25F11
Observation of Plasma Fluctuation
by a Fast Balmer-αα Line Shape Measurement in LHD
高速Balmer-α線形状計測によるLHD中プラズマ揺動の観測
Keisuke Fujii1, Taiichi Shikama1, Motoshi Goto2, Shigeru Morita2 and Masahiro Hasuo1
藤井 恵介1, 四竈 泰一1，後藤 基志2，森田 繁2，蓮尾 昌裕1
1, Graduate School of Enginerring, Kyoto University
Yoshidahonmachi, Sakyo-ward, Kyoto 606-8501, Japan
2, National Institute for fusion science, Oroshi-cho, Toki-city, Gifu, 509-5292, Japan
1,606-8501
2,509-5292

京都大学大学院工学研究科 〒核融合科学研究所 〒京都市左京区吉田本町
岐阜県土岐市下石町

We have measured the Balmer-α emission spectrum from an LHD plasma with a wavelength resolution
and a measurement frequency of 0.14 nm and 70 kHz, respectively. The plasma is generated in the outward
shifted configuration having a major axis of 3.9 m with a magnetic field strength of 2.54 T. In
synchronization with a magnetic field fluctuation, the intensity increases near the peak of the spectrum
while decreases in the line wing. The phenomena suggest that the charged particle flux to the divertor
plates and the density inside the last closed flux surface increase with the fluctuation.

1. Introduction
In magnetically confined plasmas, it has beenfound that the Balmer-α shape has substantial
wings which cannot be reproduced by a single
Maxwellian velocity distribution [1, 2]. Recently,
we reported that the wing reflects the existence ofhigh velocity neutral atoms and its shape changes
depending on the plasma parameter inside the lastclosed flux surface (LCFS) [3]. The high velocity
neutral atoms are attributed to those penetrating
inside the LCFS and being heated through charge
exchange collisions with high temperature protons
there. Recently, Goto et al. estimated thepenetrating neutral atom density from the Balmer-αline shape [4].
On the other hand, in the magnetically confinedplasmas, the line-integrated line intensity of theBalmer-α emission, which is defined as the area ofthe line shape, is known to be proportional to theneutral hydrogen influx and nearly independent on
the electron density and temperature [5]. The influx
has been also known to relate to charged particle
outflux from the plasma [6]. An increase of theBalmer-α line intensity observed in anedge-localized-mode in Tokamaks is a well-known
example. A similar intensity increase has been also
observed in Large Helical Device (LHD) in theoutward shifted configuration [7].
Here, we report on a time-resolved measurementof the Balmer-α line shape and its line intensity
during the phenomenon in LHD.

2. Experiment
A hydrogen discharge in LHD is generated undera confining magnetic field strength of 2.54 T at themagnetic axis. The major radius of the plasma is 3.9
m. The discharge is sustained by neutral beam
injections with the heating power of 3.7 MW. The
electron temperature and the density are measured
to be 900 eV and 6 x 1019 m-3 at the plasma center,and 50 eV and 3 x 1019 m-3 around the LCFS,
respectively, by the Thomson scattering method [8].
The central region of a horizontally elongatedpoloidal plasma cross section of LHD is viewed
with a single line of sight. The field of view isroughly 150 mm height and 100 mm width. Theemission from the plasma is introduced by opticalfibers into a high dispersion spectrometer (JobinYvon, THR1000, grating: 2400 grooves/mm). Thespectrum is focused on the photo cathodes of amulti-anodephoto-multiplier-tube(PMT,Hamamatsu, R-5900U-20-L16) which equips 16channels along the dispersion direction. The linear
wavelength dispersion at the photo cathodes is 0.14
nm/ch. The output currents from the anodes of thePMT are amplified by 16 Op-Amps (TexasInstruments, OPA2743) and digitized by a
16-channels analog-digital converter (NationalInstruments, USB-6251). The cut-off frequencies of
the amplifiers are 40 kHz and the sampling rate is70 kHz. An example of the Balmer-α spectrum
measured with the system is shown in Fig 1 with alogarithmic vertical axis. The Balmer-α line
intensity can be obtained from the sum of thespectral intensities in the 16 wavelength regions.

105intensity(arb. units)v (x10 m/s)
5 4 3 2 1 0 -1 -2 -3 -4 -5

1

total (line intensity)4∆λ = 0.0 nm(a)320.41 nm (x100)
0.68 nm (x200)

0.1signal(arb. units)

spectral intensity(arb. units)

10

8765

0.01

0.001
-1.0

-0.5

0.0

0.5

1.0

∆λ (nm)Fig. 1. Example of a measured Balmer-α spectrum.
The velocity of the excited hydrogen atoms along
the line of sight is indicated in the top axis.

3. Result and Discussion
Figure 2 (a) shows 400 µs averaged temporaldevelopments of the measured Balmer-α spectral
intensities in the wavelength regions of ∆λ = 0.0(line center), 0.41, and 0.68 nm, where ∆λ is thewavelength shift from the line center. The
wavelengths are indicated with the vertical dottedlines in Fig. 1. For a comparison, the spectral
intensities in ∆λ = 0.41 and 0.68 nm shown in Fig.
2 (a) are multiplied by 100 and 200, respectively.
The Balmer-α line intensity which is dominantly
contributed by the spectral intensity in ∆λ = 0.0 nm
is also shown. In Fig. 2 (b), we plot magnetic field
fluctuation measured by the magnetic flux loop
located near to the measurement port.
Periodic changes with a cycle of about 15 ms canbe seen in the line intensity. In each cycle, the line
intensity shows a sharp increase followed by agradual decrease. In Fig. 2, the local maxima for theline intensity are shown with vertical dotted lines.
The increases of the line intensity synchronize tothe magnetic field fluctuations. The spectral
intensity in ∆λ = 0.41 nm also shows a sharp
increase in a cycle, while that in ∆λ = 0.68 nm
decreases and increases gradually.
The increase of the Balmer-α line intensity
indicates the increase of the hydrogen atom influx
to the plasma [5]. Since the influx depends on therecycling flux of the atoms from a divertor plate
and also relates to the charged particle flux to it, thesharp increase in the line intensity accompanying
with a magnetic field fluctuation suggests that thecharged particles are ejected to the divertor plates
by the plasma turbulence [7].
On the other hand, the spectral intensity in ∆λ =0.68 nm represents the number of fast hydrogenatoms having the velocity along the line of sight of
3 x 105 m/s. The corresponding kinetic energy is

10magnetic field fluctuation
-1
8.60
8.70
8.80

(b)
8.90

time (s)Fig 2. (a) The time developments of the Balmer-α
spectral intensity in three wavelength regions.
The wavelengths for each trace are indicated.
The line intensity of the Balmer-α emission is
also plotted. (b) The magnetic field fluctuation
measured by a magnetic flux loop.

470 eV, which is much higher than the electron
temperature around the LCFS. Such high velocity
atoms are mainly generated deeply inside the LCFS.
The decrease of the high velocity hydrogen atoms
comparing to the neutral influx indicates that thepenetration length of the hydrogen atoms decreases.
It suggests that the plasma fluctuation triggers anelectron density increase inside the LCFS.
Acknowledgments
The help of the LHD experimental group isgreatly acknowledged. This work was supported by
the National institute for Fusion Science(NIFS11KLPP023), and also by the Sasakawa
Scientific Research Grant from The Japan ScienceSociety (23-204).
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