J. Plasma Fusion Res' SERIES, vol. 3 (2000) 44H43

Development of the Real-time lmage Data Acquisition
System for Observing the Plasma Dynamic Behavior of LHD
Long-pulse Discharges
SHOJI Mamoru*, YAMAZAKI Kozo and YAMAGUCHI Satarou
National Institute.for Fusion Science, Toki 509-5292, Japan(Received: 18 January 2000 / Accepted: 6 April 2000)

Abstract
The experimental results of plasma dynamic behavior and impurity radiation profiles measured with
tangentially viewing CCD cameras in Large Helical Device (LHD) are reported. The two dimensional
profile of visible line emission from oxygen, carbon, helium and hydrogen has been measured. Thecameras observed magnetic island structure in the plasma periphery and divertor magnetic field linestructure. These structures are consistent with the calculations obtained by a multi-filament model. The
image data taken with these cameras will give useful information for the analysis of the effect of themagnetic island on impurity transport and for designing a closed divertor configuration planned in Phase
IL The image data acquisition system using data compression technique (MPEG-2) will be also
described.

Keywords:
image data acquisition, MPEG, CCD camera, long-pulse discharge, impurity measurement

1. Introduction

Recently, image data compression technique suchas JPEG and MPEG has been developed in the Intemettechnology. We applied the compression technique tothe data acquisition of plasma images in LHD. In theCharge Coupled Device (CCD) cameras are widelyused for observing plasma dynamic behavior, impurity
radiation profiles and plasma-wall interactions, etc [-4].
The image datp taken with the cameras give much
useful information. However, huge data storage system
is needed because of the output signal from many pixels

next chapter, the image data acquisition system will beshown. We will briefly explain the data compression
technique (MPEG-2) in chapter 3, and give thedependence of the image quality on compression inchapter 4, and two-dimensional profiles of impurity line
emission acquired with this system in chapter 5.

on the CCD arrays. When we used a standard video
camera (NTSC), a video data stream (-100 Mbps) isgenerated, corresponding to (125 Mbytes/shot!) when
the camera monitor plasma discharge for l0 seconds.
Video Tape Recorders (VTR) have been widely used forimage data acquisition. This analog recording system
has some disadvantages, i.e., the recorded images
deteriorate compared to the original images and tracking
noise is inevitable. Furthermore, we cannot access thedata at random, which is inconvenient for flexible data
analysis and efficient plasma experiments.
" C o rre spondinIautho r' s e -mail : shoj i @ LH D. ntfs.a c

2. Tangentially Viewing Cameras and lmage
Data Acquisition System

In plasma discharge experiments in LHD,
tangential viewing CCD cameras have been used as
useful apparatuses for monitoring plasmas. One camera
is mounted in the tangential port (7-T), and four cameras@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Researchjp440

Shoji M. et al., Development of the Real-time Image Data Acquisition System for Observing the Plasma Dynamic Behavior(SONY DXC-LSl) with interference filters are installed

and store the image data in real-time (less than a few
hundred msec), contributing to efficient image data

in another ta'ngential port (6-T). We can choose fourinterference filters from six ones (OII: 442.5 nm, CIII:465.4 nm, CII:. 426.7 nm, HeI: 587.8 nm. HeII: 468.6

acquisition at a reasonable cost.

nm and H": 656.6 nm) for these cameras. The video
signals from the cameras are integrated with a video

3. lmage Data Compression Techniquemultiplexer. The video signal is converted to an opticalsignal with a converter for avoiding electric noises. We
have stored the video signal by a video tape recorder(SONY SVT-S5100). To overcome the defects of theanalog recording, we newly introduced a Video On
Demand (VOD) system during the third experimental
campaign. Figure I shows the configuration of the VODsystem. The video signal is received by an MPEG-2
encoder board (FutureTel PrimeView-Duet) installed ina VOD encoder server. The data of the video sequenceis compressed by an MPEG-2 image data compression
technique and is stored in hard disks. The plasma video
image data can be easily accessed from a client terminalin the Local Area Network (LAN) for a LHD control
data processing system [5]. We have determined thesequence for plasma heating devices and the gas fuelingrate for the next plasma shot by considering the plasmaimages and experimental results measured with many

the second phase of MPEG (Moving Pictures Experts

The image data compression technique MPEG-2 isGroup) standard [6]. It provides higher-quality encoding
and is designed for providing higher-resolution video.

