J. Plasma Fusion Res. SERIES, Vol.7 (2006) 174-178

Measurement of NSTX Divertor Plasma Using Fast Camera
NISHINO Nobuhiro, ROQUEMORE Lane1 , BIEWER Theodore1 , ZWEBEN Stewart J.1 ,MAQUEDA Ricard2 , MAINGI Rajesh3 , BUSH Charles3 and NSTX Team
Hiroshima University, Higashi-Hiroshima 739-8527, Japan
1 Princeton Plasma Physics Laboratory, Princeton, NJ 08543, U.S.A.
2 Nova photonics Inc., Princeton, NJ 08543, U.S.A.
3 Oak Ridge National Laboratory, U.S.A.

（Received: 5 October 2004 / Accepted: 10 January 2006）

Abstract
To visualize the ELMs, fluctuations and associated impurity line emission near the lower x-point region,a new tangential observation port has been recently installed on NSTX. Many filaments are seen along theout board leg of the x-point in L-mode. Diﬀerences between L and H-modes are clearly seen near the outer
separatrix region as well as in the inner separatrix region. Structure-like phenomena, which are not along themagnetic field line, are found in H-mode near the outer separatrix region. The propagation of at least three
types of ELMS (I, III, V) is clearly seen from this view as well as MARFE-like region formed along thecenter stack.
Keywords:
divertor tangential view, fast camera, ELM, filament, L and H-modes, L-H transition
1．Introduction
Turbulence and / or fluctuation measurement is oneof the most important research in fusion plasma physics.
Because it is widely believed that the global particle / energy confinement was determined by turbulence.
Therefore, understanding the turbulence physics is toget the good confinement plasma and this will lead us
to achieve the cheep and safety fusion device. Turbulence in the edge region of tokamaks and other device
has been studied for many years (One of recent reviews
is cited like this [1]). Recently the great progress of themeasurement systems for fluctuations and ELMs [2-6]is made in NSTX, which is a medium sized low-aspectratio tokamak. Using a fast camera is a method for thesemeasurements.
This paper describes that the initial results of the divertor plasma measurement with a tangential view using a fast camera.

dow was installed to reach near the cutout in the passive
plate. Typical field of view from this tangential port isshown in Fig.1. The x-point of the standard configuration is located at near the center of this view. A lensand mirror inside the re-entrant port and a 400 × 400 coherent fiber optic bundle (4 mm × 4 mm, 2.7 m long) isused for transferring the plasma image to a fast camera(Photron Inc. Ultima-SE). The capability of this camera
is 40500 frames per second (FPS) with 64 × 64 pixels,

2．Diagnostics
A new tangential observation port has been installedon NSTX recently. A description of this diagnostics ispublished elsewhere [2], and so only a brief description will be summarized here. NSTX has a horizontal
port, which is located at approximately the height of astandard x-point. One of the passive plate tiles was removed to view the divertor region using this port, andto increase the field of view a re-entrant sapphire win-Fig. 1 Approximate ﬁeld of view with divertor tangential
port. Yellow (gray in B/W picture) box shows approximate ﬁeld of view for Ultima-SE fast camera.
The passive plate tile limits left side.
C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchCorresponding author’s e-mail: nishino@hiroshima-u.ac.jp174Measurement of NSTX Divertor Plasma Using Fast Camera

and 4500 FPS with 256 × 256 pixels. In front of thecamera a lens and the filter wheel were set up to change
the optical filter shot by shot remotely.

3.1 Fluctuations

One of the interesting things is that many filamentsin L-mode are observed near the outer separatrix region,
but no filament phenomena is observed in the inner separatrix region. Only wave pattern (finger-like pattern)
are observed in the inner separatrix regions. In divertor
view the filaments move downward and the fast camera can catch them over the speed of 13500 FPS. Figure
2 shows these fluctuations in L-mode. Apparently thefluctuations near the outer and inner separatrix regions
are quite diﬀerent. Due to the small aspect ratio of ST
plasma the angles of the magnetic field line is quite different near the outer and inner regions. However, thefilament structure usually expanded along the magnetic
field line. The wave pattern does not look like the filament. Both filaments and wave pattern are suppressed
during L-H transition. In H-mode a little filament canbe seen (but not zero), however, the wave pattern does
not disappear (not shown in the figure). Figure 3 shows
another structure-like phenomena in H-mode near theouter separatrix region. It looks like a shower curtain,
and this shower curtain is only seen in H-mode. The
angles of the bright zone of the shower curtain are quite
diﬀerent from that of the magnetic field line. From GPI(gas puﬀ imaging) experiment no spatial structure near
the outer mid-plane was found in H-mode. Up to now it
is unclear where is the origin of this shower curtain (thestructure-like phenomena). Also, this shower curtain
and the filaments can exist together during small ELMs

3．Experimental results and discussion
Fast visible camera had been using for the divertor
plasma measurement since 2002. The data accumulated period is only two years, however, there are already many image data in NSTX. Therefore, only typical image data are presented in this paper.

Filaments

Outerstrike point(a)Center stack

Inner separatrix

Wavepattern
Bright anddark zone(b)

Fig. 3 Spatial structure in H-mode. Spatial structure-likephenomena were found near the outer separatrix
region only in H-mode. The angles of this brightzone are very different from that of the magnetic
ﬁeld line.

