23P029-P
Measurement of Electron Density Fluctuations using Microwave
Reflectometer in Heliotron J
ヘリオトロンJにおけるマイクロ波反射計を用いた電子密度揺動計測
Koji Mizuno1), Kazunobu Nagasaki2), Kiyofumi Mukai1), Yuichi Nakamura1), Takeshi Fukuda3),
Teresa Estrada4), Shinsuke Ohshima5), Tohru Mizuuchi2), Takashi Minami2), Hiroyuki Okada2),
Kai Masuda2), Shinji Kobayashi2), Satoshi Yamamoto2), Hyunyong Lee1), Linge Zang1),
Shohei Arai1), Tasuku Kagawa1), Takayuki Minami1), Hiroaki Yashiro1), Hiroto Watada1),
Yoshinobu Wada1), Yuji Nakamura1), Kiyoshi Hanatani2), Shigeru Konoshima2), Fumimichi Sano2)
水野浩志 1)，長崎百伸 2)，向井清史 1)，中村雄一 1)，福田武司 3)，T. Estrada4)，大島慎介 5)，
水内亨 2)，南貴司 2)，岡田浩之 2)，増田開 2)，小林進二 2)，山本聡 2)，H.Y. Lee1)，L. Zang1)，
荒井翔平 1)，香川輔 1)，南貴之 1)，八代浩彰 1)，和多田泰士 1)，和田善信 1)，中村祐司 1)，
花谷清 2)，木島滋 2)，佐野史道 2)1) Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto, Japan
京都大学大学院エネルギー科学研究科 〒611-0011 京都府宇治市五ヶ庄
2) Institute of Advanced Energy, Gokasho, Uji, Kyoto University, Gokasho, Uji, Kyoto, Japan京都大学エネルギー理工学研究所 〒611-0011 京都府宇治市五ヶ庄
3) Center for Atomic and Molecular Technologies, Osaka University, 2-1, Yamadaoka, Suita, Osaka, Japan
大阪大学原子分子イオン制御理工学センター 〒565-0871 大阪府吹田市山田丘 2-1
4) Laboratorio Nacional de Fusión, EURATOM-CIEMAT, Spain
5) Pioneering Research Unit for Next Generation, Kyoto University, Gokasho, Uji, Kyoto, Japan
京都大学次世代開拓ユニット 〒611-0011 京都府宇治市五ヶ庄
A microwave reflectometer for measurement of electron density fluctuations is developed in Heliotron J.
The injection frequency of microwave ranges from 24.75 to 42 GHz corresponding to the O-mode cut-offdensity from 0.8 to 2.2×1019 m-3. A characteristic test with a vibrating reflector shows that the output
signal from the emitting horn has intensity from -10 to -5 dBm over the frequency range and that thereflectometer can measure fluctuations corresponding to frequency given to the vibrating reflector.

1. Introduction
It is necessary to understand the mechanism ofanomalous transport to improve plasma
confinement. It is considered that anomaloustransport comes from fluctuations of magnetic
field, electric field, plasma density, andtemperature out of various instability of plasma.
Therefore, it is important to measure theseparameters.
Measurement of density fluctuations withLangmuir probes and a beam emission
spectroscopy has been developed and performed in
Heliotron J. However, they can apply only to edge
plasma or NBI plasma. Microwave reflectometerhas a capability of measuring fluctuations over theconfinement region for ECH and/or NBI plasmas
by choosing injection frequency. In this research,
we have been developing a microwave
reflectometer system to measure electron density
fluctuations. The goal of this research is to study
the characteristics of plasma confinement by meansof investigating MHD instabilities of energetic

particles and long-range correlation of fluctuations
by combining the reflectometer with othermeasurement methods.
2. Principle of Reflectometer
Microwaves produced by an oscillator are
injected to plasma and reflected at an electron
density corresponding to the cutoff frequency [1, 2].
When electron density fluctuations arise, theposition of cutoff layer also changes with thefluctuations. The received waves with thefluctuations are mixed with the reference waves,and then the fluctuations are extracted by a phasedetector. We measure electron density fluctuations
at various cutoff layers by changing frequency ofmicrowaves. For the measurement system reported
here, we use the O-mode of which the cutoff
frequency depends only on electron density.
3. System Configuration
We have designed and assembled a microwave
reflectometer as shown in Figure 1 and performed aVCO(8.25-14 GHz)

LPF

Antennae~~
Amp.
Local oscillator(100MHz)

Function generator(DC or Step)~

PlasmaAmp.

