23P041-P
Poloidal rotation measurements by Doppler reflectometer in LHD
ドップラー反射計によるＬＨＤプラズマのポロイダル回転速度計測
T. Tokuzawa1, A. Ejiri2, K. Kawahata1 and LHD Experiment Group
徳沢季彦1、江尻晶2、川端一男1、ＬＨＤ実験グループ
1)

2)

National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan
Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa 277-8561, Japan1)
核融合科学研究所 〒509-5292 土岐市下石町322-62)
東京大学 大学院新領域創成科学研究科 〒277-8561 千葉県柏市柏の葉5-1-5

In order to measure the poloidal rotation velocity, Doppler reflectometer has been developed in Large
Helical Device (LHD). The frequency hopping microwave reflectometer system has been utilized. For
tilting an antenna alignment, two types of antenna pair are applied. One has a fixed tilt angle and anotherhas a tunable control. In LHD last experimental campaign we have obtained first Doppler shifted signal
and its result is found to consist with the CXRS measured value. At the conference, we will show newly
obtained some experimental results and also some problems to be solved.

1. Introduction
Doppler reflectometry is a unique technique
combined with the backscattering method andreflectometry. It can measure the perpendicularvelocity of electron density fluctuations v⊥, theradial electric field Er, and the perpendicularwave number spectrum S(k⊥) in magnetized
confinement plasmas [1-8]. Especially, the Er andits shear are one of the important parameter for
the understanding of plasma turbulence andconfinement transition phenomena.
The principle of Doppler reflectometry can be
explained by the grating reflection model withsmall sinusoidal corrugation characterized by awave number k⊥. The signal wave is launched andreceived under a non-zero tilt angle θtilt with respect
to the normal onto the reflecting layer. This selects
perturbations with a finite wave vector component
in the reflecting layer k⊥ via microwave scattering
into diffraction order -1. This condition determines
the probed wave number to k⊥=-2 ki, where ki is the
local wave vector of the incident beam. For a
plasma slab, the Bragg condition ki = k0 sin(θtilt),
where k0 is the wave number of the microwave in
vacuum. By an actuation of θtilt, the k⊥-spectrum of
the density perturbations can be scanned. Theunwanted strong 0th order reflection used inconventional reflectometry is at least partiallysuppressed as it is shifted out of the receiving
antenna pattern. For the high-curvature LHDplasma, k⊥=-2 ki has to be used for determining theperpendicular wave number. To calculate ki, a 3D
beam tracing code LHDGAUSS [9] is employed.
The Doppler shift of the received signal depends

on the velocity of the plasma turbulence and on its
wave numberω D = v ⋅ k = v ⊥ k ⊥ + v|| k|| + v r k r. (1)In magnetically confined plasmas it is usuallyassumed that k⊥>> k|| and v⊥<v||, in a way that thesecond term is negligible with respect to the firstone. If in addition the turbulence does not displace
itself radially, the third term vanishes and ωD=v⊥k⊥
is obtained. Therefore, the propagation velocityv⊥(k⊥) of the selected perturbations can becalculated.
2. Microwave System
The schematic of ka-band Doppler reflectometersystem is shown in Fig. 1. A microwave synthesizer
is used as a source, because its phase noise is low
enough to apply the density fluctuationmeasurements. The utilized frequency range is from13 to 20 GHz and its output frequency is easily
Synthesizer
13-20GHz

26-40GHzf0f

2 f0x2tSSBfm

2 (f0 - fm) Mixer

2 f0 +φx2
IF AMPx2
fm =220MHz

2 fm

I.Q.
Acosφ AsinφBPF
440MHzLPF<40GHzLHD
plasma2 fm+φ+/-10MHzFig. 1. Schematic of the ka-band Doppler
reflectometer system.

+ Bt- Bt

Fig. 2. Complex frequency spectra. Each magnetic
field direction is positive (top) and negative(bottom). Solid line shows the Gaussian fittingcurve for extracting the Doppler shift.

changed by the external GPIB control. For
obtaining the complex frequency components andalso the phase fluctuation strength, the single
side-band (SSB) modulation is utilized. The source
output is split into the probe and the reference
signal. The probe signal is doubled followed by anactive multiplier to bring the launching frequency
up to 26-40 GHz (ka-band). The microwave
launches from the outboard side along inverse themajor radius direction on equatorial plane withslightly tilting angle. The polarization of launching
wave is selectable on the ordinary mode orextraordinary mode. The returning wave is received
and mixed with reference wave. The SSB
modulator driven by 220 MHz (fm) quartz oscillator
shifts the frequency of the reference signal for theheterodyne I-Q detection. The suppression levels ofimage sidebands are less than -20dB in this system.
The intermediate frequency (IF) signal is amplified
and filtered by band pass filter (BPF) which thepass frequency component is 440±10 MHz. Then,the IF signal and the modulation signal are led toI-Q detection. The output signals of I-Q
demodulator, which are described by Acosφ andAsinφ, are acquired by real-time data acquisition
system based on a compact PCI digitizer and thesampling rates are both 1 and 10 MHz during thewhole plasma discharge. Also, the part of the IF
signal is monitored by the spectrum analyzer forchecking the frequency shift.
3. Experimental Results
Here, we show a preliminary experimental resultof Doppler reflectometer with tilting angle antenna
setup in the LHD plasma. At first, we investigate
the direction of the back-scattered wave. When aLHD magnetic field direction is changed, the signof Doppler shift frequency would be changed.
Figure 2 shows complex frequency spectra obtainedin two almost same plasma discharges. The Doppler
shifted peak moves to the ion diamagnetic direction
in both graphs. The estimated each poloidal
velocity is about 5.8 (#99973) and 7.2 km/s(#100263). These values are quite reasonable
compared with CXRS.
Acknowledgments
This work was partially supported in part by
KAKENHI (Nos. 22360394, and 22017007). Alsoit was in part by a budgetary Grant-in-Aid of NIFS
LHD project and under the auspices of the NIFS
Collaboration Research program.
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