Fig. 2 Fluctuations in L-mode. (a) Many ﬁlaments appear near the outer separatrix region in L-mode.
(b) Wave pattern (ﬁnger) appears in the inner separatrix region in L-mode.
175

Measurement of NSTX Divertor Plasma Using Fast Cameraof TYPE V (not shown in the figure). To summarize theobserved results, the filaments and wave pattern weresuppressed during L-H transition. However, the filaments and wave pattern were not disappeared even inH-mode. On the other hand the shower curtain could beseen only in H-mode.

time
103

133(a)

(b)

151

158

(c)

(d)

175

185(e)(f)

200

240(g)(h)

3.2 ELMs

Many ELMs of TYPE I, III and V were observedin NSTX during experiments in 2004. The categorization of ELMs is useful, but sometimes it is imprecise[7]. In this paper, the ELMs are also categorized bythe eﬀect on the energy confinement. Sometimes giant
ELM, medium ELMs and small ELMs are used here,
however giant ELM associated with severe energy loss
(up to 30 % in NSTX) is usually labeled TYPE I, andmedium ELM associated with small energy loss (a few% in NSTX) is labeled TYPE III here. At last small
ELM associated with tiny energy loss or no energy loss
is labeled TYPE V. This TYPE V ELM is found recently in NSTX.
Figures 4-6 show that giant ELM, medium ELM, andsmall ELM in the outer separatrix region respectively.
Giant ELM (TYPE I) has no precursor of the magnetic
probe signals (0.2 – 400 kHz). Figure 4 shows giant
ELM in outer separatrix region. Type I ELMs clearlyblow oﬀ the MARFE-like region all the way to the target. The MARFE-like structure rebuilds on the orderof tens of msec. It is believed that TYPE I is probably
caused by the ideal ballooning mode.
Medium ELMs are classified into three types by theprecursor of the magnetic probe signals (0.2 – 40 kHz).
One has 2 kHz MHD odd-n (odd toroidal number) low
frequency precursor, second has a weak precursor, andthe last has no precursor. Figure 5 shows the propagation of medium ELMs labeled TYPE III, which has
the precursor clearly. In this figure ELMs occur periodically. In the other medium ELMs also similar scene
had been taken by fast camera. Among the pictures ofthree medium ELMs, a little diﬀerence of the recovery
process can be seen (not shown in the figure). However, the diﬀerence of these medium ELMs is not clear
in NSTX, but it is believed that the cause of TYPE III
ELM is the resistive ballooning mode.
Small ELMs labeled TYPE V was first reported at
2002 [8], but it is categorized recently. This ELM(TYPE V) has n = 1 precursor of the magnetic probe
signals. From soft-X ray signals it is found that TYPE
V ELMs almost originate near x-point, and propagate
upward. Figure 6 shows periodically TYPE V ELM
using the fast camera. These pictures reveal that how
TYPE V ELM does birth near x-point. The left side ofFig. 4 Giant ELMs in divertor region. The speed of shutter is 40500 FPS. Frame numbers are indicated ineach pictures. This ELM is labeled TYPE I (giantELM). The bright lines in the lower part of eachframe are outer strike point.

x-point in these pictures behaves up and down repeatedly and a shaded light cloud goes upward along themagnetic surface.
Also, it is observed that another ELMs near the inner176

Measurement of NSTX Divertor Plasma Using Fast Camera

time
242

306

(a)

(b)

324

329
Fig. 6 Small ELM in divertor region. ELM grows in orderof time (a) to ( f ); each frames are at 222 sec
intervals. This ELM is labeled TYPE V, which originate near x-point and is going up toward the innerregion.

(c)

(d)

334339(e)(f)

353482(g)bounces almost from down to up, but sometimes fromup to down. Moreover this ELM bounces from down
to near the inner midplane and goes down, and viceversa. Merely this ELM goes up to the inner midplaneand split in two; one is going up, and the other is going down. This ELM is labeled TYPE VI. TYPE VI
ELM was only observed near the center region and wasnot observed in the outer separatrix region. Due to thelimit of the camera performance it cannot catch TYPE
VI ELM with wide spatial range at high-speed. Therefore, to find the origin location of this ELM more than
two cameras are needed to measure the same ELM atthe diﬀerent positions simultaneously.
The physics of TYPE V and VI ELMs are not known.
The analysis of these ELMs will be very near futurework.
4．Conclusions
It was found that many ELMs and fluctuations wereobserved in NSTX. ELMs of TYPE I, III and V (labeledrecently) were clearly seen in the divertor region. EvenDα filter images from the fast camera these ELMs canbe distinguished. Also, L-H transition was seen near
the outer and inner separatrix regions, and new structure was found in H-mode. The angles of this structure
were very diﬀerent with that of the magnetic field line,
and sometimes the filaments and this structure were observed together. Much analysis of all image data isneeded and it will be near term work for us.

(h)Fig. 5 Medium ELM in divertor region. The speed ofshutter is 40500 FPS. Frame numbers are indicated in each pictures. This ELM is labeled TYPE
III (resistive ballooning mode). The bright lines in
the lower part of 324 to 329 frames are outer strikepoint.

region in double null (DN) configuration (not shown inthe figure). This ELM is very interesting. Because it177

Measurement of NSTX Divertor Plasma Using Fast Camera

Acknowledgements
This work is supported by US-Japan collaborative
activity.

[4]

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