J. Plasma Fusisn Res. SERIES, Vol.3 (2000) 427430

Measurement of Soft X-Ray lmage by Using CCD Gamera
for Long Pulse Discharge
LIANG Yunfeng*, IDA Katsumir, KADO Shinichiror, WATANABE Kiyomasar
YAMADA Hiroshir and LHD G2 group
Graduate University for Advanced Studies, Toki 509-5292, Japan
INational InstituteFusion Science, Toki 509-5292,

forJapan(Received: 18 January 2000 / Accepted: 18 April 2000)

Abstract
The measurement of soft x-ray image using CCD camera was installed to a tangential port in LHD.
The large number of pixels (1O24 x 512) of CCD gives a good spatial resolution of 1.8 mm. The
tangential soft x-ray image is measured during long pulse discharge on LHD with a time resolution of
0.5 s. The shift of x-ray emission profile is found for plasmas with different magnetic axes.

Keywords:
soft x-ray CCD camera, soft x-ray image, magnetic axis

1. Introduction

beam.

The x-ray CCD camera with photon counting
capability has been widely applied to measure x-ray

In the end of 1999, the soft x-ray CCD camera wasinstalled on LHD and soft x-ray image data wasmeasured during long pulse discharge with timeimage and energy spectra in space physics and laser
fusion field [,2]. Because the readout time of CCD isabout few hundred milliseconds and the time resolution
was considered to be too poor, this type of CCD has
never been applied to the experiment in magnetically

x-ray CCD camera becomes more useful diagnostics,

resolution about 0.5 s. The LHD is a superconducting
helical device (Um = 2/10) with a major radius of 3.9 m
and a minor radius of 0.65 m. One of the mission forLHD project is to achieve high-p plasma with <p> >57oand the measurement of Shafranov shift of magnetic
axis becomes very important. In this paper, the soft xray CCD camera system on LHD and the preliminary
results of soft x-ray image measured with CCD camera

even the time resolution is poor.

during long pulse discharge are described.

confined plasma. In recent years, the long pulse
discharge above a minute with a temperature of >1 keV
was achieved in Large Helical Device (LHD). Then theThe soft x-ray CCD camera system has been

2. Experimental Set-up

applied to measure magnetic axis and two-dimensional
x-ray energy spectra on compact helical system (CHS)

The soft x-ray CCD camera system was installed
on a tangential port in LHD. The distance between theport axis and LHD middle plane is 0.21 m.

with image mode and photon counting mode,
respectively t3,41. By using this soft x-ray CCD camera,

the Shafranov shift of plasma magnetic axis was

The top view and the schematic of soft x-ray CCD

measured in CHS. The Shafranov shift is larger than thatcamera optic assembly are shown in Fig. 1. The soft x-estimated from diamagnetic loop, which is due to thebeam pressure driven by tangentially injected neutral

ray CCD camera optic assembly consists of pinhole,@2000 by The Japan Society of Plasma*C o rre sp o ndin g autho r' s e - mail : Iian g @ nifs. ac,j p

Science and Nuclear Fusion Research427

Y. Liang. et al., Measurement of Soft X-Ray Image by Using CCD Camera for Long Pulse Discharge

6-T Up Port
Gate Valvelnsulator

6-T Port

V-Shape

6-0 Port Pinhole Mask
Interchangeable
Be Filters

7-O Port

Fig. 1 Top view and scheme of soft x-ray CCD camera optic assembly installed on LHD.

absorption Be foil, shutter, and soft x-ray CCD camera.
The soft x-ray CCD camera is made by Princeton
Instruments (model number SX-TE/CCD-10245) with
TEK lO24 x 1024D frame transfer back illumination
CCD detector (the imaging is 512 x 1024 pixels). TheShutter

trigger

L-0.37s.

