Proceedings of ITC/ISHW2007

Measurement of peripheral plasma turbulenceusing a fast camera in Heliotron J
N. Nishino, T. Mizuuchi1), S. Kobayashi1), K. Nagasaki1), H. Okada1), F. Sano1),
S. Yamamoto2), and K. Kondo3)
Graduate School of Engineering, Hiroshima University
1) Institute of Advanced Energy, Kyoto University
2) Kyoto University Pioneering Research Unit for Next Generation
3) Graduate School of Energy Science, Kyoto University(Received 15 Oct. 2007 / Accepted)

Keywords: turbulence, peripheral plasma, L-H transition, H-mode, Heliotron J, fast camera

1. Introduction
Study of peripheral turbulence is very important
issue for fusion research due to the relationship to theenergy confinement and H-mode physics. Fast cameras
have been installed in Heliotron J [1] to get informationon the peripheral plasma turbulence since several years
ago. In our previous work, using a combination methodof a fast camera (Ultima-SE, Photron) and a small
movable carbon target, a structure of a low frequency(5-6kHz) edge plasma oscillation in high electron density
ECH discharges was observed just after the L-Htransition [2]. Also a combination of a fast camera(FX-K4, NAC image technology), movable probe anddirectional gas puff technique gave us important
information that the spatial profile of turbulent burst in
the edge plasma were observed as a filamentary structurein the L-mode [3-5]. Each burst of the ion saturation
current was corresponding to that the filamentary
structure hit the probe simultaneously.
Recently the turbulent structures during the L-modeand H-mode were observed by the latter camera
with/without directional gas puff [6]. It was clear that thespatial structure of peripheral turbulence during theL-mode was different with that of the H-mode. Relativewide structure along the magnetic field was observed in
the H-mode however, narrower structure along themagnetic field was observed in the L-mode. Moreover,these filamentary structures were sometimes disappearedin the H-mode.
In this paper the obtained results included in thenishino@hiroshima-u.ac.jp

latest results of Heliotron J plasmas by the fast camera
are reported.

2. Experimental Setup
The tangential port was provided with the fastcamera to measure peripheral plasma behavior easily.
Fig.1 shows the tangential port position (green) andhorizontal port position (blue) for the fast camera. Thesemeasurements could not be done simultaneously becauseof the same fast camera (FX-K4, NAC imagetechnology).

Fig.1. Location of tangential view for the fast camera in
Heliotron J
Fig. 2 shows the tangential view of the fast camera.
This view is the initial phase of Heliotron J plasma
discharge. The first bright string in the images became
thicker and darker, and the equilibrium state was obtained.

Unfortunately the adjustment of the up-down direction in
the image was not completed this time, however, it was
clear to see the port and its position. In the image the grids
were each fiber of the coherent bundle fiber.

Is (A.U.)

Proceedings of ITC/ISHW2007

1.5
1
0.5
0

Fast probe

200time (ms)300

Fig.4 Ion saturation current signalup

Fig.2 Tangential view from the fast camera
A part of the original image (448x368pixels, 10000FPS)

3. Results and discussion

Wb (A.U.) I (A.U.) P (A.U.)

In high density ECH discharge of STD configuration(-86%, -87%, –88%, -89% of the magnetic field strength)
the abrupt drop of Dα signal were observed and theH-mode plasmas were obtained. In these shots thereproducibility was very good. Typical waveform of
H-mode shot is shown in Fig.3. Fig.4 shows the ion
saturation current of the movable probe of the same shot.
The power spectra of this ion saturation current show thelow frequency fluctuation were suppressed during
H-mode (not shown here).

0.15
0.1
0.05
00.4
0.3
0.2
0.1
0.2
0.1
0PECHDalphaDiamag200

time (ms)300

Fig.3 Typical waveform of H-mode in ECH discharge

From the tangential view images the plasma rotation
was observed clearly, moreover, it was observed thatplasma rotated counterclockwise in the images during theL-mode and stopped to rotate during the L-H transition,at last plasma rotated clockwise in the images during theH-mode. The magnetic field direction and the camera
viewing direction were counterclockwise from thehorizontal plan (see Fig.1) in these shots. Therefore, if
plasma would rotate by ErxB drift, Er should be positivein the L-mode and Er should be negative in the H-mode in
Heliotron J plasmas. These results were consistent withthe past results on the H-mode in Heliotron J [7]. The low
frequency of 8.75kHz signal was relative strong during
L-mode in the power spectra of the camera pixel data andno strong peak was obtained during H-mode. To evaluate
rotation effect with two-dimension this low frequency
fluctuation was chosen to see how behaves during the Land H-modes.
Fig.5 shows two-dimensional phase images using
time-dependent FFT analysis from the original camera
images. The column of the left shows images during theL-mode, the center shows images during the L-H
transition, and the right shows images during the H-mode.
The time progresses downward where time difference
between each frame is 25µs at each column.
The frequency of each frame was 8.75kHz and thefast camera operated with 40000 frames per second (FPS).
We used 32 frames for time-dependent FFT. The color iscorresponding to the phase –π to π. The same color
region has the same phase. Therefore, these images showthe propagation phenomena and/or wave at the selectedfrequency.
The rotation speed of 3000m/s was roughly
estimated in the images during H-mode and it was about
half during L-mode. Therefore, according to this roughestimation from the images Er is -2kV/m during theH-mode, and Er is +1kV/m during the L-mode. The
magnetic field is about 1.5T. These results were consistent
with Ref[7].
Transition period of the L-H transition was also
deduced in the images. The number of the camera framesthat rotation stopped is only one at 40000 frames persecond. That shows the transition period in Heliotron Jplasma is below than 50µs, because to get the lightemission the shutter was open in these shots.

Proceedings of ITC/ISHW2007

L-modeL-H transition
H-modeFig. 5 Phase images during the L-mode, the L-H transition, and the H-mode
Time progresses downward, and time difference between each frame is 25µs.

Proceedings of ITC/ISHW2007
4. Conclusion
Using fast camera peripheral plasma turbulence wasmeasured successfully. In particular, it was observed thatthe direction of plasma rotation was changed due toH-mode. Moreover, the transition period was estimatedby fast camera images because the plasma stopped torotate during the transition.
The fast camera usage for two-dimensional plasma
diagnostics is started recently in Heliotron J and webelieve this method will be very hopeful to get the imageof the plasma parameters in the future.

References
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