. The measurements of plasma parameterson the divertor leg and these dependencies on the main
plasma density are qualitatively consistent with the

2. the finite difference between the center3.

wavelengths of transmittance of the interference
filters and these of the observed HeI lines,
the ambiguity of the relative sensitivities of theCCD cameras,

4. the instrumental uncertainty for acquiring anddigitizing the output signal from the cameras,
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Shoji M. et ul., CCD Camera Anay System for Electron Density and Temperature Measurement in the LHD Plasma Periphery

5. the conversion error of the HeI line intensity ratios

in n" measurement and about 40 Vo in T" measurement

to the plasma parameters (n" and T").
The first and second experimental erors can cause theless than 8 7o reduction of the intensity of the HeI lines,
leading to the about lO Vo reduction and rise of the neand r", respectively. We estimated about 10 Vo enor ofon the divertor legs, respectively. The reason why theexperimental error for the electron temperature issmaller than that for the electron density is ascribed tothe significant variation of the HeI line intensity ratios
to the change of the electron temperature in typicalplasma parameters on the LHD divertor legs.

the plasma parameters due to the third and forth experi-mental enors fiom the intensity plots of the cameras in

the calibration experiment in the intensity rage (> 60A.U.) which was actually used in plasma measurement.
The fifth conversion error definitely determines the accuracy of the plasma parameters in our camera system.
The data to convert the HeI line intensity ratios to theplasma parameters was derived under some idealized
theoretical assumptions (no high energy electrons, andReferences

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t2l M. Shoji er a/., J. Plasma Fusion Res. SERIES 3,440 (2000).

[3] E de la Cal et al., Nucl. Instrum. And Methods

no He* recombination, etc.). Considering the theoretical

A403, 490 (1998).
[4] F. Meo et al.,F.ev. Sci.Instrum. 6E,3426 (1997).

assumptions and the ambiguity of the dependence of the[5] S. Masuzaki et a/., J. Nucl. Mater. 290-293, 12(2001).

intensity ratios on the plasma parameters, we estimatedthe overall measurement error to be a factor of about 2.5441