23P087-P
Behavior of the Temperature Anisotropy Driven ICRF Wave in the Inner
Region of the GAMMA10 Plasma
温度非等方性が駆動するICRF波動のGAMMA10プラズマ内部領域における挙動
Ryuya Ikezoe, Makoto Ichimura, Tatsunori Sato, Mafumi Hirata, Toru Iwai,
Takuro Yokoyama, Yumeto Ugajin, Takuma Iimura, Yu-ki Saito, Yoriko Shima,
Junko Kohagura, Masayuki Yoshikawa and Tsuyoshi Imai
池添竜也，市村真，佐藤達典，平田真史，岩井透，横山拓郎，宇賀神ゆめと，
飯村拓真，齋藤裕希，嶋頼子，小波蔵純子，吉川正志，今井剛
Plasma Research Center, University of Tsukuba
1-1-1 Tennodai, Tsukukba 305-8577, Japan筑波大学プラズマ研究センター 〒305-8577 つくば市天王台1-1-1
Behavior of a temperature anisotropy driven ICRF wave, named Alfvén ion cyclotron (AIC) wave, isinvestigated in GAMMA10 by using a microwave reflectometer and edge magnetic probes. The frequency
spectrum of the AIC wave has several discrete peaks in GAMMA10. Simultaneous measurement of theinternal density fluctuation and edge magnetic fluctuation shows that AIC waves grow as the same radial
eigenmode, and so the separation of peaks arises from some axial boundary condition. To investigate theaxial behavior of AIC waves, reflectometer system is being upgraded.

1. Introduction
GAMMA10 is the world’s largest tandem
mirror machine and characterized with strong
temperature anisotropy because of loss corn
nature of mirror trap and strong ICRF heating of afew hundred kW. Here, temperature anisotropy isdefined as the ratio of component perpendicular
to the magnetic field line to that parallel, T /T|| .
In the main confinement region named central
cell, ion temperature and its anisotropy becomes
greater than a few keV and 10, respectively. Insuch plasma, an electromagnetic wave named
Alfvén ion cyclotron (AIC) wave becomes
unstable and spontaneously excited. Theoretically
predicted driving force of AIC wave isproportional to  (T / T|| ) 2 .
In GAMMA10, AIC waves with several
discrete peaks in the frequency spectrum areexcited simultaneously and those frequencies andthe number of peaks vary with the change ofplasma parameters [1]. Previous studies using
adjacent edge magnetic probe sets have revealed
that the axial wave numbers k∥ of the AIC waves
vary from finite (propagating) to zero (standing)[2]. From these edge measurements, following
behaviors have been deduced; the AIC waves areexcited in the center region of the central cell
where both of  and temperature anisotropy
seems to be the highest in GAMMA10 and thenpropagate to the mirror throats. As the high and the strong anisotropy region extends, theboundary between standing and propagatingwaves moves to both in axial and radial directions.
The detailed spatial structure and the time
evolution of the boundary have remained to beclarified, partly because of the lack of internalmeasurements.
2. Set-up
To investigate the spatiotemporal behavior of
the AIC waves in detail, we have developed theinternal measurement using a reflectometer. Withthe use of a reflectometer, we can observe thedensity fluctuation arising from AIC waves
and/or the magnetic fluctuation in the inner
region of the plasma [3,4]. At first, weconstructed a simple reflectometer system, wherea pyramidal horn is used as both transmitter andreceiver, which reduces the number of hornantennas that must be installed inside the vacuum
chamber. Reflected wave from plasma isseparated from the incident wave by a circulator.
We used a frequency-variable yttrium-iron-garnet(YIG) oscillator as the microwave source, whichenables us to survey the measuring position inradial direction by appropriately tuning thefrequency. In this study, the incident frequency isfixed during a shot so that we can measure a time
evolution of the density fluctuation at a fixed
cut-off layer, and varied shot-by-shot.

3. Radial Evolution of AIC Wave
We have measured inner density fluctuations
and also edge magnetic fluctuations, which aredigitized in synchronization and then Fouriertransformed. We successfully observed several
peaks of AIC waves in the frequency spectra ofboth signals. The coherence between them isfound to be well above the statistical noise levelin those frequency ranges. Therefore, we canevaluate the phase difference of those two signals.
In Figs. 1(b) and (c), we show the radial
variations of the phase differences at two time
periods shown on Fig. 1(a), which were obtainedby varying the incident frequency of thereflectometer shot-by-shot in a series of plasma
discharges with discharge conditions fixed. The
horizontal axis can be regarded as radius because
the cutoff radius is monotonically related to theincident frequency for the GAMMA10 density
profile. Figure 1(b) shows that in the initial
excitation phase, when the peak frequencies rise
rapidly, the radial profiles of the phasedifferences differ each other except some peaks.
In the later steady phase, shown in Fig. 1(c), thedifference is diminished, and those radial profiles
approach each other. This behavior indicates thatseveral AIC waves grow as the same structure inradial direction, namely as the same radialeigenmode [5].

Fig. 1. Frequencies of spontaneously excited AIC
waves (a), and phase differences of AIC waves between
inner density fluctuation measure with a reflectometerand edge magnetic fluctuation, (b) and (c), which aretime averaged over the time periods denoted on (a).

4. Measurement of Axial Correlation
To investigate the axial evolution of the AIC
waves in the inner region of GAMMA10 plasma,

we have installed new horn antennas at z = 112 and137 cm with the same azimuthal angle. We havesuccessfully
observed
densityfluctuations
simultaneously at these two axial positions bymodifying the reflectometer system so as toincident microwaves through two antennas bydividing the power of an oscillator with a 2-way
power splitter, and mixes the received reflected
waves from two cut-off positions with the reference
waves separately. Although, the similar time
evolutions of the AIC waves have been obtained,
the correlation between them has found to be weakcompared with the correlation of signals from areflectometer and a magnetic probe. Therefore, we
couldn’t clearly distinguish the phase difference
between them. Reasons for poor correlation may
include small gap between magnetic field lines on
which the cut-off layers lie, and weak amplitudes ofdetected signals, and some fluctuations disturbing
cut-off layer as to violate clear measurement with areflectometer.
To carry away the above mentioned causes, weare further upgrading the reflectometer system. The
main upgrade is modification of the detecting
system from previous homodyne to heterodyne,
which enables us to discriminate the phasefluctuation arising from the oscillation of the cut-off
layer from the variation of the amplitude ofreflected signal, which is neglected on the previous
homodyne system. This discrimination might help
to improve clarity of density fluctuation from AIC
waves. The experiment using this new system is inprogress. Furthermore, we plan to use two
oscillators so that we can simultaneously measure at
two positions which exactly lie on the same field
line by appropriately altering the two incident
frequencies.
Acknowledgments
This work was supported in part by aGrant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science andTechnology of Japan (No. 21540506) and in part by
the NIFS Collaborative Research Program(NIFS09KUGM040). The authors thank themembers of the GAMMA10 groups for theircollaboration in the experiments.
References[1] M. Ichimura et al.: Phys. Rev. Lett. 70 (1993) 2734.
[2] R. Katsumata et al.: Phys. Plasmas 3 (1996) 4489.
[3] A. Mase et al.: Phys. Fluids B 5 (1993) 1677.
[4] H. Hojo et al.:J. Plasma Fusion Res. 69 (1993) 1043.
[5] R. Ikezoe et al.: Plasma Fusion Res. 6 (2011)
2402047.