23P097-P
Effect of fast ion driven modes on radial electric fieldin the Large Helical Device
大型ヘリカル装置における高速イオン励起モードの電場に及ぼす影響
Takeshi Ido1, Akihiro Shimizu1, Masaki Nishiura1, Mitsuki Nakamura2, Kazuo Toi1,Yasuo Yoshimura1, Ryohei Makino2, Shinji Kato1, LHD experiment group1
井戸 毅1, 清水昭博1, 西浦正樹1,中村充希2, 東井和夫1, 吉村泰夫1, 牧野良平2, 加藤眞治1,
LHD実験グループ1
1

National Institute for Fusion Science322-6, Oroshi, Toki, Gifu 509-5292, Japan
2
Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-0073, Japan1
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-62
名古屋大学大学院工学研究科 〒464-8603 名古屋市千種区不老町Electrostatic potential fluctuations associated with fast-ion driven modes are observed by a heavy ion beamprobe in LHD plasmas. Characteristics of a mode with the toroidal mode number(n) of 0 are presented. The
initial frequency of the n=0 mode is in the range of the predicted GAM frequency, but it is higher than thepredicted frequency. Judging from the temperature dependence of the frequency and mode structure (n=0),
a candidate of the n=0 mode is energetic-particle driven geodesic acoustic mode(EGAM). When the n=0
mode is excited, other burst of energetic-particle driven mode also occurs simultaneously. In addition that,drop of the averaged electrostatic potential is observed. It may indicate outward flux of fast ions by theexcited modes.

1. Introduction
In future nuclear fusion reactor, there is aconcern that the energetic-particle driven Alfvén
eigenmodes(AEs) will enhance the radial transport
of energetic particles such as alpha particles in aD-T reaction and the loss of the energetic particles
will deteriorate the performance of the fusion
reactors. On the other hands, it is suggested that theradial transport of the energetic particles may form
radial electric field(Er), and may trigger improved
confinement of bulk plasmas[1].
Recently, in addition to that, geodesic acousticmode(GAM) driven by energetic particles has been
observed [2-6]. The GAM is a branch of zonal flow
and attracts much attention in the area of
turbulent-transport study[7] because the zonal flow
is considered to be a key phenomena regulating
turbulence level.
Therefore, the behaviors of energetic particle
modes including the influence on the radial electricfield should be investigated in order to predict theperformance of nuclear fusion reactors.
In recent LHD experiment, energetic-particle
driven modes with the toroidal mode number(n) ofzero and with the up-chirping frequency areobserved by Mirnov coils and a heavy ion beamprobe (HIBP). In this presentation, the electrostatic
potential fluctuations associated with theup-chirping n=0 mode are reported. The
electrostatic potential drop synchronizing with theexcitation of the mode is also presented.
2. Apparatus and experimental condition
In order to measure the electrostatic potential(),its fluctuations(  ) and density fluctuations( ne ),HIBP using a gold ion beam is installed on LHD.
The temporal and spatial resolution is 2 s and afew centimeters, respectively. Since the beamenergy(Eb) is 1.134 MeV in this experiment and theresolution of the electrostatic potential is /Eb =1.6 x 10-5, the potential fluctuation with theamplitude of 18 V or more can be measured. The
potential profiles can be measured from the center
to the half of the minor radius by scanning theprobing beam.
The plasmas are produced and sustained bytangential neutral beam injections (NBIs) in whichhydrogen beams with the energy of 170 keV are
injected in co and/or counter directions. The line
averaged electron density is about 0.1 x 1019 (m-3),
and the central electron temperature is about 2 keV
and it increases to 10 keV by superposition of ECH.
3. Characteristics of fast-ion driven n=0 modes
The spectrogram of the potential fluctuation isshown in Fig.1. The frequency of n=0 mode shifts
upward from 60 kHz to 80 kHz with a time constantof a few 10 ms. The time constant, the amount of
the frequency shift, repetition frequency changes
increase during the superposition of ECH. Since thetotoidal mode number is 0 and the frequency tends
to relate the electron temperature, a candidate of then=0 chirping mode is EGAM[4]. However, thefrequency of the mode is larger than the predicted
GAM frequency (40 kHz and 64 kHz for hydrogenplasmas without and with ECH, respectively).

Fig.1 Spectrogram of the electrostatic potential
measured by HIBP. The observed position is at thenormalized minor radius of about 0.3.
Spatial profiles of the mode can be measured
through scanning the probe beams of the HIBP.
According to the results, the n=0 mode exists in thecentral region like the GAM with a constantfrequency observed in reversed shear plasmas[6].
The frequency shift does not seem to correlate withthe observed position. Thus, it is not attributed tothe excitation position. The results may consistent
with the hole-and-clump model[8] in which thenonlinear evolution of the hole-and-clump pair inthe velocity space causes the frequency shift.
4. Effect of fast-ion driven modes on Er
Figure 2 indicates a temporal evolution of theaveraged electrostatic potential. The potential drops
by a few hundred volts when n=0 mode excited.
The magnitude of the drop correlates with theamplitude of the mode. Note that other bursts offast-ion driven modes are also exited in thefrequency range from 140 to 200 kHz as shown inFig.2(c).
Since a drop in the electrostatic potential meansthat an outward radial current is induced, the dropin Fig.2(a) may indicate outward flux of fast ionsby the n=0 mode and/or another fast-ion driven
mode. At present, however, a role of each mode is
not clarified because the both modes excited
simultaneously. The mechanism of the potential
drop remains an open question.

Fig.2 (a)Electrostatic potential at the normalizedminor radius of about 0.4. (b) Spectrogram of thepotential fluctuation. (c)Spectrogram of magnetic
fluctuation measured by a Mirnov coil.
Acknowledgments
This work was supported by MEXT Japan underGrant-in-Aid for Young Scientists (Nos 18760640
and 20686062), and NIFS/NINS under the projectof Formation of International Network for
Scientific Collaborations, NIFS09ULBB505, 515
and NIFS10ULHH020.
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