MPEG-2 is a lossy compression in which some of theoriginal data is lost. Although image fidelity may becompromised, up to 95 percent of the data representing
a particular image may be discarded without an apparent

loss in resolution. Images are compressed by Discrete
Cosine Transform (DCT), which is a form of encoding
to transform spatial information, typically an 8 x 8 blockof pixels, into digital frequency information. In thisprocess, high-frequency components are reduced,
leaving a softer picture but offering greater compression.

An MPEG-2 video sequence consists of a Group ofPictures (GOP) which is a series of one or more picturesintended to allow random access into the video
sequence. A GOP is composed of three kinds of frame:
I-, P- and B-frame. I-frame is a reference picture using
only the information present in the image itself andcoded using only the DCT compression. P-frame iscoded from the information contained in the nearest

plasma diagnostics. The camera system has also been
useful for controlling the gas fuelling rate in long-pulse
discharges. Since there are no abrupt plasma disruptionsin helical systems, we can easily control the plasma
density by optimizing the fuelling rate manually while

previous reference frame: either an I-frame or another Pframe. B-frames utilize information from both previous
and future frames.

monitoring the plasma. Our final goal is to sustain
plasmas for several hours. In this experiment, the realtime image data acquisition is essential, unlessextraordinary huge buffer memories for storing theimage data are needed. The VOD system can encode

Video Monitor V T R

4. Optimization of lmage Data Compression
Before applying the MPEG-2 standard to plasma
image data acquisition, we optimized the video sequence

bit rate acquired in the VOD system. Figure 2 shows thetime evolution of the images of the impurity radiation
observed with the cameras mounted in the 6-T port.
Figure 2 (a), (b) and (c) are the images in the case wherethe video bit rate is 0.58, 2.31 and 5.77 Mbps,respectively. Prominent block noises appear in the firstframe in the lowest video bit rates. The block noise is atypical noise appearing in compression process by DCT.
The noise diminishes as the progress of frames, which is
ascribed to the fact that the plasma shape becomessteady as the progress of time. As a result, the accuracyof the prediction of the images from I-pictures becomes
higher, leading the reduction of the noises under thelimitation of a specified video bit rate. Considering thequality of the plasma images in the plasma start-up
phase and comparing with the video images recorded inVideo Encoder Server

LHD

Fig. 1 Configuration of the VOD system for monitoring
LHD plasmas.
441

Shoji M. et al., Development of the Real-time Image Data Acquisition System for Observing the Plasma DynamicGGre*,rlrc rc*,GG rc
MPEC*2 (0.58 Mtrrs)(a)Behavior(b)

MPRC-Z (2,lll Mbp*)

MPEc-g (5.?7 Vbps)

Fig. 3 (a) Radiation profile of carbon ions (Cll) observedin the plasma start-up phase. (b) Calculated

magnetic island structure seen from the tangentialport (6-T).

Fig. 2 Dependence of the video streaming bit rate on the(a)plasma images taken by the CCD cameras(b)installed in 6-T port for (a) 0.58, (b) 2.31, lcl 5.77
Mbps. respectively.

the VTR, the optimized video bit rate is set to be 5.77
Mbps. Consequently, we have succeeded in compressing
the video image data into a reasonable size which is as
small as about 1/17 of that without compression.
Fig. 4 (a) Radiation profile of carbon ions (Cll) observed

5. Measurement of lmpurity Emission

in an NB|-heated plasma. (b) Calculated divertor
magnetic field line configuration seen from theProfiles
5.1 Magnetic lsland Strusturetangential port (6-T).