0.25s

r-*
iT-

9,37sr 0.25,

t-Shutter
openShutter idetector is cooled down to -40"C using multistage
Peltier devices to reduce the dark current of CCD. The
readout time of CCD is about 5 s with 100 kHz A/D rate
and 0.5 s with I MHz A,/D rate.
Four sets of pinhole with different diameter of 0.4mm, 0.2 mm, 0.1 mm, 0.05 mm, are installed to adjust
the flux of soft x-ray. The stainless-steel strip is fixed onmonitor
Not scanFini Enosure

Second Emsue

0123456

the pinhole selector. The size ofpinhole can be changedt (s)by using a stepping motor. The distance from pinhole to
CCD surface is 87 mm. The CCD camera views theplasma at a tangency radius of 3.65 m with the viewing
angle of+8.0o (horizontal directions) and 14.0o (verticalFig. 2 Timing chart of CCD camera with Ay'D rate 100 kHz
and752 x 320 pixels transferred.

direction). The observation region at horizontallyelongated cross section of LHD plasma is from R =2.710 m to 4.590 m horizontally and from Z = 4.780 m

controlled by a stepping motor. The x-ray flux isadjusted to the level enough for the imaging mode orphoton counting mode by choosing various

to +0.160 m vertically. Since the thickness of pinhole
plate is only lmm, it is too thin to stop the hard x-ray(>30 keV). Therefore a l2-mm-thick tantalum mask (Vshape hole) with pinhole diameter of 0.5 mm had been
installed in front of the pinhole plate.

combinations of pinhole and absorption foils. The lowenergy cut-off of soft x-ray measured depends on thethickness of Be filter. The cut-off energy is about I keV
for 50 pm Be filter. The upper limit of x-ray energy isl0 keV, which is determined by the quantum efficiency

A 50-pm-thick Be window is arranged between
LHD plasma and CCD to prevent visible light fromof CCD.

LHD chamber. Between the pinhole and the CCDsurface, a Be filter disk is arranged. The filter disk has

Because of strong magnetic field near the LHD
device during experiments, a shutter controlled bymagnet does not work. An air-controlled shutter isinstalled between pinhole and CCD to prevent x-ray

five Be filters with different thickness of 30 pm, 70 pm,
140 pm, 350 pm, 800 pm, respectively, and it isphotons coming to CCD surface during the readout ofimage data. The minimum pulse width of shutter trigger

plasma. The Be window isolates the CCD chamber from428

Y. Liang. et al., Measurement of Soft X-Ray Image by Using CCD Camera for Long Pulse Discharge

is about 0.3 s.

23.93 s, from 33.04 s to 33.53 s and from 61.84 s to62.33 s, respectively, show that the x-ray emission
profile is peaked, flat, then peaked again while both

Another important thing for CCD camera system isthe timing setting of external sync pulse and shutter
trigger. Figure 2 shows an example of the CCD camera
timing option with A/D rate of 100 kHz and 752 x 320

plasma electron density and stored energy areincreasing, decreasing and increasing again, as seen inFig. 3(a). It is seen that not only the x-ray intensity but
also the shape of x-ray profile varies as discharge timein this experiment.

pixels transferred. The CCD camera reads out one frame

for every external sync pulse received. The exposure
time setting and the readout time indicated by shutter
monitor output and not scan output is 0.8 s and 2.385 s,
respectively. By reason of the slow speed of aircontroller, the shutter requires 0.335 s to start open
action and another 0.(R7 s to open completely after
every shutter trigger received. To avoid the image
smearing which is due to continuous exposure during

(a)

150*

3.0
2.5

i: 1002.09.,

areadout, the timing of shutter open should be inside ofexposure time setting and the second exposure time

XlStr

F 50

1.0 rG

setting should start after the first frame readout

0.5ri

completed. This can be done by adjusting the delay time

l0 20 30 40 50 60 70 80

and pulse width of external sync pulse and shutter
trigger. In the case shown in Fig. 2, the start trigger isreceived before discharge 12 s with pulse width of 2 s.
The external sync pulse and the shutter trigger aredelayed 0.7 42 s and 0.665 s with pulse width 0. I s andO.623 s, respectively. The time region for the first x-ray

0t(s)

(b)

0,/rr(J>\ AN-o.2

1.04s-1.53s

-o.4

3.5

image frame is from 1.037 s to 1.52'l s, and the4.O

4.s

R(m)

repetition time is 3.2 s.

a0
N -o.,

3. Experimental Resultsf,-o.4A) Tangential soft x-ray images during long pulse

B3.5

4.5

D

61.84s-62.33s-o.44.0R(m)discharge

f'oN -0.;

3.0

3.5

4.O

R(m)

