22P112-P
Density fluctuation measurements of GAMMA10 plasma by using theFraunhofer Diffraction method
フラウンホーファー回折法によるGAMMA10プラズマの密度揺動計測
Yusuke Morikawa Masayuki Yoshikawa, Junko Kohagura, Yoriko Shima, Tomohiro Yamada,
Daichi Akita, Mizuki Sakamoto and Tsuyoshi Imai
森川裕亮、吉川正志、小波蔵純子、嶋頼子、山田智博、秋田大地、坂本瑞樹、今井剛
Plasma Research center, University of Tsukuba
1-1-1-1F, Tennouji, Tsukuba, Ibaraki 3058577, 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寺1-1-1-1
One of the techniques to study plasma instabilities in nuclear fusion plasmas is Fraunhofer diffraction(FD) method. We applied FD method to plasma density fluctuation measurement in GAMMA10.
Measuring frequency spectra of the density fluctuation by measuring of FD signal are carried out.

1. Introduction
Instabilities in plasma are one of the major causeof plasma confinement degradation in nuclear
fusion plasma. There is a density fluctuation
measurement method using dispersion ofelectromagnetic wave. Fraunhofer diffraction (FD)method is one of the forward scattering methods. In
this study, we applied FD method system to thedensity fluctuation measurements in the central cell
plasma of the GAMMA10 tandem mirror.
2. FD method system
Scattering methods with microwave to infraredlasers have been used for the study of low
frequency instabilities. In these conventional
scattering methods the frequency spectrum isobtained by frequency analysis of the scatteringwave. The wave number spectrum is obtained byvarying the scattering angle and using the Bragg
relation. The scattering angle has to be larger thanthe divergence angle of the incident beam in order
to avoid stray light. In the FD method, on the otherhand, a heterodyne detection method is applied tothe scattered wave and to the incident beam so that
detection is possible within the undeviated incident
beam, i.e. within the divergence of the probing
beam. Therefore, this technique makes it possible toinvestigate long wavelength waves, which areconsidered to be more relevant to anomaloustransport. We show a diagram of FD method system
in Fig.1.The FD method system is located at the eastside of the central cell, at z = 2.4 m, where z is theaxial distance from the central cell midplane. A beamof 70 GHz frequency is focused on the plasma centre
by a fused quartz lens. The frequency shifted FD
signal and the unshifted transmitted wave are focusedvia another lens onto beam directed GaAs Schottky

barrier diode mixers bonded to gold bow-tie antennas
which form a monolith with a fused quartz substrate.
Rectangular waveguide antennas in the TE10 mode areinstalled on the reverse side of the diode detectors.
The aperture of the waveguide antenna functions as aspectrometric slit in the plane of observation. Theintermediate frequency signals from the mixers areamplified by low noise amplifiers and fed to dataprocessing systems.

Fig.1. FD method system.

3. Experimental results
When fundamental electron cyclotron heating(ECH) is applied in the plug region (P-ECH), an ion
confining potential is created owing to theformation of a potential depression in the thermalbarrier. The P-ECH produces warm electrons at theend mirror cells. A fraction of heated electrons isdriven out of the end mirror through the loss cone
along the magnetic field lines. The plug potential
increases owing to the existence of warm electrons,
while the potential of the floating end platesdecreases.
Figure 2 shows the time evolution of FD signalof center cord of channel (a) and the density
fluctuation of it (b) at the central cell. The samplingtime is 1 μs. We can clearly observe the density
fluctuations. P-ECH and barrier-ECH are applied
between 151 ms and 174 ms and between 151 msand 200 ms, respectively.

FD Signal[a.u.]2000(a)Shot Number = 217743

1000
0-1000-2000
50

12000

100150Time[ms]

200250(b)Frequency[Hz]10000
8000
6000
4000
2000
00.02

0.04

0.06

0.08

0.1
0.12
Time[ms]

0.14

0.16

0.18

0.2

Fig.2. Time evolution of the FD signal (a) and thedensity fluctuation (b).

In Fig.2 (b), two frequency peaks of density
fluctuation, 5kHz and 10kHz, are confirmed
without P-ECH. When P-ECH is applied, density
fluctuation is suppressed.
4. Summary
The FD method was applied to the GAMMA10
tandem mirror in order to measure the density
fluctuations in the central cell plasma. Then, weobserved the low frequency fluctuation withoutapplication of P-ECH. The fluctuation suppression
was observed with application of P-ECH. We are
now reconstructing the total FD method measuringsystem to obtain the eight channels of FD signals,
simultaneously, and to get the wave number of thedensity fluctuations.
References[1] A.Mase, A. Itakura, M. Inutake, K. Ishii, J.H.
Jeong, K. Hattori, S. Miyoshi , Nucl. Fusion
31(1991) 1727