We used the image data acquisition system forstoring impurity emission profiles measured with thefour cameras in the 6-T port. We have observed many

that the magnetic island structure can be measured
during the plasma discharge by using the tangentiallyinteresting plasma images. For an example, the double
layer structure of the line emission from carbon ions(CII) in the plasma periphery in a plasma start-up phaseis shown in Figure 3 (a). We have already found thepresence of an m(poloidal mode number) = I magnetic
island in the plasma periphery by magnetic surface

viewing cameras. The camera system will contribute theinvestigation of impurity and particle transport in a
Local Island Divertor (LID) configuration in LHD.

5.2 Divertor magnetic field line structure
Divertor magnetic field line structure is essential

measurement using a fluorescent technique under highfor particle and impurity removal in long-pulse or steadystate plasmas. It is important to clarify the structure ofthe divertor plasma before designing the closed divertor
configuration planed in Phase II in LHD. The positionof the divertor plasma has been locally measured withmagnetic fields [7]. We thought that the double layer

was ascribed to the effect of the magnetic island:
trapped carbon ions in the island spread along themagnetic field lines to form the emission profile along

the magnetic island. We calculated the three
dimensional magnetic island structure by a multifilament model (HSD code) including the error field
which can explain the experimental results of thean electrostatic probe. Formation of the divertor

magnetic surface measurement. Figure 3 (b) illustrates
the calculated structure seen from the 6-T port, which
agrees with the experimental result. We have confirmed
the presence of the double layer structure by anothercamera in the 7-T port. These experimental results show

which firstly appeared in the third experimentalstructure, however, has not yet been clarified in detail.
In NBl-heated plasmas, we observed a clear light trace
campaign as shown in Figure 4 (a). We though that thelight trace was ascribed to the formation of the plasmaalong the divertor magnetic field lines. Using the multifilament model, we calculated the three dimensional442

Shoji M. et al., Development of the Real-time Image Data Acquisition System for Observing the Plasma Dynamic Behavior

structure of the divertor magnetic field lines. Figure 4(b) shows th6 calculated image of the field lines seenfrom the 6-T port, which agrees with the measured
image (Figure 4 (a)). The calculated image seen fromto be useful for investigating the effect of the magnetic
island on impurity transport in the plasma periphery andfor designing the closed divertor configuration toachieve steady-state plasmas in the future.

the 7-T port also proves the consistency with the image
observed by the camera in the 7-T port. Consequently,

Acknowledgements

we have succeeded in clarifying the presence of thedivertor structure from the image taken with thetangentially viewing cameras. It is expected that theThe authors are grateful to Mr. Nagae for his
contribution to construction of the image dataacquisition system, and we would like to acknowledge
the members of LHD experimental groups for theircollaboration in plasma discharge experiments.

camera systems give us useful information on impurity
transport and the divertor plasma physics for achieving
steady-state plasma opefation.

Referencestll S.J. Zweben et al., Phys. Fluids B I 10, 2058

6. Summary
The image data compression technique (MPEG-2)(1989).

has been successfully applied to acquisition of theplasma image data by optimizing the acquired video bitrates. The image data have been compressed to be areasonable size (about lllT

[2] F.Sardei et al., J. Nucl. Mater. 241-243, 135(1997').

[3] D.G. Nilson et al., Rev. Sci. Instrum.70,738

of that without(l999).

compression). The tangentially viewing CCD cameras
observed the magnetic island structure in the plasma
start-up phase and the divertor structure in the NBI-

[4] R.J. Maqueda et a/., Nucl. Fusion, 39,629 (1999).
[5] S. Yamaguchi et a/., J. Plasma Fusion Res. 73, 335

heated plasmas. These structures are in agreement with

[6] http://drogo.cselt.stet.itlmpeg/[7] M. Fujiwara et al., Proc.26th EPS Conf. on Contr.
Fus. and Plasma Phys., Mastricht (1999).

(1997).

images of the 3-dimensional magnetic field line
structure calculated by the multi-filament model
including the error field. These image data are expected443