The tangential soft x-ray image during long pulse
discharge on LHD is measured in the last two weeks of-a- 1.037s-1.527s

LHD 1999 experimental campaign. The time evolution-+- 23.437s-23.927so- 33.037s-33.52

of various signals relevant to the present experiment isshown in Fig. 3(a) for a long pulse LHD plasma heatedby ICRF with vacuum magnetic axis of 3.6 m andmagnetic field of 2.750T. The line-averaged electronh6000

orttrq)density is measured using microwave interferometer at
R = 3.69 m, and the stored energy is measured using
diamagnetic loop.
Figures 3(b) and (c) show four contour plots andhorizontal profiles of soft x-ray emission measured byusing soft x-ray CCD camera with a 5O-pm-thick Be
filter and a 0.2-mm-diameter pinhole in different time
region which have been indicated in Fig. 3(a). The x-ray

4000,ru%^X/\image accumulated from 1.04 s to 1.53 s shows theweak x-ray intensity and flat x-ray emission profile
which are due to the low electron temperature andelectron density in the beginning of discharge. Other
three x-ray contour plots accumulated frbm 23.44 s toFig. 3 (a) The time evolution of plasma electron density
and stored energy for a long pulse LHD plasma.
(b) Contour plots of soft x-ray image measured bysoft x-ray CCD camera in four different timeregions. (c) Horizontal profiles of soft x-rayemission.
429

Y. Liang. er al., Measurement of Soft X-Ray Image by Using CCD Camera for Long Pulse Discharge

electron densities are 0.5 x 10le m-3 and 3.5 x l0le m-3,
respectively.
Figure 4 shows the contour plots and horizontal
profiles of soft x-ray emission measured with soft x-rayB) Soft x-ray image with different magnetic axes
The soft x-ray image is also measured from LHD

plasmas with different magnetic axes. In theseexperiments, target plasmas are produced by ECH

CCD camera from LHD plasmas with different

during 0.15-0.75 s. The plasma, with vacuum magnetic
axis of 3.6 m and magnetic field of 2.893 T, is heated bymagnetic axes. The soft x-ray image data are integrated
from 6.0 s to 6.75 s with 50-pm-thick Be filter and 0.2mm-diameter pinhole. Although the number of pixel is

ICRF from 0.3 s to 10.5 s. Another plasma, withmagnetic axis of 3.75 m and magnetic field of 1.5 T, isheated by NBI from 0.3 s to 9.22 s. Line-averaged

1024 x 512, the x-ray image measured is divided to 64 x

32 spatial channels (averaged by 16 x 16 pixels) toreduce noise. The two of steep gradient of x-ray
intensity in the contour plot indicate the shadow ofcarbon plate and cooling pipe, which are installed at #6T port. The third steep gradient of x-ray intensity at0+-0.2
N

about R = 3.3 m indicates the shadow of inner wall,
which gives the excellent reference for the position. The
shift of x-ray intensity center, which is due to different
magnetic axis for these two discharges, is measured
about 11.0 cm in major radius.

Ru*.'-:3.6m-0.4

B:2.8937-0.604. Summary
The soft x-ray CCD camera system has beendeveloped for long pulse discharge experiments onc -o)

NR*,.,",:3.75m
ts:1.51'-0.64.0I.6 104

LHD. The soft x-ray image with time resolution of 0.5 s
has been measured during long pulse ICRF and NBI
heating experiments. The shift of x-ray intensity centerfor LHD plasmas with different magnetic axes is

4.5-e- Rar=3.75m Bt=1.57

measured by using soft x-ray CCD camera.

Rax:3.6m AF2.ll9-iT

1.2 104

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g 8.0 rol[]

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4.0 t03

t2l S.P. Hirshman and J.C. Whitson, Phys. Fluids 26(12), December 1983.

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0.0
3.0

4.0

3.5

4.5

2nd IAEA Technical Committee Meeting on Steady-R(m)State Operation of Magnetic Fusion, pp.736-751,
1999.
t4l Y. Liang, K. Ida, S. Kado and I. Nomura, Rev. Sci.
Instrum (Submitted)

Fig. 4 Contour plots and horizontal profiles of soft x-ray

emission measured by soft x-ray CCD camera fordifferent magnetic axis LHD plasmas.

430