22P125-P
Evaluation of the Non-Axisymmetry of the GAMMA10 Plasma
by Using a Segmented Limiter
分割リミターを用いたGAMMA10プラズマの非軸対称性の評価
Toru Iwai, Makoto Ichimura, Mafumi Hirata, Ryuya Ikezoe, Takuro Yokoyama, Yumeto Ugajin,
Tatsunori Sato, Takuma Iimura, Yu-ki Saito, Yousuke Nakashima and Tsuyoshi Imai
岩井透, 市村真, 平田真史, 池添竜也, 横山拓郎, 宇賀神ゆめと,佐藤達典, 飯村拓真, 齋藤裕希, 中嶋洋輔, 今井剛
Plasma Research Center,University of Tsukuba 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan筑波大学プラズマ研究センター 〒305-8577 茨城県つくば市天王台1-1-1プラズマ研究センター
Production of high-density and high-temperature plasmas is investigated as one of the main
purposes in the GAMA10 tandem mirror. Non-axisymmetry of the plasmas causes radial transport
in a mirror configuration. In this study, floating potentials in the vicinity of segmented limiter wasmeasured to evaluate the non-axisymmetry of plasmas. The limiter divided into 8 equal parts inazimuthal direction can measure the azimuthal distribution of the floating potentials on eachelement installed near the midplane of the GAMMA10 central cell. It becomes clear that
non-axisymmetry of the central cell plasma depends strongly on heating systems and their power
levels.

1. Introduction
IntheGAMMA10
tandemmirror,electromagnetic waves with two different
frequencies in the ion-cyclotron range of frequency(ICRF) are used for plasma production and ion
heating. For the magneto-hydrodynamic (MHD)
stabilization of the whole plasma, ICRF waves arealso used at the anchor cells, which are composedof minimum B field and located at both ends of thecentral cell. Waves in the electron-cyclotron
resonance (ECR) frequency are used for theelectron heating and the formation of the plasma
potential along the magnetic field line. In typicaldischarges, the formation of non-axisymmetricplasmas has been observed with a medium speed
camera placed of the midplane of the central cellthough the magnetic field has an axisymmetricconfiguration[1] Non-axisymmetry of the plasmas
causes radial transport of confined particles anddeterioration of plasma parameters. In this paper,
non-axisymmetry of plasmas in the central cell of

GAMMA10 is evaluated from the distribution ofthe floating potentials on a segmented limiter.
2. Experimental Setup
Figure 1 shows the schematic drawing of theGAMMA10 tandem mirror. A segmented limiter ofwhich diameter is 36cm is installed near themidplane of the central cell. The segmented limiter
is divided into 8 equal parts in the azimuthal
direction, as shown in Fig. 2. The segmented
elements are electrically-insulated from the ground.
Thus, the floating potentials and its azimuthal
distribution in the peripheral region can bemeasured. By connecting all segmented elements
electrically, we can change the boundary conditionsin the peripheral region.
xNo.1
No.2

No.8

No.3
No.7

y360 mmNo.4

No.62MΩNo.5100Ω
OSC（１ＭΩ）

Fig. 2. Configuration of the segmented limiter.
Fig. 1. Magnetic field structure of GAMMA10.

Fig. 3. Temporal evolution of floating potentials on thesegmented limiter.

Figure 3 shows the temporal evolution of thefloating potentials on each element (from No.1 toNo.8). Potential difference more than 100V isobserved between No.5 and No.8 elements. We canstudy the non-axisymmetry of the plasmas from thebehavior of these floating potentials.
3. Experimental Results(a) Reduction of the amplitude of low frequency
fluctuations
Two types of low-frequency fluctuations aredetected on the floating potential signal. These aredrift-type and flute-type fluctuations. Azimuthalmode structure of these fluctuations are analyzed by
the phase difference between floating potential
signals on segmented elements. The degradation ofthe plasma parameters due to low-frequency
fluctuations is observed in GAMMA10. Thesefluctuations are also observed with electrostatic
probes installed in the central cell. Therefore, weare able to analyze the amplitude and their mode
numbers. Drift-type fluctuations of which
frequency is around 10 kHz rotate in the directionof the election gyro-motion. As a result, it is clearlyobserved that the amplitude of the drift-type
fluctuation with low-azimuthal mode numbers
becomes small, when all segmented elements areconnected electrically. The potentials in theperipheral region would become uniform by theconnection.
(b) Evaluation of non-axisymmetry of the central
cell plasma using standard deviationThe azimuthal distribution of the floating
potential is a good indication of non-axisymmetryof the plasmas. Non-axisymmetry of the plasmas ina simple mirror configuration will cause diffusionin the radial direction and degrade plasma
parameters. We have evaluated non-axisymmetry of
the plasma by calculating the standard deviation of
the edge floating potentials from the axisymmetricprofile. We have observed the axisymmetry of theplasmas are affected by the heating systems.
(c) Evaluation of mode numbers of thenon-axisymmetry in the central cellIn this experiment, non-axisymmetry of theplasmas in the central cell of GAMMA 10 isevaluated with the azimuthal distribution of thefloating potentials on each element of thesegmented limiter. When ICRF and ECR are used
for the experiments, the azimuthal distribution of
the floating potentials depends on their injecting
powers. We measure the influence of ICRF andECR on non-axisymmetry of the plasma. The mode
numbers of the floating potential in the azimuthal
direction are analyzed. We evaluated thenon-axisymmetry of plasma by the application ofspatial Fourier transformation to 8 floatingpotentials. Each mode can be represented by 2variables, which are amplitude and phase, as shownin Fig.4. We can resolve the mode number from
m=0 to m=4. Figure 4 shows the temporalevaluation of the amplitude and phase of each modenumber.
Central ECR

Plug ECR

Fig. 4. Temporal evolution of the mode amplitudes andphases.

4. Summary
We have measured the floating potentials on the8 elements of the segmented limiter, and resolved
the non-axisymmetric behavior in azimuthal
direction up to m=4 mode by using Spatial Fouriertransform. It is found that m=1 mode is thestrongest and its phase is shifted with the increaseof ICRF power.
Acknowledgement
The authors would like to thank the members ofthe GAMMA10 groups for their collaboration in theexperiments and for helpful discussion.
References[1] Y. Nakashima et al., Trans. Fusion Sci. Technol.
55, No.2T, (2009) 38-45.