Q

Figure 1 Schematic of reflectometercharacteristic test. The injection frequency ofmicrowaves to plasma ranges from 24.75 to 42 GHz(Ka-band), corresponding to the O-mode cut-off
density from 0.8 to 2.2×1019 m-3.
A function generator supplies DC or step
function to a voltage controlled oscillator (VCO).
The frequency band of 24.75-42 GHz is generated
by the VCO and a x3 frequency multiplier. The
oversized waveguides of WR-90 are used fortransmission of 5.8 m long in order to suppress
transmission loss. The microwaves are then divided
into the waves injected to plasma and the reference
waves by using a directional coupler. After theinjection waves are up-converted with a signal of
100 MHz generated by a local oscillator, they areinjected into the Heliotron J plasma through apyramidal horn. Two antennae are used as one forinjecting waves and the other for receiving waves,located inside the vacuum vessel to remove
parasitic reflection. At the receiver section, alow-pass filter is assembled to remove noise
coming from a high-power 70 GHz gyrotron. The
received microwaves are mixed with the otherwaves divided by the directional coupler todown-convert to the frequency of 100 MHz withfluctuations, delivering to an I/Q detector. We canestimate the complex phase difference between thereceived waves and the reference waves through theI and Q signals. The sampling frequency of a dataacquisition system is 1 MHz.

Intensity [dBm]

Intensity [dBm]

4. Characteristic Test
We have examined the performance of thereflectometer in a test stand by using a vibratingreflector to simulate plasma fluctuations. Figure 2shows the intensity of microwaves in the test stand.

100-10-20
2025303540Frequency [GHz]4500.0 0.2 0.4 0.6 0.8 1.01005000.020

Input to directional coupler
Input to amplifier
Input to mixer

1005002461.0

1.52.0(b) 1 kHz

10000.5

Frequency [kHz](a) 0.2 kHz

I/Q detector

201000150

Frequency [kHz]I

Input to directional coupler
Input to up-converter
Output from up-converter

2000

Intensity [a.u.]~Mixer

3000

Intensity [a.u.]

Waveguide(WR-28, WR-90)

FlangeIntensity [a.u.]

Up-converter

Attenuator

Intensity [a.u.]

Freq. X3 multiplier(24.75-42 GHz)

810

Frequency [kHz]1086420051015

20

Frequency [kHz](c) 6 kHz(d) 10 kHz
Figure 3 Spectrum of I signal in test stand. The
frequency of incident microwaves is 29.5 GHz.

The intensity of microwaves at each point isadjusted by an attenuator located before thedirectional coupler so that the input signal to themixer is less than 15 dBm to avoid the damage. The
average signal intensity and the flatness after theup-converter are -5.6 dBm and 5.2 dB, respectively.
Figure 3 shows the FFT result of the I signal ofI/Q detector. It is found that a peak appears at thefrequency of the vibrating reflector, indicating thatthe simulating fluctuations are measured asdesigned. The maximum measurable frequency of
the fluctuations is 10 kHz in the test stand. The
amplitude of fluctuations at the frequency higher
than 10 kHz from the vibrating reflector may be too
low to be measured. It is expected that this system
can measure fluctuations of higher frequency
because the I/Q detector can measure up to 1 MHz
signals as the specifications. In fact, we confirmedin last experimental campaign that fluctuations of
200 kHz could be measured in Heliotron J plasma
with a Q-band reflectometer with the same I/Q
detector. We will start the density fluctuation
measurement using the new Ka-band reflectometerin ongoing experimental campaign.
Acknowledgments
The authors are grateful to the Heliotron J staff
for conducting the experiments. This work wasperformed with the support and under the auspicesof the Collaboration Program of the Laboratory forComplex Energy Processes, IAE, Kyoto University,and the NIFS Collaborative Research Program(NIFS08KOAR010,
NIFS10KUHL030),
theNIFS/NINS project of Formation of International
Network for Scientific Collaborations.

-10-202025303540

Frequency [GHz](a)(b)
Figure 2 Intensity of microwave in test stand45References[1] K. Mukai, et al., Contrib. Plasma Phys. 50, No. 6-7(2010) 646.
[2] K. Mukai, et al., this conference.